use std::collections::{HashMap, HashSet};
use std::ops::ControlFlow;
use std::rc::Rc;

use crate::model::category::{DIM_CATEGORY, INDEX_CATEGORY, MEASURE_CATEGORY};
use crate::model::cell::{CellKey, CellValue};
use crate::model::types::FoldMemo;
use crate::model::{Aggregation, Model};
use crate::view::{Axis, View};

/// The trailing column that creates a new measure: typing a name into it
/// adds that item to `_Measure` (improvise-c5v).
pub const NEW_MEASURE_COL: &str = "+";

/// The column that creates a new **category** (improvise-2zq). Its position
/// — end of the category block, not of the grid — is argued in core.md §6.
pub const NEW_CATEGORY_COL: &str = "+cat";

/// Extract (record_index, column_index) from a synthetic records-mode key;
/// `None` for a pivot key. `_Dim` carries an index, not a label (§6).
pub fn synthetic_record_info(key: &CellKey) -> Option<(usize, usize)> {
    let idx: usize = key.get(INDEX_CATEGORY)?.parse().ok()?;
    let col: usize = key.get(DIM_CATEGORY)?.parse().ok()?;
    Some((idx, col))
}

/// One row of the records grid: the coordinates its cells share (never
/// `_Measure`) and the measure values against them. Wide format (§6).
#[derive(Debug, Clone, PartialEq)]
pub struct RecordRow {
    pub coords: CellKey,
    pub values: Vec<(String, CellValue)>,
}

impl RecordRow {
    /// Collapse loose cells into wide rows, one per coordinate tuple, in
    /// first-appearance order so the store's order survives (§8).
    pub fn group(cells: impl IntoIterator<Item = (CellKey, CellValue)>) -> Vec<RecordRow> {
        let mut rows: Vec<RecordRow> = Vec::new();
        for (key, value) in cells {
            let measure = key.get(MEASURE_CATEGORY).map(String::from);
            let coords = key.without(MEASURE_CATEGORY);
            let row = match rows.iter().position(|r| r.coords == coords) {
                Some(i) => &mut rows[i],
                None => {
                    rows.push(RecordRow {
                        coords,
                        values: Vec::new(),
                    });
                    rows.last_mut().expect("just pushed")
                }
            };
            // A cell with no `_Measure` still anchors a row (that is how
            // `add-record-row` seeds one) but holds no column value.
            if let Some(measure) = measure {
                row.values.push((measure, value));
            }
        }
        rows
    }

    /// The value recorded for `measure` in this row, if any.
    pub fn value(&self, measure: &str) -> Option<&CellValue> {
        self.values
            .iter()
            .find(|(m, _)| m == measure)
            .map(|(_, v)| v)
    }

    /// The model cell holding this row's `measure` value.
    pub fn cell_key(&self, measure: &str) -> CellKey {
        self.coords.clone().with(MEASURE_CATEGORY, measure)
    }
}

/// The key a drill into `key` actually constrains by: a formula target is
/// stripped, or `matching_cells` returns nothing (gotcha 10).
pub fn drill_constraint(model: &Model, key: &CellKey) -> CellKey {
    let names_a_formula = key.get(MEASURE_CATEGORY).is_some_and(|target| {
        model.formulas().iter().any(|f| {
            crate::model::category::CategoryKind::of_name(&f.target_category).is_measure()
                && f.target == target
        })
    });
    match names_a_formula {
        true => key.without(MEASURE_CATEGORY),
        false => key.clone(),
    }
}

/// The frozen record list a drill into `constraint` shows. **One home**: a
/// second derivation would freeze a different order — core.md §8, gotcha 18.
pub fn drill_snapshot(model: &Model, constraint: &CellKey) -> Vec<RecordRow> {
    let cells: Vec<(CellKey, CellValue)> = match constraint.0.is_empty() {
        true => model
            .data
            .iter_cells()
            .map(|(k, v)| (k, v.clone()))
            .collect(),
        false => model
            .data
            .matching_cells(&constraint.0)
            .into_iter()
            .map(|(k, v)| (k, v.clone()))
            .collect(),
    };
    RecordRow::group(cells)
}

/// The records row `record` is, as the records grid groups it; empty when no
/// cell stores it. The one lookup a rename by record uses (improvise-fdk1).
pub fn record_row(model: &Model, record: &CellKey) -> RecordRow {
    drill_snapshot(model, record)
        .into_iter()
        .find(|row| &row.coords == record)
        .unwrap_or_else(|| RecordRow {
            coords: record.clone(),
            values: Vec::new(),
        })
}

/// One step of a view configuration: **what** a view must be, with no answer
/// to how a front end gets there. The model half of drill — core.md §8.
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum ViewStep {
    Axis { category: String, axis: Axis },
    PageSelection { category: String, item: String },
}

/// The records-mode spine: `_Index` on Row and `_Dim` on Column, which is
/// what `GridLayout` auto-detects records mode by (§6).
pub fn records_spine() -> Vec<ViewStep> {
    vec![
        ViewStep::Axis {
            category: INDEX_CATEGORY.to_string(),
            axis: Axis::Row,
        },
        ViewStep::Axis {
            category: DIM_CATEGORY.to_string(),
            axis: Axis::Column,
        },
    ]
}

/// The `_Records` view over `model`: the spine, and every other category
/// aggregated away onto `Axis::None`.
pub fn records_steps(model: &Model) -> Vec<ViewStep> {
    let mut steps = records_spine();
    let rest = model
        .categories
        .keys()
        .filter(|name| !crate::model::category::CategoryKind::of_name(name).is_index_or_dim());
    steps.extend(rest.map(|name| ViewStep::Axis {
        category: name.clone(),
        axis: Axis::None,
    }));
    steps
}

/// The `_Drill` view a drill into `key` needs, given the categories the grid
/// it was launched from was aggregating over. Coordinates fix on **Filter**.
pub fn drill_steps(key: &CellKey, none_cats: &[String]) -> Vec<ViewStep> {
    let mut steps = records_spine();
    // A drilled coordinate is an immutable constraint of this drill, not a
    // dimension to page through (improvise-dz8, gotcha 27).
    for (cat, item) in &key.0 {
        steps.push(ViewStep::Axis {
            category: cat.clone(),
            axis: Axis::Filter,
        });
        steps.push(ViewStep::PageSelection {
            category: cat.clone(),
            item: item.clone(),
        });
    }
    // Previously-aggregated categories stay on `Axis::None` so they do not
    // filter records; they appear as columns.
    let fixed: std::collections::HashSet<&str> = key.0.iter().map(|(c, _)| c.as_str()).collect();
    let free = none_cats
        .iter()
        .filter(|cat| !fixed.contains(cat.as_str()) && !cat.starts_with('_'));
    steps.extend(free.map(|cat| ViewStep::Axis {
        category: cat.clone(),
        axis: Axis::None,
    }));
    steps
}

/// What a records-mode column holds, resolved from the column index so a
/// category and a measure sharing a name stay distinguishable.
#[derive(Debug, Clone, PartialEq, Eq, Hash)]
pub enum RecordCol {
    /// A coordinate of the row; editing it moves every measure cell in it.
    Category(String),
    /// A measure; editing it writes that one cell.
    Measure(String),
    /// The trailing column that creates a new measure.
    NewMeasure,
    /// The column that creates a new category, sitting where the column it
    /// creates will appear (improvise-2zq).
    NewCategory,
}

/// One entry on a grid axis. A `GroupHeader` is always visible so its label
/// and collapse state show; a collapsed group is ONE `DataItem` (core.md §14).
#[derive(Debug, Clone, PartialEq)]
pub enum AxisEntry {
    /// Opens a run at `(level, depth)` — the category's place on the axis, then the
    /// group's depth in its hierarchy — and closes every run inside it (§6, §19).
    GroupHeader {
        level: usize,
        depth: usize,
        cat_name: String,
        group_name: String,
        /// The view's answer for this group, so no reader re-derives it.
        collapsed: bool,
    },
    /// Ends the run at `(level, depth)` and every run inside it (improvise-gd83,
    /// e265). Nothing is drawn for it; it exists so membership is not inherited.
    GroupEnd { level: usize, depth: usize },
    /// `collapsed` is not derivable from `tuple`: a group and an item may share a
    /// name. Set for a folded group and for everything expanded beneath one.
    DataItem { tuple: Vec<String>, collapsed: bool },
}

impl AxisEntry {
    /// An ordinary, uncollapsed data item.
    pub fn item(tuple: Vec<String>) -> Self {
        AxisEntry::DataItem {
            tuple,
            collapsed: false,
        }
    }
}

/// The data-item tuples on an axis, skipping group headers. Every public
/// `row`/`col` argument indexes this sequence, not the visual entry list.
#[cfg(test)]
fn data_items(items: &[AxisEntry]) -> impl Iterator<Item = &Vec<String>> {
    items
        .iter()
        .filter_map(|e| match e {
            AxisEntry::DataItem { tuple, .. } => Some(tuple),
            AxisEntry::GroupHeader { .. } | AxisEntry::GroupEnd { .. } => None,
        })
        .inspect(|_| DATA_ITEM_STEPS.with(|n| n.set(n.get() + 1)))
}

#[cfg(test)]
thread_local! {
    /// Items `data_items` has yielded on this thread: the work a lookup did.
    static DATA_ITEM_STEPS: std::cell::Cell<usize> = const { std::cell::Cell::new(0) };
}

/// Entry `positions(items)[n]` is data item `n`: a lookup by data index is O(1),
/// not a walk past every header before it (layout-cost.md).
fn positions(items: &[AxisEntry]) -> Vec<usize> {
    items
        .iter()
        .enumerate()
        .filter_map(|(at, e)| matches!(e, AxisEntry::DataItem { .. }).then_some(at))
        .collect()
}

/// The data item at `idx`, by the axis's position index.
fn data_item<'a>(items: &'a [AxisEntry], pos: &[usize], idx: usize) -> Option<&'a Vec<String>> {
    match items.get(*pos.get(idx)?)? {
        AxisEntry::DataItem { tuple, .. } => Some(tuple),
        AxisEntry::GroupHeader { .. } | AxisEntry::GroupEnd { .. } => None,
    }
}

/// Whether the data item at `idx` folds a collapsed group.
fn folds(items: &[AxisEntry], pos: &[usize], idx: usize) -> bool {
    match pos.get(idx).and_then(|at| items.get(*at)) {
        Some(AxisEntry::DataItem { collapsed, .. }) => *collapsed,
        Some(AxisEntry::GroupHeader { .. } | AxisEntry::GroupEnd { .. }) | None => false,
    }
}

/// The data index of the first item in the run `cat`/`group` opens under
/// `prefix` — where a toggle's cursor lands, collapsing or expanding (§14).
fn run_start(
    cats: &[String],
    items: &[AxisEntry],
    cat: &str,
    group: &str,
    prefix: &[String],
) -> Option<usize> {
    let level = cats.iter().position(|c| c == cat)?;
    let mut data_idx = 0usize;
    let mut armed = false;
    for entry in items {
        match entry {
            AxisEntry::GroupHeader {
                cat_name,
                group_name,
                ..
            } => armed |= cat_name == cat && group_name == group,
            AxisEntry::GroupEnd { .. } => {}
            AxisEntry::DataItem { tuple, .. } => {
                if armed && tuple.get(..level) == Some(prefix) {
                    return Some(data_idx);
                }
                armed = false;
                data_idx += 1;
            }
        }
    }
    None
}

/// The display label of the data item at `idx`, or empty when out of bounds.
fn axis_label(items: &[AxisEntry], pos: &[usize], idx: usize) -> String {
    data_item(items, pos, idx)
        .map(|v| v.join("/"))
        .unwrap_or_default()
}

/// Visual index of the nth data entry (skipping group headers).
fn data_index_to_visual(pos: &[usize], data_idx: usize) -> Option<usize> {
    pos.get(data_idx).copied()
}

/// Where a run sits: `(level, depth)`, ordered, so "inside" is "greater than".
/// Across categories the level decides; within one, the depth (§19).
pub type RunKey = (usize, usize);

/// One open run: its key and the position of the header that opened it.
pub type OpenRun = (RunKey, usize);

/// Each open run, innermost last, with its header. A header at K replaces K
/// and closes every deeper run; a `GroupEnd` at K closes K and deeper.
type OpenRuns = Vec<(RunKey, usize, AxisEntry)>;

/// Close every run at or inside `key`, then open `run` there if one is given.
fn open_at(runs: &mut OpenRuns, key: RunKey, run: Option<(usize, AxisEntry)>) {
    let outside = runs.partition_point(|(k, _, _)| *k < key);
    runs.truncate(outside);
    runs.extend(run.map(|(at, header)| (key, at, header)));
}

/// Drop the data items `keep` rejects, then re-mark the survivors so each keeps
/// exactly the runs it was in. Why re-marked, not filtered: core.md §6, e265.
fn prune_axis_items(items: &mut Vec<AxisEntry>, keep: impl Fn(usize) -> bool) {
    let mut kept: Vec<AxisEntry> = Vec::new();
    // What the input has open, and what `kept` has open so far.
    let mut input: OpenRuns = Vec::new();
    let mut output: Vec<OpenRun> = Vec::new();
    let mut data_idx = 0usize;
    for (at, entry) in items.drain(..).enumerate() {
        match entry {
            AxisEntry::GroupHeader { level, depth, .. } => {
                open_at(&mut input, (level, depth), Some((at, entry)))
            }
            AxisEntry::GroupEnd { level, depth } => open_at(&mut input, (level, depth), None),
            AxisEntry::DataItem { .. } => {
                if keep(data_idx) {
                    remark(&input, &mut output, &mut kept);
                    kept.push(entry);
                }
                data_idx += 1;
            }
        }
    }
    *items = kept;
}

fn group_end((level, depth): RunKey) -> AxisEntry {
    AxisEntry::GroupEnd { level, depth }
}

/// Emit the markers that make `kept`'s open runs equal the input's: past the
/// runs they share, end the deeper ones innermost out, then open what differs.
fn remark(input: &OpenRuns, output: &mut Vec<OpenRun>, kept: &mut Vec<AxisEntry>) {
    let shared = input
        .iter()
        .zip(output.iter())
        .take_while(|((_, want, _), (_, have))| want == have)
        .count();
    let want = input.get(shared);
    let have = output.get(shared).copied();
    let Some(slot) = [want.map(|w| w.0), have.map(|h| h.0)]
        .into_iter()
        .flatten()
        .min()
    else {
        return;
    };
    // Deeper runs end first, innermost out, as `expand_category` ends them.
    for (key, _) in output[shared..].iter().rev().filter(|(k, _)| *k > slot) {
        kept.push(group_end(*key));
    }
    output.truncate(shared);
    let replaces = want.is_some_and(|w| w.0 == slot);
    if !replaces && have.is_some_and(|h| h.0 == slot) {
        kept.push(group_end(slot));
    }
    for (key, at, header) in &input[shared..] {
        kept.push(header.clone());
        output.push((*key, *at));
    }
}

/// The one reading of the markers: each data item's open runs, innermost last,
/// lent to `visit` — one stack, never copied (0r01).
fn walk_open_runs<T>(
    items: &[AxisEntry],
    mut visit: impl FnMut(&[OpenRun]) -> ControlFlow<T>,
) -> Option<T> {
    let mut open: Vec<OpenRun> = Vec::new();
    for (at, entry) in items.iter().enumerate() {
        match entry {
            AxisEntry::GroupHeader { level, depth, .. } => {
                open.truncate(open.partition_point(|(k, _)| *k < (*level, *depth)));
                open.push(((*level, *depth), at));
            }
            AxisEntry::GroupEnd { level, depth } => {
                open.truncate(open.partition_point(|(k, _)| *k < (*level, *depth)))
            }
            AxisEntry::DataItem { .. } => {
                if let ControlFlow::Break(found) = visit(&open) {
                    return Some(found);
                }
            }
        }
    }
    None
}

/// The innermost run open at the data item `idx` — the group nearest it, which
/// is what `z` toggles (improvise-e265's ruling).
fn innermost_group(items: &[AxisEntry], idx: usize) -> Option<(String, String)> {
    let mut data_idx = 0usize;
    let at = walk_open_runs(items, |open| match data_idx == idx {
        true => ControlFlow::Break(open.last().map(|(_, at)| *at)),
        false => {
            data_idx += 1;
            ControlFlow::Continue(())
        }
    })??;
    match &items[at] {
        AxisEntry::GroupHeader {
            cat_name,
            group_name,
            ..
        } => Some((cat_name.clone(), group_name.clone())),
        AxisEntry::GroupEnd { .. } | AxisEntry::DataItem { .. } => None,
    }
}

/// One category constraining which records are visible: what it is fixed to,
/// and — when the user may page it — what `[`/`]` cycle. core.md §7.
#[derive(Debug, Clone, PartialEq, Eq, serde::Serialize, serde::Deserialize)]
pub struct PageAxis {
    pub cat: String,
    pub selected: String,
    /// `Some` exactly for `Axis::Page`. A `Filter` axis constrains without
    /// being navigable, so a client can compose no cycle for one (gotcha 27).
    pub items: Option<Vec<String>>,
}

/// The page axes as the plain coordinate pairs `matching_cells` and
/// `CellKey::new` take.
pub fn page_constraints(axes: &[PageAxis]) -> Vec<(String, String)> {
    axes.iter()
        .map(|p| (p.cat.clone(), p.selected.clone()))
        .collect()
}

impl PageAxis {
    /// The items to cycle and the index `selected` sits at, for the two
    /// callers that page. `None` where the axis is not navigable.
    pub fn cycle(&self) -> Option<(&[String], usize)> {
        let items = self.items.as_deref().filter(|i| !i.is_empty())?;
        let at = items.iter().position(|i| *i == self.selected).unwrap_or(0);
        Some((items, at))
    }
}

/// The resolved 2-D layout of a view, and the one place the multi-dimensional
/// model becomes the flat grid the renderer and the CSV exporter consume.
pub struct GridLayout {
    pub row_cats: Vec<String>,
    pub col_cats: Vec<String>,
    pub page_coords: Vec<PageAxis>,
    pub row_items: Vec<AxisEntry>,
    pub col_items: Vec<AxisEntry>,
    /// `positions(row_items)` / `positions(col_items)`: derived, so every writer of
    /// the items calls `reindex`, and nothing outside this module writes them.
    row_pos: Vec<usize>,
    col_pos: Vec<usize>,
    /// Categories on `Axis::None` — hidden, implicitly aggregated.
    pub none_cats: Vec<String>,
    /// Records mode: one entry per row, grouping the cells that share every
    /// coordinate but `_Measure`. `None` for a pivot view; `Rc` to share.
    pub records: Option<Rc<Vec<RecordRow>>>,
    /// In records mode: what each column holds, parallel to `col_items`.
    /// Empty for normal pivot views.
    record_cols: Vec<RecordCol>,
    /// The view's hidden items: a fold covers only the members this grid draws.
    hidden: HashMap<String, HashSet<String>>,
}

impl GridLayout {
    /// Build a layout, using `frozen_records` instead of re-querying the
    /// store when records mode is active and a snapshot is supplied.
    pub fn with_frozen_records(
        model: &Model,
        view: &View,
        frozen_records: Option<Rc<Vec<RecordRow>>>,
    ) -> Self {
        let mut layout = Self::new(model, view);
        if layout.is_records_mode()
            && let Some(records) = frozen_records
        {
            let row_items: Vec<AxisEntry> = (0..records.len())
                .map(|i| AxisEntry::item(vec![i.to_string()]))
                .collect();
            layout.row_items = row_items;
            layout.records = Some(records);
            layout.reindex();
        }
        if view.prune_empty {
            layout.prune_empty(model);
        }
        layout
    }

    pub fn new(model: &Model, view: &View) -> Self {
        let row_cats: Vec<String> = view
            .categories_on(Axis::Row)
            .into_iter()
            .map(String::from)
            .collect();
        let col_cats: Vec<String> = view
            .categories_on(Axis::Column)
            .into_iter()
            .map(String::from)
            .collect();
        // Page *and* Filter: both constrain the visible records, and the
        // layout does not care which the user can navigate (§7).
        let page_cats: Vec<String> = view
            .filtering_cats()
            .into_iter()
            .map(String::from)
            .collect();
        let none_cats: Vec<String> = view
            .categories_on(Axis::None)
            .into_iter()
            .map(String::from)
            .collect();

        let page_coords = page_cats
            .iter()
            .filter_map(|cat| {
                let items: Vec<String> = model.effective_item_names(cat);
                let selected = view
                    .page_selection(cat)
                    .map(String::from)
                    .or_else(|| items.first().cloned())?;
                let navigable = view.try_axis_of(cat) == Some(Axis::Page);
                Some(PageAxis {
                    cat: cat.clone(),
                    selected,
                    items: navigable.then_some(items),
                })
            })
            .collect();

        // `View` owns the one definition of records mode (improvise-2zq):
        // `on_category_added` asks it too, and two derivations would drift.
        if view.is_records_layout() {
            Self::build_records_mode(model, view, page_coords, none_cats)
        } else {
            let row_items = cross_product(model, view, &row_cats);
            let col_items = cross_product(model, view, &col_cats);
            Self {
                row_cats,
                col_cats,
                page_coords,
                row_pos: positions(&row_items),
                col_pos: positions(&col_items),
                row_items,
                col_items,
                none_cats,
                records: None,
                record_cols: Vec::new(),
                hidden: view.hidden_items.clone(),
            }
        }
    }

    /// Build a records-mode layout in wide format, each measure getting its
    /// own value column so data entry is direct (improvise-c5v, §6).
    fn build_records_mode(
        model: &Model,
        _view: &View,
        page_coords: Vec<PageAxis>,
        none_cats: Vec<String>,
    ) -> Self {
        let records = Self::group_records(model, &page_constraints(&page_coords));

        let row_items: Vec<AxisEntry> = (0..records.len())
            .map(|i| AxisEntry::item(vec![i.to_string()]))
            .collect();

        let record_cols = Self::records_columns(model);
        let col_items: Vec<AxisEntry> = record_cols
            .iter()
            .map(|c| AxisEntry::item(vec![Self::record_col_label(c)]))
            .collect();

        Self {
            row_cats: vec![INDEX_CATEGORY.to_string()],
            col_cats: vec![DIM_CATEGORY.to_string()],
            page_coords,
            row_pos: positions(&row_items),
            col_pos: positions(&col_items),
            row_items,
            col_items,
            none_cats,
            records: Some(Rc::new(records)),
            record_cols,
            hidden: HashMap::new(),
        }
    }

    fn group_records(model: &Model, page_coords: &[(String, String)]) -> Vec<RecordRow> {
        RecordRow::group(
            model
                .data
                .matching_cells(page_coords)
                .into_iter()
                .map(|(k, v)| (k, v.clone())),
        )
    }

    /// `[coordinates…, +cat, measures…, +]`. Formula targets are not columns:
    /// they are computed, so there is nothing to type into them (§6).
    fn records_columns(model: &Model) -> Vec<RecordCol> {
        let mut cols: Vec<RecordCol> = model
            .category_names()
            .into_iter()
            .filter(|c| {
                model
                    .category(c)
                    .map(|cat| cat.kind.is_regular())
                    .unwrap_or(false)
            })
            .map(|c| RecordCol::Category(c.to_string()))
            .collect();
        cols.push(RecordCol::NewCategory);
        cols.extend(
            model
                .category(MEASURE_CATEGORY)
                .map(|c| c.ordered_item_names())
                .unwrap_or_default()
                .into_iter()
                .map(|m| RecordCol::Measure(m.to_string())),
        );
        cols.push(RecordCol::NewMeasure);
        cols
    }

    fn record_col_label(col: &RecordCol) -> String {
        match col {
            RecordCol::Category(name) | RecordCol::Measure(name) => name.clone(),
            RecordCol::NewMeasure => NEW_MEASURE_COL.to_string(),
            RecordCol::NewCategory => NEW_CATEGORY_COL.to_string(),
        }
    }

    /// What the records-mode column at `col` holds; `None` outside records
    /// mode or past the last column.
    pub fn record_col(&self, col: usize) -> Option<&RecordCol> {
        self.record_cols.get(col)
    }

    /// The grouped row at `row`, in records mode.
    pub fn record_row(&self, row: usize) -> Option<&RecordRow> {
        self.records.as_ref()?.get(row)
    }

    /// The model cell a records position addresses, for the callers that must
    /// ask the *document* about it. `None` where the column stores no value.
    pub fn record_cell_key(&self, row: usize, col: usize) -> Option<CellKey> {
        let record = self.record_row(row)?;
        match self.record_col(col)? {
            RecordCol::Measure(name) => Some(record.cell_key(name)),
            RecordCol::Category(_) | RecordCol::NewMeasure | RecordCol::NewCategory => None,
        }
    }

    /// The same, from a synthetic `(_Index, _Dim)` key — the shape a
    /// `CmdContext` carries in records mode (gotcha 1).
    pub fn record_cell_key_of(&self, key: &CellKey) -> Option<CellKey> {
        let (row, col) = synthetic_record_info(key)?;
        self.record_cell_key(row, col)
    }

    /// The **one** resolution of a records-mode cell (improvise-tom1). `None`
    /// for a pivot layout and out of bounds, symmetric with `cell_key`.
    pub fn records_value(&self, row: usize, col: usize) -> Option<CellValue> {
        let record = self.record_row(row)?;
        let blank = || CellValue::Text(String::new());
        match self.record_col(col)? {
            // A coordinate is an item NAME, so it is text whatever it looks
            // like: `2024` is a label, not a number.
            RecordCol::Category(name) => Some(CellValue::Text(
                record.coords.get(name).unwrap_or_default().to_string(),
            )),
            RecordCol::Measure(name) => Some(record.value(name).cloned().unwrap_or_else(blank)),
            // Always blank: where a new name is typed, never where a value
            // is stored.
            RecordCol::NewMeasure | RecordCol::NewCategory => Some(blank()),
        }
    }

    /// The same cell as the string the editor and the terminal want. One
    /// `to_string`, so the text can never disagree with the value.
    pub fn records_display(&self, row: usize, col: usize) -> Option<String> {
        self.records_value(row, col).map(|v| v.to_string())
    }

    /// Drop entirely empty data rows and columns, keeping a group header if
    /// an item under it survives. Records mode is exempt (§6).
    pub fn prune_empty(&mut self, model: &Model) {
        if self.is_records_mode() {
            return;
        }
        let rc = self.row_count();
        let cc = self.col_count();
        if rc == 0 || cc == 0 {
            return;
        }

        // Build a row×col grid of "has content?", one fold memo for the whole
        // walk (improvise-metf). It is dropped before the axes are pruned.
        let mut has_value = vec![vec![false; cc]; rc];
        {
            let eval = self.evaluator(model);
            for (ri, row) in has_value.iter_mut().enumerate() {
                for (ci, cell) in row.iter_mut().enumerate() {
                    *cell = self.cell_key(ri, ci).and_then(|k| eval.cell(&k)).is_some();
                }
            }
        }

        // Pruning rows touches neither `col_items` nor `has_value`, so the
        // column pass still reads the original grid.
        prune_axis_items(&mut self.row_items, |ri| {
            (0..cc).any(|ci| has_value[ri][ci])
        });
        prune_axis_items(&mut self.col_items, |ci| {
            (0..rc).any(|ri| has_value[ri][ci])
        });
        self.reindex();
    }

    /// Rebuild the position indexes after the items change.
    fn reindex(&mut self) {
        self.row_pos = positions(&self.row_items);
        self.col_pos = positions(&self.col_items);
    }

    /// Whether this layout is in records mode.
    pub fn is_records_mode(&self) -> bool {
        self.records.is_some()
    }

    /// Number of data rows (group headers excluded).
    pub fn row_count(&self) -> usize {
        self.row_pos.len()
    }

    /// Number of data columns (group headers excluded).
    pub fn col_count(&self) -> usize {
        self.col_pos.len()
    }

    pub fn row_label(&self, row: usize) -> String {
        axis_label(&self.row_items, &self.row_pos, row)
    }

    pub fn col_label(&self, col: usize) -> String {
        axis_label(&self.col_items, &self.col_pos, col)
    }

    /// The value of a pivot key as THIS grid reads it: aggregated over its None
    /// axis, and a fold over the members it draws (core.md §14). One evaluation.
    pub fn evaluate(&self, model: &Model, key: &CellKey) -> Option<CellValue> {
        self.evaluator(model).cell(key)
    }

    /// One grid build's evaluator, whose fold memo this layout built, so no other
    /// grid's memo can reach its cells or totals (improvise-h6z1, pntu, opbo).
    pub fn evaluator<'a>(&'a self, model: &'a Model) -> GridEval<'a> {
        let memo = FoldMemo::new(Aggregation::of(&self.none_cats), |cat, item| {
            !self.hidden.get(cat).is_some_and(|h| h.contains(item))
        });
        GridEval {
            layout: self,
            model,
            memo,
        }
    }

    /// Display text for (row, col) in either mode.
    pub fn display_text(
        &self,
        model: &Model,
        row: usize,
        col: usize,
        plan: &crate::format::FormatPlan,
    ) -> String {
        if self.is_records_mode() {
            self.records_display(row, col).unwrap_or_default()
        } else {
            self.cell_key(row, col)
                .and_then(|key| {
                    let value = self.evaluate(model, &key)?;
                    Some(plan.at(Some(&key)).value(Some(&value)))
                })
                .unwrap_or_default()
        }
    }

    /// The `CellKey` at (row, col) including the active page/filter coords;
    /// `None` out of bounds. Records mode yields a synthetic key (§6).
    pub fn cell_key(&self, row: usize, col: usize) -> Option<CellKey> {
        if let Some(records) = &self.records {
            if row >= records.len() || col >= self.record_cols.len() {
                return None;
            }
            return Some(CellKey::new(vec![
                (INDEX_CATEGORY.to_string(), row.to_string()),
                (DIM_CATEGORY.to_string(), col.to_string()),
            ]));
        }
        let row_item = data_item(&self.row_items, &self.row_pos, row)?;
        let col_item = data_item(&self.col_items, &self.col_pos, col)?;
        let mut coords = page_constraints(&self.page_coords);
        for (cat, item) in self.row_cats.iter().zip(row_item.iter()) {
            coords.push((cat.clone(), item.clone()));
        }
        for (cat, item) in self.col_cats.iter().zip(col_item.iter()) {
            coords.push((cat.clone(), item.clone()));
        }
        Some(CellKey::new(coords))
    }

    /// The coordinates a COLUMN answers for once its rows are aggregated away:
    /// the page/filter constraints and the column's own tuple (core.md §6).
    pub fn column_key(&self, col: usize) -> Option<CellKey> {
        let col_item = data_item(&self.col_items, &self.col_pos, col)?;
        let mut coords = page_constraints(&self.page_coords);
        for (cat, item) in self.col_cats.iter().zip(col_item.iter()) {
            coords.push((cat.clone(), item.clone()));
        }
        Some(CellKey::new(coords))
    }

    /// Visual index of the nth data row (skipping group headers).
    pub fn data_row_to_visual(&self, data_row: usize) -> Option<usize> {
        data_index_to_visual(&self.row_pos, data_row)
    }

    /// Visual index of the nth data column (skipping group headers).
    pub fn data_col_to_visual(&self, data_col: usize) -> Option<usize> {
        data_index_to_visual(&self.col_pos, data_col)
    }

    /// The `(cat_name, group_name)` of the innermost group the Nth data row
    /// is in — what `z` toggles (improvise-e265).
    pub fn row_group_for(&self, data_row: usize) -> Option<(String, String)> {
        innermost_group(&self.row_items, data_row)
    }

    /// Whether the cell at `(row, col)` folds a collapsed group: an aggregate
    /// with no record under it. The one home of that answer (§14).
    pub fn folds_a_group(&self, row: usize, col: usize) -> bool {
        self.row_folds_a_group(row) || folds(&self.col_items, &self.col_pos, col)
    }

    /// The row half of [`Self::folds_a_group`], for a command about the row alone.
    pub fn row_folds_a_group(&self, row: usize) -> bool {
        folds(&self.row_items, &self.row_pos, row)
    }

    /// The first data row of `cat`/`group`'s run under `prefix` — where a toggle
    /// of that group puts the cursor, asked of the layout AFTER the toggle (§14).
    pub fn row_run_start(&self, cat: &str, group: &str, prefix: &[String]) -> Option<usize> {
        run_start(&self.row_cats, &self.row_items, cat, group, prefix)
    }

    /// [`Self::row_run_start`] for a column group.
    pub fn col_run_start(&self, cat: &str, group: &str, prefix: &[String]) -> Option<usize> {
        run_start(&self.col_cats, &self.col_items, cat, group, prefix)
    }

    /// The coordinate tuple of the Nth data row.
    pub fn row_tuple(&self, row: usize) -> Option<&[String]> {
        data_item(&self.row_items, &self.row_pos, row).map(Vec::as_slice)
    }

    /// The coordinate tuple of the Nth data column.
    pub fn col_tuple(&self, col: usize) -> Option<&[String]> {
        data_item(&self.col_items, &self.col_pos, col).map(Vec::as_slice)
    }

    /// The `(cat_name, group_name)` of the innermost group the Nth data
    /// column is in.
    pub fn col_group_for(&self, data_col: usize) -> Option<(String, String)> {
        innermost_group(&self.col_items, data_col)
    }
}

/// Expand one category into `AxisEntry` values under a coordinate prefix: a
/// `GroupHeader` per boundary, then each visible item or collapsed group (§14).
fn expand_category(
    model: &Model,
    view: &View,
    cat_name: &str,
    prefix: Vec<String>,
    prefix_collapsed: bool,
) -> Vec<AxisEntry> {
    let Some(cat) = model.category(cat_name) else {
        return vec![];
    };
    // The prefix holds one item per outer category, so its length is this level.
    let mut runs = CategoryRuns {
        level: prefix.len(),
        cat_name,
        view,
        open: Vec::new(),
        out: Vec::new(),
    };
    // A group's items need not be contiguous, so a collapsed one folds ONCE.
    let mut folded: Vec<&str> = Vec::new();

    // effective_item_names, so _Measure picks up formula targets.
    let effective_names = model.effective_item_names(cat_name);
    for item_name in &effective_names {
        if view.is_hidden(cat_name, item_name) {
            continue;
        }
        let chain = cat.item_chain(item_name);
        // The OUTERMOST collapsed group folds every leaf beneath it (§19).
        let fold_depth =
            (0..chain.len()).find(|&depth| view.is_group_collapsed(cat_name, &chain[..=depth]));
        let fold = fold_depth.map(|depth| chain[depth]);
        if fold.is_some_and(|g| folded.contains(&g)) {
            continue;
        }
        runs.enter(&chain[..fold_depth.map_or(chain.len(), |d| d + 1)]);

        let mut tuple = prefix.clone();
        match fold {
            Some(g) => {
                folded.push(g);
                tuple.push(g.to_string());
            }
            None => tuple.push(item_name.to_string()),
        }
        runs.out.push(AxisEntry::DataItem {
            tuple,
            collapsed: prefix_collapsed || fold.is_some(),
        });
    }
    // The expansion is the end of the run too: the next prefix's items follow
    // immediately and must not inherit it (improvise-gd83).
    runs.enter(&[]);
    runs.out
}

/// One category's group runs as `expand_category` emits them: the chain of groups
/// open, root first, and the markers that move it from item to item.
struct CategoryRuns<'a> {
    level: usize,
    cat_name: &'a str,
    view: &'a View,
    open: Vec<&'a str>,
    out: Vec<AxisEntry>,
}

impl<'a> CategoryRuns<'a> {
    /// Move the open chain to `chain` in `remark`'s own convention, so an unpruned
    /// axis survives pruning marker for marker (core.md §6).
    fn enter(&mut self, chain: &[&'a str]) {
        let shared = self
            .open
            .iter()
            .zip(chain)
            .take_while(|(open, want)| open == want)
            .count();
        let level = self.level;
        for depth in (shared + 1..self.open.len()).rev() {
            self.out.push(AxisEntry::GroupEnd { level, depth });
        }
        if chain.len() == shared && self.open.len() > shared {
            self.out.push(AxisEntry::GroupEnd {
                level,
                depth: shared,
            });
        }
        self.open.truncate(shared);
        for (depth, group) in chain.iter().enumerate().skip(shared) {
            self.out.push(AxisEntry::GroupHeader {
                level,
                depth,
                cat_name: self.cat_name.to_string(),
                group_name: group.to_string(),
                collapsed: self
                    .view
                    .is_group_collapsed(self.cat_name, &chain[..=depth]),
            });
            self.open.push(group);
        }
    }
}

/// Cartesian product of visible items across `cats`, headers always emitted.
/// `vec![DataItem(vec![])]` when `cats` is empty.
fn cross_product(model: &Model, view: &View, cats: &[String]) -> Vec<AxisEntry> {
    if cats.is_empty() {
        return vec![AxisEntry::item(vec![])];
    }
    let mut result: Vec<AxisEntry> = vec![AxisEntry::item(vec![])];
    for cat_name in cats {
        result = result
            .into_iter()
            .flat_map(|entry| match entry {
                AxisEntry::DataItem { tuple, collapsed } => {
                    expand_category(model, view, cat_name, tuple, collapsed)
                }
                marker @ (AxisEntry::GroupHeader { .. } | AxisEntry::GroupEnd { .. }) => {
                    vec![marker]
                }
            })
            .collect();
    }
    // The axis itself ends the last run; a trailing marker names nothing.
    while matches!(result.last(), Some(AxisEntry::GroupEnd { .. })) {
        result.pop();
    }
    result
}

/// One grid build's evaluations under one fold memo its own layout built, valid
/// for one model state (improvise-h6z1, opbo). `context/code/core.md` §14.
pub struct GridEval<'a> {
    layout: &'a GridLayout,
    model: &'a Model,
    memo: FoldMemo<'a>,
}

impl GridEval<'_> {
    /// A pivot key's value as this grid reads it: [`GridLayout::evaluate`], shared.
    pub fn cell(&self, key: &CellKey) -> Option<CellValue> {
        self.model.evaluate_covering_in(key, &self.memo)
    }

    /// A column's total: its cell as if every drawn row were ONE fold, so a
    /// formula is evaluated over the pooled rows (improvise-wm20, core.md §14).
    pub fn column_total(&self, col: usize) -> Option<CellValue> {
        let key = self.layout.column_key(col)?;
        self.model
            .evaluate_spanning_in(&key, &self.layout.row_cats, &self.memo)
    }
}

#[cfg(test)]
mod tests {
    use super::{
        AxisEntry, GridLayout, NEW_CATEGORY_COL, NEW_MEASURE_COL, RecordCol, data_items,
        prune_axis_items, synthetic_record_info,
    };
    use crate::model::Aggregation;
    use crate::model::cell::{CellKey, CellValue};
    use crate::model::types::{FOLD_FORMULA_EVALUATIONS, FoldMemo};
    use crate::view::Axis;
    use crate::workbook::Workbook;

    fn records_workbook() -> Workbook {
        let mut wb = Workbook::new("T");
        wb.add_category("Region").unwrap();
        wb.add_category("_Measure").unwrap();
        wb.add_item("Region", "North").unwrap();
        wb.add_item("_Measure", "Revenue").unwrap();
        wb.add_item("_Measure", "Cost").unwrap();
        wb.model.set_cell(
            CellKey::new(vec![
                ("Region".into(), "North".into()),
                ("_Measure".into(), "Revenue".into()),
            ]),
            CellValue::Number(100.0),
        );
        wb.model.set_cell(
            CellKey::new(vec![
                ("Region".into(), "North".into()),
                ("_Measure".into(), "Cost".into()),
            ]),
            CellValue::Number(50.0),
        );
        // Records mode setup: _Measure must not filter.
        wb.active_view_mut().set_axis("_Measure", Axis::None);
        wb
    }

    /// The predicate sees *data* indices, not visual ones — see
    /// context/test-evidence/core.md.
    #[test]
    fn prune_axis_items_indexes_data_positions_not_visual_ones() {
        let mut items = vec![
            AxisEntry::item(vec!["A".into()]),
            AxisEntry::GroupHeader {
                level: 0,
                depth: 0,
                cat_name: "C".into(),
                group_name: "G".into(),
                collapsed: false,
            },
            AxisEntry::item(vec!["B".into()]),
            AxisEntry::item(vec!["C".into()]),
        ];
        // Data index 1 is "B": the header does not consume an index. If the
        // predicate saw visual indices, "A" would be the one dropped.
        prune_axis_items(&mut items, |idx| idx != 1);

        let survivors: Vec<&str> = data_items(&items).map(|v| v[0].as_str()).collect();
        assert_eq!(survivors, vec!["A", "C"]);
        // "C" survived, so its group header is retained.
        assert!(items.iter().any(|e| matches!(
            e,
            AxisEntry::GroupHeader { group_name, .. } if group_name == "G"
        )));
    }

    /// A group header is dropped when every data item under it is pruned.
    #[test]
    fn prune_axis_items_drops_a_header_whose_items_all_go() {
        let mut items = vec![
            AxisEntry::GroupHeader {
                level: 0,
                depth: 0,
                cat_name: "C".into(),
                group_name: "G".into(),
                collapsed: false,
            },
            AxisEntry::item(vec!["A".into()]),
            AxisEntry::item(vec!["B".into()]),
        ];
        prune_axis_items(&mut items, |_| false);
        assert!(items.is_empty());
    }

    /// improvise-l4qx seen at the layout — see context/test-evidence/core.md.
    #[test]
    fn a_text_cell_renders_in_a_default_view() {
        let mut wb = Workbook::new("T");
        wb.add_category("Region").unwrap();
        wb.add_item("Region", "West").unwrap();
        wb.add_item("_Measure", "Revenue").unwrap();
        wb.model.set_cell(
            CellKey::new(vec![
                ("Region".into(), "West".into()),
                ("_Measure".into(), "Revenue".into()),
            ]),
            CellValue::Text("pending".into()),
        );

        let layout = GridLayout::new(&wb.model, wb.active_view());
        // A default view leaves _Index and _Dim on Axis::None.
        assert!(
            !layout.none_cats.is_empty(),
            "a default view must have None-axis categories for this to be the bug"
        );
        assert_eq!(
            layout.display_text(&wb.model, 0, 0, &crate::format::FormatPlan::uniform("")),
            "pending"
        );
    }

    #[test]
    fn prune_empty_removes_all_empty_columns_in_pivot_mode() {
        let mut wb = Workbook::new("T");
        wb.add_category("Row").unwrap();
        wb.add_category("Col").unwrap();
        wb.add_item("Row", "A").unwrap();
        wb.add_item("Col", "X").unwrap();
        wb.add_item("Col", "Y").unwrap();
        // Only X has data; Y is entirely empty.
        wb.model.set_cell(
            CellKey::new(vec![("Row".into(), "A".into()), ("Col".into(), "X".into())]),
            CellValue::Number(1.0),
        );

        let mut layout = GridLayout::new(&wb.model, wb.active_view());
        assert_eq!(layout.col_count(), 2); // X and Y before pruning
        layout.prune_empty(&wb.model);
        assert_eq!(layout.col_count(), 1); // only X after pruning
        assert_eq!(layout.col_label(0), "X");
    }

    #[test]
    fn records_mode_activated_when_index_and_dim_on_axes() {
        let mut wb = records_workbook();
        let v = wb.active_view_mut();
        v.set_axis("_Index", Axis::Row);
        v.set_axis("_Dim", Axis::Column);
        let layout = GridLayout::new(&wb.model, wb.active_view());
        assert!(layout.is_records_mode());
        // Wide format: both measures of Region=North share one row.
        assert_eq!(layout.row_count(), 1);
    }

    #[test]
    fn records_mode_cell_key_returns_synthetic_for_all_columns() {
        let mut wb = records_workbook();
        let v = wb.active_view_mut();
        v.set_axis("_Index", Axis::Row);
        v.set_axis("_Dim", Axis::Column);
        let layout = GridLayout::new(&wb.model, wb.active_view());
        assert!(layout.is_records_mode());
        // Every column yields a synthetic key carrying the row and the column
        // *index* — see `synthetic_record_info`.
        for col in 0..layout.col_count() {
            let key = layout.cell_key(0, col).unwrap();
            assert_eq!(key.get("_Index"), Some("0"));
            assert_eq!(key.get("_Dim"), Some(col.to_string().as_str()));
        }
        assert!(
            layout.cell_key(0, layout.col_count()).is_none(),
            "past the last column there is no cell"
        );
    }

    #[test]
    fn records_mode_records_display_returns_values() {
        let mut wb = records_workbook();
        let v = wb.active_view_mut();
        v.set_axis("_Index", Axis::Row);
        v.set_axis("_Dim", Axis::Column);
        let layout = GridLayout::new(&wb.model, wb.active_view());
        let cols: Vec<String> = (0..layout.col_count())
            .map(|i| layout.col_label(i))
            .collect();

        // A measure column resolves to that measure's value
        let value_col = cols.iter().position(|c| c == "Revenue").unwrap();
        assert_eq!(layout.records_display(0, value_col).as_deref(), Some("100"));

        // Category column resolves to the coordinate value
        let region_col = cols.iter().position(|c| c == "Region").unwrap();
        assert_eq!(
            layout.records_display(0, region_col).as_deref(),
            Some("North")
        );
    }

    #[test]
    fn synthetic_record_info_returns_none_for_pivot_keys() {
        let key = CellKey::new(vec![
            ("Region".to_string(), "East".to_string()),
            ("Product".to_string(), "Shoes".to_string()),
        ]);
        assert!(synthetic_record_info(&key).is_none());
    }

    /// `_Dim` carries the column *index*, not its label: a category and a
    /// measure may share a name (improvise-c5v).
    #[test]
    fn synthetic_record_info_extracts_index_and_column() {
        let key = CellKey::new(vec![
            ("_Index".to_string(), "3".to_string()),
            ("_Dim".to_string(), "2".to_string()),
        ]);
        let (idx, col) = synthetic_record_info(&key).unwrap();
        assert_eq!(idx, 3);
        assert_eq!(col, 2);
    }

    /// Pins the whole column vocabulary in order — see
    /// context/test-evidence/core.md.
    #[test]
    fn records_mode_gives_each_measure_its_own_value_column() {
        let mut wb = records_workbook();
        let v = wb.active_view_mut();
        v.set_axis("_Index", Axis::Row);
        v.set_axis("_Dim", Axis::Column);
        let layout = GridLayout::new(&wb.model, wb.active_view());
        assert!(layout.is_records_mode());
        let cols: Vec<String> = (0..layout.col_count())
            .map(|i| layout.col_label(i))
            .collect();
        assert_eq!(
            cols,
            vec![
                "Region",
                NEW_CATEGORY_COL,
                "Revenue",
                "Cost",
                NEW_MEASURE_COL
            ],
            "categories, the new-category column, one column per measure, \
             then the new-measure column — each `+` adjacent to what it adds \
             to (improvise-2zq)"
        );
        assert_eq!(
            layout.record_col(0),
            Some(&RecordCol::Category("Region".to_string()))
        );
        assert_eq!(layout.record_col(1), Some(&RecordCol::NewCategory));
        assert_eq!(
            layout.record_col(2),
            Some(&RecordCol::Measure("Revenue".to_string()))
        );
        assert_eq!(layout.record_col(4), Some(&RecordCol::NewMeasure));

        // Both measures are readable side by side, and `+cat` is blank.
        assert_eq!(layout.records_display(0, 1).as_deref(), Some(""));
        assert_eq!(layout.records_display(0, 2).as_deref(), Some("100"));
        assert_eq!(layout.records_display(0, 3).as_deref(), Some("50"));
    }

    /// On entry into records mode rows are `CellKey`-sorted regardless of
    /// insertion order — see context/test-evidence/core.md.
    #[test]
    fn records_mode_initial_layout_is_sorted() {
        let mut wb = Workbook::new("T");
        wb.add_category("Region").unwrap();
        wb.add_item("Region", "North").unwrap();
        wb.add_item("Region", "East").unwrap();
        // Insert East before North — opposite of alphabetical
        wb.model.set_cell(
            CellKey::new(vec![("Region".into(), "East".into())]),
            CellValue::Number(2.0),
        );
        wb.model.set_cell(
            CellKey::new(vec![("Region".into(), "North".into())]),
            CellValue::Number(1.0),
        );
        // Sort the store before entering records mode, as ToggleRecordsMode would
        wb.model.data.sort_by_key();
        let v = wb.active_view_mut();
        v.set_axis("_Index", Axis::Row);
        v.set_axis("_Dim", Axis::Column);
        let layout = GridLayout::new(&wb.model, wb.active_view());
        assert_eq!(layout.row_count(), 2);
        let region_col = (0..layout.col_count())
            .find(|&c| layout.col_label(c) == "Region")
            .unwrap();
        let row0_region = layout.records_display(0, region_col).unwrap();
        let row1_region = layout.records_display(1, region_col).unwrap();
        assert_eq!(
            row0_region, "East",
            "first row should be East (alphabetical)"
        );
        assert_eq!(
            row1_region, "North",
            "second row should be North (alphabetical)"
        );
    }

    /// A record added later appends rather than re-sorting into the middle —
    /// see context/test-evidence/core.md.
    #[test]
    fn records_mode_new_record_appends_at_bottom() {
        let mut wb = records_workbook();
        let v = wb.active_view_mut();
        v.set_axis("_Index", Axis::Row);
        v.set_axis("_Dim", Axis::Column);
        let layout = GridLayout::new(&wb.model, wb.active_view());
        assert_eq!(layout.row_count(), 1);
        // The Cost column, not the trailing new-measure column.
        let cost_col = (0..layout.col_count())
            .find(|&c| layout.record_col(c) == Some(&RecordCol::Measure("Cost".to_string())))
            .unwrap();
        let first_value = layout.records_display(0, cost_col).unwrap();

        // Add a record whose CellKey sorts BEFORE the existing ones
        wb.model.set_cell(
            CellKey::new(vec![
                ("Region".into(), "AAA".into()),
                ("_Measure".into(), "Cost".into()),
            ]),
            CellValue::Number(999.0),
        );
        let layout2 = GridLayout::new(&wb.model, wb.active_view());
        assert_eq!(layout2.row_count(), 2);
        // First row should still be the same record — new one appends at bottom
        let first_value2 = layout2.records_display(0, cost_col).unwrap();
        assert_eq!(
            first_value, first_value2,
            "first row should be unchanged after adding a new record; \
             new record should append at bottom, not re-sort"
        );
        // New record should be the last row
        let last_value = layout2.records_display(1, cost_col).unwrap();
        assert_eq!(last_value, "999", "new record should be the last row");
    }

    /// improvise-byz regression guard — see context/test-evidence/core.md.
    #[test]
    fn records_display_out_of_bounds_col_returns_none() {
        let mut wb = records_workbook();
        let v = wb.active_view_mut();
        v.set_axis("_Index", Axis::Row);
        v.set_axis("_Dim", Axis::Column);
        let layout = GridLayout::new(&wb.model, wb.active_view());
        assert!(layout.is_records_mode());
        assert_eq!(layout.records_display(0, layout.col_count()), None);
    }

    fn coord(pairs: &[(&str, &str)]) -> CellKey {
        CellKey::new(
            pairs
                .iter()
                .map(|(c, i)| (c.to_string(), i.to_string()))
                .collect(),
        )
    }

    fn two_cat_workbook() -> Workbook {
        let mut wb = Workbook::new("T");
        wb.add_category("Type").unwrap();
        wb.add_category("Month").unwrap();
        for item in ["Food", "Clothing"] {
            wb.add_item("Type", item).unwrap();
        }
        for item in ["Jan", "Feb"] {
            wb.add_item("Month", item).unwrap();
        }
        wb
    }

    #[test]
    fn row_and_col_counts_match_item_counts() {
        let wb = two_cat_workbook();
        let layout = GridLayout::new(&wb.model, wb.active_view());
        assert_eq!(layout.row_count(), 2); // Food, Clothing
        assert_eq!(layout.col_count(), 2); // Jan, Feb
    }

    #[test]
    fn cell_key_encodes_correct_coordinates() {
        let wb = two_cat_workbook();
        let layout = GridLayout::new(&wb.model, wb.active_view());
        // row 0 = Food, col 1 = Feb
        let key = layout.cell_key(0, 1).unwrap();
        assert_eq!(key, coord(&[("Month", "Feb"), ("Type", "Food")]));
    }

    #[test]
    fn cell_key_out_of_bounds_returns_none() {
        let wb = two_cat_workbook();
        let layout = GridLayout::new(&wb.model, wb.active_view());
        assert!(layout.cell_key(99, 0).is_none());
        assert!(layout.cell_key(0, 99).is_none());
    }

    #[test]
    fn cell_key_includes_page_coords() {
        let mut wb = Workbook::new("T");
        wb.add_category("Type").unwrap();
        wb.add_category("Month").unwrap();
        wb.add_category("Region").unwrap();
        wb.add_item("Type", "Food").unwrap();
        wb.add_item("Month", "Jan").unwrap();
        wb.add_item("Region", "East").unwrap();
        wb.add_item("Region", "West").unwrap();
        wb.active_view_mut().set_page_selection("Region", "West");
        let layout = GridLayout::new(&wb.model, wb.active_view());
        let key = layout.cell_key(0, 0).unwrap();
        assert_eq!(key.get("Region"), Some("West"));
    }

    #[test]
    fn cell_key_round_trips_through_model_evaluate() {
        let mut wb = two_cat_workbook();
        wb.model.set_cell(
            coord(&[("Month", "Feb"), ("Type", "Clothing")]),
            CellValue::Number(42.0),
        );
        let layout = GridLayout::new(&wb.model, wb.active_view());
        // Clothing = row 1, Feb = col 1
        let key = layout.cell_key(1, 1).unwrap();
        assert_eq!(wb.model.evaluate(&key), Some(CellValue::Number(42.0)));
    }

    #[test]
    fn labels_join_with_slash_for_multi_cat_axis() {
        let mut wb = Workbook::new("T");
        wb.add_category("Type").unwrap();
        wb.add_category("Month").unwrap();
        wb.add_category("Year").unwrap();
        wb.add_item("Type", "Food").unwrap();
        wb.add_item("Month", "Jan").unwrap();
        wb.add_item("Year", "2025").unwrap();
        wb.active_view_mut()
            .set_axis("Year", crate::view::Axis::Column);
        let layout = GridLayout::new(&wb.model, wb.active_view());
        assert_eq!(layout.col_label(0), "Jan/2025");
    }

    #[test]
    fn row_count_excludes_group_headers() {
        let mut wb = Workbook::new("T");
        wb.add_category("Month").unwrap();
        wb.add_category("Type").unwrap();
        wb.add_item_in_group("Month", "Jan", "Q1").unwrap();
        wb.add_item_in_group("Month", "Feb", "Q1").unwrap();
        wb.add_item_in_group("Month", "Apr", "Q2").unwrap();
        wb.active_view_mut().set_expand_depth("Month", 1);
        wb.add_item("Type", "Food").unwrap();
        let layout = GridLayout::new(&wb.model, wb.active_view());
        assert_eq!(layout.row_count(), 3); // Jan, Feb, Apr — headers don't count
    }

    #[test]
    fn group_header_emitted_at_group_boundary() {
        let mut wb = Workbook::new("T");
        wb.add_category("Month").unwrap();
        wb.add_category("Type").unwrap();
        wb.add_item_in_group("Month", "Jan", "Q1").unwrap();
        wb.add_item_in_group("Month", "Apr", "Q2").unwrap();
        wb.active_view_mut().set_expand_depth("Month", 1);
        wb.add_item("Type", "Food").unwrap();
        let layout = GridLayout::new(&wb.model, wb.active_view());
        let headers: Vec<_> = layout
            .row_items
            .iter()
            .filter(|e| matches!(e, AxisEntry::GroupHeader { .. }))
            .collect();
        assert_eq!(headers.len(), 2);
        assert!(
            matches!(&headers[0], AxisEntry::GroupHeader { group_name, .. } if group_name == "Q1")
        );
        assert!(
            matches!(&headers[1], AxisEntry::GroupHeader { group_name, .. } if group_name == "Q2")
        );
    }

    /// The inversion of `collapsed_group_has_header_but_no_data_items`: a
    /// collapsed group is ONE row. Evidence: context/test-evidence/core.md.
    #[test]
    fn a_collapsed_group_contributes_one_aggregate_row() {
        let mut wb = Workbook::new("T");
        wb.add_category("Month").unwrap();
        wb.add_category("Type").unwrap();
        wb.add_item_in_group("Month", "Jan", "Q1").unwrap();
        wb.add_item_in_group("Month", "Feb", "Q1").unwrap();
        wb.add_item_in_group("Month", "Apr", "Q2").unwrap();
        wb.add_item_in_group("Month", "May", "Q2").unwrap();
        wb.active_view_mut().set_expand_depth("Month", 1);
        wb.add_item("Type", "Food").unwrap();
        wb.active_view_mut().toggle_group_collapse("Month", &["Q1"]);
        let layout = GridLayout::new(&wb.model, wb.active_view());
        let labels: Vec<String> = (0..layout.row_count())
            .map(|r| layout.row_label(r))
            .collect();
        assert_eq!(
            labels,
            ["Q1", "Apr", "May"],
            "Q1 folds into one row; Q2 stays open"
        );
        assert!(layout.folds_a_group(0, 0), "the Q1 row is a fold");
        assert!(
            !layout.folds_a_group(1, 0),
            "Apr, under the OPEN group, is not"
        );
        assert_eq!(
            layout.row_group_for(0),
            Some(("Month".into(), "Q1".into())),
            "the fold sits under its own header, so z can name it"
        );
    }

    /// Evidence: context/test-evidence/core.md.
    #[test]
    fn a_non_contiguous_collapsed_group_folds_once() {
        let mut wb = Workbook::new("T");
        wb.add_category("Month").unwrap();
        wb.add_category("Type").unwrap();
        wb.add_item_in_group("Month", "A", "G1").unwrap();
        wb.add_item_in_group("Month", "B", "G2").unwrap();
        wb.add_item_in_group("Month", "C", "G1").unwrap();
        wb.active_view_mut().set_expand_depth("Month", 1);
        wb.add_item("Type", "Food").unwrap();
        wb.active_view_mut().toggle_group_collapse("Month", &["G1"]);
        let layout = GridLayout::new(&wb.model, wb.active_view());
        let labels: Vec<String> = (0..layout.row_count())
            .map(|r| layout.row_label(r))
            .collect();
        assert_eq!(labels, ["G1", "B"], "one G1 row, not one per run");
        let g1_headers = layout
            .row_items
            .iter()
            .filter(
                |e| matches!(e, AxisEntry::GroupHeader { group_name, .. } if group_name == "G1"),
            )
            .count();
        assert_eq!(
            g1_headers, 1,
            "a second G1 header would open a run with no row"
        );
    }

    /// Evidence: context/test-evidence/core.md.
    #[test]
    fn a_category_nested_under_a_collapsed_group_is_folded_too() {
        let mut wb = Workbook::new("T");
        wb.add_category("Month").unwrap();
        wb.add_category("Region").unwrap();
        wb.add_category("Type").unwrap();
        wb.add_item_in_group("Month", "Jan", "Q1").unwrap();
        wb.add_item_in_group("Month", "Apr", "Q2").unwrap();
        wb.active_view_mut().set_expand_depth("Month", 1);
        wb.add_item("Region", "North").unwrap();
        wb.add_item("Type", "Food").unwrap();
        wb.active_view_mut().set_axis("Region", Axis::Row);
        wb.active_view_mut().toggle_group_collapse("Month", &["Q1"]);
        let layout = GridLayout::new(&wb.model, wb.active_view());
        assert_eq!(
            layout.row_tuple(0),
            Some(&["Q1".to_string(), "North".to_string()][..])
        );
        assert!(
            layout.folds_a_group(0, 0),
            "Q1/North has no record under it either"
        );
        assert!(!layout.folds_a_group(1, 0), "Apr/North does");
    }

    /// improvise-gd83: two arms — an ungrouped item AFTER a grouped run, and
    /// the same item under a later prefix. context/test-evidence/core.md.
    #[test]
    fn an_ungrouped_item_after_a_grouped_run_belongs_to_no_group() {
        let mut wb = Workbook::new("T");
        wb.add_category("Month").unwrap();
        wb.add_category("Type").unwrap();
        wb.add_item_in_group("Month", "Jan", "Q1").unwrap();
        wb.add_item_in_group("Month", "Feb", "Q1").unwrap();
        wb.active_view_mut().set_expand_depth("Month", 1);
        wb.add_item("Month", "Sep").unwrap();
        wb.add_item("Type", "Food").unwrap();
        let layout = GridLayout::new(&wb.model, wb.active_view());

        assert_eq!(
            layout.row_group_for(0),
            Some(("Month".into(), "Q1".into())),
            "precondition: Jan really is in Q1"
        );
        assert_eq!(
            layout.row_group_for(2),
            None,
            "Sep is in no group — the run ended before it"
        );
    }

    /// The second arm: the run ends at the end of ONE expansion, so the next
    /// prefix's first item does not inherit it. context/test-evidence/core.md.
    #[test]
    fn a_grouped_run_does_not_leak_into_the_next_prefix() {
        let mut wb = Workbook::new("T");
        wb.add_category("Type").unwrap();
        wb.add_category("Recipient").unwrap();
        wb.add_item("Type", "Food").unwrap();
        wb.add_item("Type", "Clothing").unwrap();
        wb.add_item("Recipient", "Carol").unwrap();
        wb.add_item_in_group("Recipient", "Alice", "Family")
            .unwrap();
        wb.add_item_in_group("Recipient", "Bob", "Family").unwrap();
        wb.active_view_mut().set_expand_depth("Recipient", 1);
        wb.active_view_mut().set_axis("Recipient", Axis::Row);
        let layout = GridLayout::new(&wb.model, wb.active_view());

        assert_eq!(
            layout.row_group_for(2),
            Some(("Recipient".into(), "Family".into())),
            "precondition: Food/Bob is in Family"
        );
        assert_eq!(
            layout.row_group_for(3),
            None,
            "Clothing/Carol opens a new prefix and is in no group"
        );
    }

    #[test]
    fn ungrouped_items_produce_no_headers() {
        let wb = two_cat_workbook();
        let layout = GridLayout::new(&wb.model, wb.active_view());
        assert!(
            !layout
                .row_items
                .iter()
                .any(|e| matches!(e, AxisEntry::GroupHeader { .. }))
        );
        assert!(
            !layout
                .col_items
                .iter()
                .any(|e| matches!(e, AxisEntry::GroupHeader { .. }))
        );
    }

    #[test]
    fn cell_key_correct_with_grouped_items() {
        let mut wb = Workbook::new("T");
        wb.add_category("Month").unwrap();
        wb.add_category("Type").unwrap();
        wb.add_item_in_group("Month", "Jan", "Q1").unwrap();
        wb.add_item_in_group("Month", "Apr", "Q2").unwrap();
        wb.active_view_mut().set_expand_depth("Month", 1);
        wb.add_item("Type", "Food").unwrap();
        wb.model.set_cell(
            coord(&[("Month", "Apr"), ("Type", "Food")]),
            CellValue::Number(99.0),
        );
        let layout = GridLayout::new(&wb.model, wb.active_view());
        // data row 0 = Jan, data row 1 = Apr
        let key = layout.cell_key(1, 0).unwrap();
        assert_eq!(wb.model.evaluate(&key), Some(CellValue::Number(99.0)));
    }

    #[test]
    fn data_row_to_visual_skips_headers() {
        let mut wb = Workbook::new("T");
        wb.add_category("Month").unwrap();
        wb.add_category("Type").unwrap();
        wb.add_item_in_group("Month", "Jan", "Q1").unwrap();
        wb.add_item_in_group("Month", "Apr", "Q2").unwrap();
        wb.active_view_mut().set_expand_depth("Month", 1);
        wb.add_item("Type", "Food").unwrap();
        let layout = GridLayout::new(&wb.model, wb.active_view());
        // visual: [GroupHeader(Q1), DataItem(Jan), GroupHeader(Q2), DataItem(Apr)]
        assert_eq!(layout.data_row_to_visual(0), Some(1)); // Jan is at visual index 1
        assert_eq!(layout.data_row_to_visual(1), Some(3)); // Apr is at visual index 3
        assert_eq!(layout.data_row_to_visual(2), None);
    }

    #[test]
    fn data_col_to_visual_skips_headers() {
        let mut wb = Workbook::new("T");
        wb.add_category("Type").unwrap(); // Row
        wb.add_category("Month").unwrap(); // Column
        wb.add_item("Type", "Food").unwrap();
        wb.add_item_in_group("Month", "Jan", "Q1").unwrap();
        wb.add_item_in_group("Month", "Apr", "Q2").unwrap();
        wb.active_view_mut().set_expand_depth("Month", 1);
        let layout = GridLayout::new(&wb.model, wb.active_view());
        // col_items: [GroupHeader(Q1), DataItem(Jan), GroupHeader(Q2), DataItem(Apr)]
        assert_eq!(layout.data_col_to_visual(0), Some(1));
        assert_eq!(layout.data_col_to_visual(1), Some(3));
        assert_eq!(layout.data_col_to_visual(2), None);
    }

    #[test]
    fn row_group_for_finds_enclosing_group() {
        let mut wb = Workbook::new("T");
        wb.add_category("Month").unwrap();
        wb.add_category("Type").unwrap();
        wb.add_item_in_group("Month", "Jan", "Q1").unwrap();
        wb.add_item_in_group("Month", "Apr", "Q2").unwrap();
        wb.active_view_mut().set_expand_depth("Month", 1);
        wb.add_item("Type", "Food").unwrap();
        let layout = GridLayout::new(&wb.model, wb.active_view());
        assert_eq!(
            layout.row_group_for(0),
            Some(("Month".to_string(), "Q1".to_string()))
        );
        assert_eq!(
            layout.row_group_for(1),
            Some(("Month".to_string(), "Q2".to_string()))
        );
    }

    #[test]
    fn row_group_for_returns_none_for_ungrouped() {
        let mut wb = Workbook::new("T");
        wb.add_category("Type").unwrap();
        wb.add_category("Month").unwrap();
        wb.add_item("Type", "Food").unwrap();
        wb.add_item("Month", "Jan").unwrap();
        let layout = GridLayout::new(&wb.model, wb.active_view());
        assert_eq!(layout.row_group_for(0), None);
    }

    #[test]
    fn col_group_for_finds_enclosing_group() {
        let mut wb = Workbook::new("T");
        wb.add_category("Type").unwrap(); // Row
        wb.add_category("Month").unwrap(); // Column
        wb.add_item("Type", "Food").unwrap();
        wb.add_item_in_group("Month", "Jan", "Q1").unwrap();
        wb.add_item_in_group("Month", "Apr", "Q2").unwrap();
        wb.active_view_mut().set_expand_depth("Month", 1);
        let layout = GridLayout::new(&wb.model, wb.active_view());
        assert_eq!(
            layout.col_group_for(0),
            Some(("Month".to_string(), "Q1".to_string()))
        );
        assert_eq!(
            layout.col_group_for(1),
            Some(("Month".to_string(), "Q2".to_string()))
        );
    }

    #[test]
    fn col_group_for_returns_none_for_ungrouped() {
        let mut wb = Workbook::new("T");
        wb.add_category("Type").unwrap();
        wb.add_category("Month").unwrap();
        wb.add_item("Type", "Food").unwrap();
        wb.add_item("Month", "Jan").unwrap();
        let layout = GridLayout::new(&wb.model, wb.active_view());
        assert_eq!(layout.col_group_for(0), None);
    }

    /// Fund exactly the `(Month, Type=Food)` cells named, so `prune_empty`
    /// keeps the rows the fixture pays for. context/test-evidence/core.md.
    fn grouped_month_book(months: &[(&str, Option<&str>)], funded: &[&str]) -> Workbook {
        let mut wb = Workbook::new("T");
        wb.add_category("Month").unwrap();
        wb.add_category("Type").unwrap();
        for (name, group) in months {
            match group {
                Some(g) => wb.add_item_in_group("Month", *name, *g).unwrap(),
                None => wb.add_item("Month", *name).unwrap(),
            };
        }
        wb.active_view_mut().set_expand_depth("Month", 1);
        wb.add_item("Type", "Food").unwrap();
        for m in funded {
            wb.model.set_cell(
                CellKey::new(vec![
                    ("Month".into(), (*m).into()),
                    ("Type".into(), "Food".into()),
                ]),
                CellValue::Number(1.0),
            );
        }
        wb.active_view_mut().prune_empty = true;
        wb
    }

    fn has_header(items: &[AxisEntry], group: &str) -> bool {
        items
            .iter()
            .any(|e| matches!(e, AxisEntry::GroupHeader { group_name, .. } if group_name == group))
    }

    /// improvise-lhi4: a COLLAPSED group contributes zero data items, so the
    /// pruner must not read its empty run as "pruned away".
    #[test]
    fn a_collapsed_groups_header_survives_pruning() {
        // `Mar` is unfunded, so pruning demonstrably runs: with `prune_empty`
        // off this fixture keeps Mar and the assertion below fails.
        let mut wb = grouped_month_book(
            &[("Jan", Some("Q1")), ("Mar", None), ("Sep", None)],
            &["Jan", "Sep"],
        );
        wb.active_view_mut().toggle_group_collapse("Month", &["Q1"]);
        let layout = GridLayout::with_frozen_records(&wb.model, wb.active_view(), None);
        // The whole sequence, not `any(is Q1)`: that cannot see a header
        // pushed in the wrong place, nor a stray `GroupEnd`.
        assert_eq!(
            layout.row_items,
            vec![
                AxisEntry::GroupHeader {
                    level: 0,
                    depth: 0,
                    cat_name: "Month".into(),
                    group_name: "Q1".into(),
                    collapsed: true,
                },
                AxisEntry::DataItem {
                    tuple: vec!["Q1".into()],
                    collapsed: true
                },
                AxisEntry::GroupEnd { level: 0, depth: 0 },
                AxisEntry::item(vec!["Sep".into()]),
            ],
            "Q1 folds to one funded row under its header, and Mar pruned"
        );
        assert_eq!(
            layout.row_group_for(0),
            Some(("Month".into(), "Q1".into())),
            "the fold is where z names the collapsed group from"
        );
        assert_eq!(
            layout.row_group_for(1),
            None,
            "Sep must not inherit the collapsed run"
        );
    }

    /// improvise-lhi4's other half, and what separates the two cases: a header
    /// whose items really did prune away still goes.
    #[test]
    fn a_header_whose_items_all_prune_away_is_dropped() {
        let wb = grouped_month_book(&[("Jan", Some("Q1")), ("Mar", Some("Q2"))], &["Jan"]);
        let layout = GridLayout::with_frozen_records(&wb.model, wb.active_view(), None);
        assert_eq!(layout.row_count(), 1, "only Jan is funded");
        assert!(has_header(&layout.row_items, "Q1"), "Jan survives under Q1");
        assert!(
            !has_header(&layout.row_items, "Q2"),
            "Q2's only item pruned away, so its header goes with it"
        );
    }

    /// improvise-b5zh: an OUTER header is followed by an inner one, not by an item
    /// — see context/test-evidence/core.md.
    #[test]
    fn an_outer_group_whose_rows_all_prune_loses_its_header() {
        let mut wb = Workbook::new("T");
        wb.add_category("Region").unwrap();
        wb.add_category("Month").unwrap();
        wb.add_category("Type").unwrap();
        wb.add_item_in_group("Region", "North", "Upper").unwrap();
        wb.add_item_in_group("Region", "South", "Lower").unwrap();
        wb.add_item_in_group("Month", "Jan", "Q1").unwrap();
        wb.add_item_in_group("Month", "Feb", "Q1").unwrap();
        wb.active_view_mut().set_expand_depth("Region", 1);
        wb.active_view_mut().set_expand_depth("Month", 1);
        wb.add_item("Type", "Food").unwrap();
        for month in ["Jan", "Feb"] {
            wb.model.set_cell(
                CellKey::new(vec![
                    ("Region".into(), "South".into()),
                    ("Month".into(), month.into()),
                    ("Type".into(), "Food".into()),
                ]),
                CellValue::Number(1.0),
            );
        }
        wb.active_view_mut().set_axis("Region", Axis::Row);
        wb.active_view_mut().set_axis("Month", Axis::Row);
        wb.active_view_mut().set_axis("Type", Axis::Column);
        wb.active_view_mut().prune_empty = true;
        let layout = GridLayout::with_frozen_records(&wb.model, wb.active_view(), None);
        assert_eq!(layout.row_count(), 2, "only South's rows are funded");
        assert!(
            has_header(&layout.row_items, "Lower"),
            "South's group stays"
        );
        assert!(
            !has_header(&layout.row_items, "Upper"),
            "every North row pruned, so Upper has nothing under it: {:?}",
            layout.row_items
        );
    }

    /// The commonest case and the one the other three cannot see: SOME of a
    /// group's items prune. context/test-evidence/core.md.
    #[test]
    fn a_partly_pruned_group_keeps_the_header_its_survivors_sit_under() {
        let wb = grouped_month_book(
            &[("Jan", Some("Q1")), ("Feb", Some("Q1")), ("Sep", None)],
            &["Jan", "Sep"],
        );
        let layout = GridLayout::with_frozen_records(&wb.model, wb.active_view(), None);
        assert_eq!(layout.row_count(), 2, "Feb pruned; Jan and Sep remain");
        assert_eq!(
            layout.row_group_for(0),
            Some(("Month".into(), "Q1".into())),
            "Jan survived, so Q1 still has a member and must keep its header"
        );
        assert_eq!(layout.row_group_for(1), None, "Sep is in no group");
    }

    /// improvise-u6gc: pruning the intervening header re-opens the earlier
    /// run, so the `GroupEnd` that closed it must survive. Regresses gd83.
    #[test]
    fn an_ungrouped_item_after_a_pruned_group_belongs_to_no_group() {
        let wb = grouped_month_book(
            &[
                ("Jan", Some("Q1")),
                ("Feb", Some("Q1")),
                ("Mar", Some("Q2")),
                ("Sep", None),
            ],
            &["Jan", "Feb", "Sep"],
        );
        let layout = GridLayout::with_frozen_records(&wb.model, wb.active_view(), None);
        assert_eq!(layout.row_count(), 3, "Mar pruned; Jan, Feb, Sep remain");
        assert_eq!(
            layout.row_group_for(0),
            Some(("Month".into(), "Q1".into())),
            "precondition: Jan is in Q1, so the walk has somewhere wrong to land"
        );
        assert_eq!(
            layout.row_group_for(2),
            None,
            "Sep is in no group — Q2's GroupEnd closed Q1's re-opened run"
        );
    }

    /// Q2 = {Apr}, Q1 = {Jan, Feb}, so the Q1 fold is row 1. Jan and Feb carry
    /// ratios 0.5 and 0.9 whose sum, mean and pooled ratio all differ; Apr is empty.
    fn margin_book(formulas: &[&str]) -> Workbook {
        let mut wb = Workbook::new("T");
        wb.add_category("Month").unwrap();
        wb.add_item_in_group("Month", "Apr", "Q2").unwrap();
        wb.add_item_in_group("Month", "Jan", "Q1").unwrap();
        wb.add_item_in_group("Month", "Feb", "Q1").unwrap();
        wb.active_view_mut().set_expand_depth("Month", 1);
        wb.add_item("_Measure", "Revenue").unwrap();
        wb.add_item("_Measure", "Cost").unwrap();
        for (month, revenue, cost) in [("Jan", 10.0, 5.0), ("Feb", 30.0, 3.0)] {
            for (measure, n) in [("Revenue", revenue), ("Cost", cost)] {
                wb.model.set_cell(
                    CellKey::new(vec![
                        ("Month".into(), month.into()),
                        ("_Measure".into(), measure.into()),
                    ]),
                    CellValue::Number(n),
                );
            }
        }
        for f in formulas {
            wb.model
                .add_formula(crate::formula::parse_formula(f, "_Measure").unwrap());
        }
        wb.active_view_mut().set_axis("Month", Axis::Row);
        wb.active_view_mut().set_axis("_Measure", Axis::Column);
        wb
    }

    /// `measure` at the row labelled `row_label`, as the grid reads it.
    fn grid_value(wb: &mut Workbook, row_label: &str, measure: &str) -> Option<f64> {
        wb.model.recompute_formulas(&wb.active_view().none_cats());
        let layout = GridLayout::new(&wb.model, wb.active_view());
        let row = (0..layout.row_count()).find(|&r| layout.row_label(r) == row_label)?;
        let col = (0..layout.col_count()).find(|&c| layout.col_label(c) == measure)?;
        layout
            .evaluate(&wb.model, &layout.cell_key(row, col)?)
            .and_then(|v| v.as_f64())
    }

    /// improvise-metf: pruning evaluates each formula a fold row reaches once, and
    /// keeps what a fresh evaluation per cell keeps. context/test-evidence/core.md.
    #[test]
    fn pruning_evaluates_each_fold_formula_once_and_keeps_the_same_rows() {
        let chain: Vec<String> = std::iter::once("F0 = Revenue".to_string())
            .chain((1..=8).map(|i| format!("F{i} = F{p} + F{p}", p = i - 1)))
            .collect();
        let chain: Vec<&str> = chain.iter().map(String::as_str).collect();
        let mut wb = margin_book(&chain);
        wb.active_view_mut().toggle_group_collapse("Month", &["Q1"]);
        wb.model.recompute_formulas(&wb.active_view().none_cats());
        let unpruned = GridLayout::new(&wb.model, wb.active_view());
        let kept_by_fresh_evaluation: Vec<String> = (0..unpruned.row_count())
            .filter(|&r| {
                (0..unpruned.col_count()).any(|c| {
                    let k = unpruned.cell_key(r, c).unwrap();
                    unpruned.evaluate(&wb.model, &k).is_some()
                })
            })
            .map(|r| unpruned.row_label(r))
            .collect();
        wb.active_view_mut().prune_empty = true;
        FOLD_FORMULA_EVALUATIONS.with(|n| n.set(0));
        let pruned = GridLayout::with_frozen_records(&wb.model, wb.active_view(), None);
        let evaluations = FOLD_FORMULA_EVALUATIONS.with(|n| n.get());
        let rows: Vec<String> = (0..pruned.row_count())
            .map(|r| pruned.row_label(r))
            .collect();
        assert_eq!(
            rows, kept_by_fresh_evaluation,
            "sharing the memo changed what is pruned"
        );
        assert!(
            rows.iter().any(|r| r == "Q1"),
            "setup: the fold row survives pruning"
        );
        assert_eq!(evaluations, 9, "F0..F8 once each for the fold row, not 45");
    }

    /// improvise-h6z1: one grid build evaluates each formula a fold row reaches
    /// once, not once per column. context/test-evidence/core.md.
    #[test]
    fn a_fold_row_evaluates_each_formula_once_across_the_row() {
        let chain: Vec<String> = std::iter::once("F0 = Revenue".to_string())
            .chain((1..=8).map(|i| format!("F{i} = F{p} + F{p}", p = i - 1)))
            .collect();
        let chain: Vec<&str> = chain.iter().map(String::as_str).collect();
        let mut wb = margin_book(&chain);
        wb.active_view_mut().toggle_group_collapse("Month", &["Q1"]);
        wb.model.recompute_formulas(&wb.active_view().none_cats());
        let layout = GridLayout::new(&wb.model, wb.active_view());
        let q1 = (0..layout.row_count())
            .find(|&r| layout.row_label(r) == "Q1")
            .expect("a Q1 fold row");
        let row_keys: Vec<CellKey> = (0..layout.col_count())
            .filter_map(|c| layout.cell_key(q1, c))
            .collect();
        let evaluations = |shared: bool| {
            let eval = layout.evaluator(&wb.model);
            FOLD_FORMULA_EVALUATIONS.with(|n| n.set(0));
            let values: Vec<_> = row_keys
                .iter()
                .map(|k| match shared {
                    true => eval.cell(k),
                    false => layout.evaluate(&wb.model, k),
                })
                .collect();
            (FOLD_FORMULA_EVALUATIONS.with(|n| n.get()), values)
        };
        let (apart, separately) = evaluations(false);
        let (together, shared) = evaluations(true);
        assert_eq!(shared, separately, "sharing the memo changed a value");
        assert_eq!(
            apart, 45,
            "setup: each column re-derives the chain below it"
        );
        assert_eq!(together, 9, "F0..F8 once each across the whole row");
    }

    /// improvise-pntu: context/test-evidence/core.md.
    #[test]
    fn a_memo_answers_for_the_covers_it_was_built_with() {
        let wb = margin_book(&["R = Revenue / Cost"]);
        let key = CellKey::new(vec![
            ("Month".into(), "Q1".into()),
            ("_Measure".into(), "R".into()),
        ]);
        let all = FoldMemo::new(Aggregation::Exact, |_, _| true);
        let no_feb = FoldMemo::new(Aggregation::Exact, |_, item| item != "Feb");
        let pooled = wb.model.evaluate_covering_in(&key, &all);
        let jan_only = wb.model.evaluate_covering_in(&key, &no_feb);
        assert_eq!(
            pooled,
            Some(CellValue::Number(5.0)),
            "40 / 8 over Jan and Feb"
        );
        assert_eq!(
            jan_only,
            Some(CellValue::Number(2.0)),
            "10 / 5 over Jan alone"
        );
        let fresh = wb
            .model
            .evaluate_covering(&key, Aggregation::Exact, &|_, item| item != "Feb");
        assert_eq!(jan_only, fresh, "a memo agrees with a fresh evaluation");
        assert_eq!(wb.model.evaluate_covering_in(&key, &all), pooled);
    }

    /// improvise-pntu's other half: the aggregation is the memo's too.
    #[test]
    fn a_memo_answers_for_the_aggregation_it_was_built_with() {
        let wb = margin_book(&[]);
        let key = CellKey::new(vec![("_Measure".into(), "Revenue".into())]);
        let exact = FoldMemo::new(Aggregation::Exact, |_, _| true);
        let over = FoldMemo::new(Aggregation::OverHidden, |_, _| true);
        assert_eq!(wb.model.evaluate_covering_in(&key, &exact), None);
        assert_eq!(
            wb.model.evaluate_covering_in(&key, &over),
            Some(CellValue::Number(40.0)),
            "Jan and Feb summed"
        );
    }

    /// improvise-opbo: a grid's evaluation cannot be handed another grid's memo.
    #[test]
    fn a_grid_evaluates_under_its_own_covers_not_another_grids() {
        let mut wb = margin_book(&["R = Revenue / Cost"]);
        wb.active_view_mut().toggle_group_collapse("Month", &["Q1"]);
        let all = GridLayout::new(&wb.model, wb.active_view());
        let mut no_feb_view = wb.active_view().clone();
        no_feb_view.hide_item("Month", "Feb");
        let no_feb = GridLayout::new(&wb.model, &no_feb_view);
        let key = CellKey::new(vec![
            ("Month".into(), "Q1".into()),
            ("_Measure".into(), "R".into()),
        ]);
        let on_all = all.evaluator(&wb.model);
        let on_no_feb = no_feb.evaluator(&wb.model);
        assert_eq!(
            on_all.cell(&key),
            Some(CellValue::Number(5.0)),
            "Jan and Feb"
        );
        assert_eq!(
            on_no_feb.cell(&key),
            Some(CellValue::Number(2.0)),
            "the grid hiding Feb reads Jan alone"
        );
        assert_eq!(
            on_all.cell(&key),
            Some(CellValue::Number(5.0)),
            "and neither moved the other"
        );
    }

    /// improvise-eccl — see context/test-evidence/core.md.
    #[test]
    fn a_folds_ratio_formula_is_the_ratio_of_its_folded_inputs() {
        for formulas in [
            &["Margin = (Revenue - Cost) / Revenue"][..],
            &["Profit = Revenue - Cost", "Margin = Profit / Revenue"][..],
        ] {
            let mut folded = margin_book(formulas);
            folded
                .active_view_mut()
                .toggle_group_collapse("Month", &["Q1"]);
            let fold = grid_value(&mut folded, "Q1", "Margin").expect("a Q1 fold");

            let mut none = margin_book(formulas);
            none.active_view_mut().set_axis("Month", Axis::None);
            let over_none = grid_value(&mut none, "", "Margin").expect("a None-axis cell");

            assert!((fold - 0.8).abs() < 1e-9, "{formulas:?}: fold {fold}");
            assert!(
                (fold - over_none).abs() < 1e-9,
                "{formulas:?}: {fold} vs {over_none}"
            );
        }
    }

    /// improvise-lnkb: the folded category reads as absent, as on `Axis::None` — see
    /// context/test-evidence/core.md.
    #[test]
    fn a_fold_answers_is_and_where_on_its_category_as_the_none_axis_does() {
        let formulas = [
            "Flag = Month IS \"Q1\"",
            "Marked = Revenue * 2 WHERE Month = \"Q1\"",
        ];
        for measure in ["Flag", "Marked"] {
            let mut folded = margin_book(&formulas);
            folded
                .active_view_mut()
                .toggle_group_collapse("Month", &["Q1"]);
            let fold = grid_value(&mut folded, "Q1", measure);
            let mut none = margin_book(&formulas);
            none.active_view_mut().set_axis("Month", Axis::None);
            let over_none = grid_value(&mut none, "", measure);
            assert_eq!(
                fold, over_none,
                "{measure}: the fold and the None axis differ"
            );
        }
        let mut folded = margin_book(&formulas);
        folded
            .active_view_mut()
            .toggle_group_collapse("Month", &["Q1"]);
        assert_eq!(
            grid_value(&mut folded, "Q1", "Flag"),
            Some(0.0),
            "false, not absent"
        );
    }

    /// improvise-eccl's guard on the other side: an aggregate INSIDE the formula
    /// still pools every record the fold covers.
    #[test]
    fn a_folds_aggregate_formula_pools_its_members_records() {
        let mut wb = margin_book(&["Top = MAX(Revenue)", "Total = SUM(Revenue)"]);
        wb.active_view_mut().toggle_group_collapse("Month", &["Q1"]);
        assert_eq!(grid_value(&mut wb, "Q1", "Total"), Some(40.0));
        assert_eq!(grid_value(&mut wb, "Q1", "Top"), Some(30.0));
    }

    /// An inline `WHERE` on the folded category makes every member context the
    /// same record set — see context/test-evidence/core.md.
    #[test]
    fn a_folds_aggregate_filtered_on_the_folded_category_counts_each_record_once() {
        let formula = ["Jans = SUM(Revenue WHERE Month = \"Jan\")"];
        let mut folded = margin_book(&formula);
        folded
            .active_view_mut()
            .toggle_group_collapse("Month", &["Q1"]);
        let fold = grid_value(&mut folded, "Q1", "Jans");

        let mut none = margin_book(&formula);
        none.active_view_mut().set_axis("Month", Axis::None);
        assert_eq!(fold, Some(10.0), "Jan's revenue, once");
        assert_eq!(
            fold,
            grid_value(&mut none, "", "Jans"),
            "the None-axis answer"
        );
    }

    /// Every row's value in column 0, summed: the totals strip's arithmetic.
    fn column_total(wb: &Workbook) -> f64 {
        let layout = GridLayout::new(&wb.model, wb.active_view());
        (0..layout.row_count())
            .filter_map(|r| layout.evaluate(&wb.model, &layout.cell_key(r, 0)?))
            .filter_map(|v| v.as_f64())
            .sum()
    }

    /// improvise-f2oo — see context/test-evidence/core.md.
    #[test]
    fn collapsing_a_group_with_a_hidden_member_leaves_the_total_where_it_was() {
        let mut wb = Workbook::new("T");
        wb.add_category("Month").unwrap();
        wb.add_category("Type").unwrap();
        wb.add_item("Type", "Food").unwrap();
        for (month, quarter, n) in [("Jan", "Q1", 3.0), ("Feb", "Q1", 4.0), ("Apr", "Q2", 100.0)] {
            wb.add_item_in_group("Month", month, quarter).unwrap();
            wb.model.set_cell(
                CellKey::new(vec![
                    ("Month".into(), month.into()),
                    ("Type".into(), "Food".into()),
                ]),
                CellValue::Number(n),
            );
        }
        wb.active_view_mut().set_expand_depth("Month", 1);
        wb.active_view_mut().set_axis("Month", Axis::Row);
        wb.active_view_mut().set_axis("Type", Axis::Column);
        wb.active_view_mut().hide_item("Month", "Feb");
        assert_eq!(column_total(&wb), 103.0, "expanded: Jan and Apr are drawn");

        wb.active_view_mut().toggle_group_collapse("Month", &["Q1"]);
        assert_eq!(column_total(&wb), 103.0, "z moved the total");
        assert_eq!(
            grid_value(&mut wb, "Q1", "Food"),
            Some(3.0),
            "Feb is hidden"
        );

        // Every member hidden: no fold is drawn, and the total still agrees.
        wb.active_view_mut().hide_item("Month", "Jan");
        assert_eq!(grid_value(&mut wb, "Q1", "Food"), None, "no Q1 row");
        assert_eq!(column_total(&wb), 100.0);
    }

    /// improvise-t0ma: the steps themselves, not their translation into effects —
    /// see context/test-evidence/core.md.
    #[test]
    fn records_steps_are_the_spine_then_every_non_index_dim_category_on_none() {
        use super::{ViewStep, records_spine, records_steps};
        let mut m = crate::model::Model::new("T");
        m.add_category("Region").unwrap();
        m.add_category("Product").unwrap();
        let none = |c: &str| ViewStep::Axis {
            category: c.to_string(),
            axis: Axis::None,
        };
        let mut expected = records_spine();
        expected.extend(
            m.categories
                .keys()
                .filter(|c| !matches!(c.as_str(), "_Index" | "_Dim"))
                .map(|c| none(c)),
        );
        assert!(
            expected.contains(&none("_Measure")) && expected.contains(&none("Region")),
            "setup: the model carries _Measure and a regular category"
        );
        assert_eq!(records_steps(&m), expected);
    }

    /// improvise-t0ma: a drill FIXES its coordinates on Filter and frees only the
    /// unfixed, non-virtual aggregated categories.
    #[test]
    fn drill_steps_filter_each_coordinate_and_free_the_rest() {
        use super::{ViewStep, drill_steps, records_spine};
        let key = CellKey::new(vec![
            ("Region".into(), "North".into()),
            ("Month".into(), "Feb".into()),
            ("_Measure".into(), "Revenue".into()),
        ]);
        let none_cats: Vec<String> = ["Product", "Region", "_Index", "Customer"]
            .map(String::from)
            .to_vec();
        let fixed = |c: &str, i: &str| {
            [
                ViewStep::Axis {
                    category: c.into(),
                    axis: Axis::Filter,
                },
                ViewStep::PageSelection {
                    category: c.into(),
                    item: i.into(),
                },
            ]
        };
        let mut expected = records_spine();
        expected.extend(fixed("Month", "Feb"));
        expected.extend(fixed("Region", "North"));
        expected.extend(fixed("_Measure", "Revenue"));
        expected.extend(["Product", "Customer"].map(|c| ViewStep::Axis {
            category: c.into(),
            axis: Axis::None,
        }));
        assert_eq!(drill_steps(&key, &none_cats), expected);
    }
}

/// Group membership on a multi-level axis is per level, and pruning cannot move
/// it (improvise-e265, improvise-6u6s). `context/test-evidence/core.md`.
#[cfg(test)]
mod group_membership {
    use super::{AxisEntry, GridLayout, data_items, prune_axis_items, walk_open_runs};
    use crate::view::Axis;
    use crate::workbook::Workbook;
    use std::ops::ControlFlow;

    /// Every data item's open runs, collected through the one reader.
    fn open_runs(items: &[AxisEntry]) -> Vec<Vec<super::OpenRun>> {
        let mut all = Vec::new();
        walk_open_runs(items, |open| {
            all.push(open.to_vec());
            ControlFlow::<()>::Continue(())
        });
        all
    }

    type Level<'a> = (&'a str, &'a [(&'a str, Option<&'a str>)]);
    type Fixture<'a> = (&'a [Level<'a>], &'a [(&'a str, &'a str)], &'a [Parent<'a>]);
    /// `(category, group, parent)`: a `>` line.
    type Parent<'a> = (&'a str, &'a str, &'a str);

    const TWO: &[Level] = &[
        ("Region", &[("North", Some("Upper")), ("South", None)]),
        (
            "Month",
            &[
                ("Jan", Some("Q1")),
                ("Feb", Some("Q1")),
                ("Mar", None),
                ("Apr", Some("Q2")),
            ],
        ),
    ];

    const THREE: &[Level] = &[
        ("Region", &[("North", Some("Upper")), ("South", None)]),
        ("Month", &[("Jan", Some("Q1")), ("Mar", None)]),
        ("Kind", &[("Food", Some("Goods")), ("Tools", None)]),
    ];

    /// `Q1` is not contiguous, so one parent holds two `Q1` runs (h0iv).
    const NONCONTIGUOUS: &[Level] = &[
        ("Region", &[("North", Some("Upper")), ("South", None)]),
        (
            "Month",
            &[("Jan", Some("Q1")), ("Mar", None), ("Feb", Some("Q1"))],
        ),
    ];

    /// A middle level with no groups at all.
    const HOLE: &[Level] = &[
        ("Region", &[("North", Some("Upper")), ("South", None)]),
        ("Kind", &[("Food", None), ("Tools", None)]),
        ("Month", &[("Jan", Some("Q1")), ("Mar", None)]),
    ];

    /// Two outer groups side by side, with no ungrouped item between them.
    const ADJACENT: &[Level] = &[
        (
            "Region",
            &[
                ("North", Some("Upper")),
                ("South", Some("Lower")),
                ("West", None),
            ],
        ),
        ("Month", &[("Jan", Some("Q1")), ("Mar", None)]),
    ];

    const FIXTURES: &[Fixture] = &[
        (
            TWO,
            &[("Region", "Upper"), ("Month", "Q1"), ("Month", "Q2")],
            &[],
        ),
        (
            THREE,
            &[("Region", "Upper"), ("Month", "Q1"), ("Kind", "Goods")],
            &[],
        ),
        (NONCONTIGUOUS, &[("Region", "Upper"), ("Month", "Q1")], &[]),
        (HOLE, &[("Region", "Upper"), ("Month", "Q1")], &[]),
        (
            ADJACENT,
            &[("Region", "Upper"), ("Region", "Lower"), ("Month", "Q1")],
            &[],
        ),
        (
            NESTED,
            &[
                ("Region", "Upper"),
                ("Month", "H1"),
                ("Month", "Q1"),
                ("Month", "H2"),
            ],
            NESTED_PARENTS,
        ),
    ];

    /// improvise-felo.6: a hierarchy INSIDE one category, under an outer group.
    /// Q1 is non-contiguous, and Jun sits directly in H1 after its child Q2.
    const NESTED: &[Level] = &[
        ("Region", &[("North", Some("Upper")), ("South", None)]),
        (
            "Month",
            &[
                ("Jan", Some("Q1")),
                ("Apr", Some("Q2")),
                ("Feb", Some("Q1")),
                ("Jun", Some("H1")),
                ("Jul", Some("Q3")),
            ],
        ),
    ];

    const NESTED_PARENTS: &[Parent] = &[
        ("Month", "Q1", "H1"),
        ("Month", "Q2", "H1"),
        ("Month", "Q3", "H2"),
    ];

    /// [`book`] with `parents` declared and every group OPEN before `collapsed`
    /// folds its own, so a subset of groups is exactly what is collapsed.
    fn nested_book(levels: &[Level], parents: &[Parent], collapsed: &[(&str, &str)]) -> Workbook {
        let mut wb = book(levels, &[]);
        for (cat, group, parent) in parents {
            let c = wb.model.category_mut(cat).unwrap();
            c.add_group(crate::model::category::Group::new(*group).with_parent(*parent));
        }
        for (cat, _) in levels {
            wb.active_view_mut().set_expand_depth(cat, 8);
        }
        for (cat, group) in collapsed {
            let chain: Vec<String> = wb
                .model
                .category(cat)
                .unwrap()
                .group_chain(group)
                .into_iter()
                .map(String::from)
                .collect();
            let chain: Vec<&str> = chain.iter().map(String::as_str).collect();
            wb.active_view_mut().toggle_group_collapse(cat, &chain);
        }
        wb
    }

    /// `levels` on Row, one column, the groups `collapsed` names folded. No cell
    /// is set: the unpruned layout has every row, and pruning is driven directly.
    fn book(levels: &[Level], collapsed: &[(&str, &str)]) -> Workbook {
        let mut wb = Workbook::new("T");
        for (cat, items) in levels {
            wb.add_category(*cat).unwrap();
            for (item, group) in items.iter() {
                match group {
                    Some(g) => wb.add_item_in_group(cat, *item, *g).unwrap(),
                    None => wb.add_item(cat, *item).unwrap(),
                };
            }
            wb.active_view_mut().set_axis(cat, Axis::Row);
            wb.active_view_mut().set_expand_depth(cat, 1);
        }
        wb.add_category("Col").unwrap();
        wb.add_item("Col", "X").unwrap();
        wb.active_view_mut().set_axis("Col", Axis::Column);
        for (cat, group) in collapsed {
            wb.active_view_mut().toggle_group_collapse(cat, &[group]);
        }
        wb
    }

    fn layout(wb: &Workbook) -> GridLayout {
        GridLayout::with_frozen_records(&wb.model, wb.active_view(), None)
    }

    /// One case: its name, the kept-row mask, and the unpruned and pruned rows.
    struct Case {
        name: String,
        mask: u32,
        full: Vec<AxisEntry>,
        pruned: Vec<AxisEntry>,
    }

    /// Every collapse subset of every fixture, one layout each, pruned by every
    /// non-empty set of kept rows. The layout is the cost, so it is built once.
    fn cases() -> Vec<Case> {
        let mut out = Vec::new();
        for (fixture, (levels, groups, parents)) in FIXTURES.iter().enumerate() {
            for folds in 0..(1u32 << groups.len()) {
                let collapsed: Vec<(&str, &str)> = (0..groups.len())
                    .filter(|g| folds & (1 << g) != 0)
                    .map(|g| groups[g])
                    .collect();
                let full = layout(&nested_book(levels, parents, &collapsed)).row_items;
                let rows = data_items(&full).count();
                for mask in 1..(1u32 << rows) {
                    let mut pruned = full.clone();
                    prune_axis_items(&mut pruned, |i| mask & (1 << i) != 0);
                    out.push(Case {
                        name: format!("fixture {fixture}, mask {mask:#b}, collapsed {collapsed:?}"),
                        mask,
                        full: full.clone(),
                        pruned,
                    });
                }
            }
        }
        out
    }

    /// The `(category, group)` a run's header names.
    fn named(items: &[AxisEntry], run: Option<usize>) -> Option<(&str, &str)> {
        match &items[run?] {
            AxisEntry::GroupHeader {
                cat_name,
                group_name,
                ..
            } => Some((cat_name, group_name)),
            AxisEntry::GroupEnd { .. } | AxisEntry::DataItem { .. } => None,
        }
    }

    fn at(runs: &[super::OpenRun], key: super::RunKey) -> Option<usize> {
        runs.iter().find(|(k, _)| *k == key).map(|(_, at)| *at)
    }

    #[test]
    fn pruning_keeps_every_surviving_rows_runs_at_every_level() {
        for case in cases() {
            let Case {
                name,
                mask,
                full,
                pruned,
            } = &case;
            let kept: Vec<usize> = (0..data_items(full).count())
                .filter(|i| mask & (1 << i) != 0)
                .collect();
            let full_runs = open_runs(full);
            let pruned_runs = open_runs(pruned);
            assert_eq!(pruned_runs.len(), kept.len(), "{name}: the survivors");
            if kept.len() == full_runs.len() {
                assert_eq!(pruned, full, "{name}: nothing pruned");
            }
            let levels: std::collections::BTreeSet<super::RunKey> =
                full_runs.iter().flatten().map(|(k, _)| *k).collect();
            for (r, &u) in kept.iter().enumerate() {
                for &level in &levels {
                    assert_eq!(
                        named(pruned, at(&pruned_runs[r], level)),
                        named(full, at(&full_runs[u], level)),
                        "{name}: row {r}, level {level:?}\n pruned {pruned:?}\n full {full:?}"
                    );
                    if r == 0 {
                        continue;
                    }
                    let same = |runs: &[Vec<super::OpenRun>], a: usize, b: usize| {
                        at(&runs[a], level).is_some() && at(&runs[a], level) == at(&runs[b], level)
                    };
                    assert_eq!(
                        same(&pruned_runs, r - 1, r),
                        same(&full_runs, kept[r - 1], u),
                        "{name}: run identity at level {level:?}, row {r}\n pruned {pruned:?}\n full {full:?}"
                    );
                }
            }
        }
    }

    #[test]
    fn a_pruned_axis_draws_no_header_over_nothing_and_ends_on_no_marker() {
        for Case {
            name,
            pruned: items,
            ..
        } in cases()
        {
            for (i, entry) in items.iter().enumerate() {
                if let AxisEntry::GroupHeader { .. } = entry {
                    let next = items[i + 1..]
                        .iter()
                        .find(|e| !matches!(e, AxisEntry::GroupHeader { .. }));
                    assert!(
                        matches!(next, Some(AxisEntry::DataItem { .. })),
                        "{name}: header {entry:?} has no member in {items:?}"
                    );
                }
            }
            assert!(
                !matches!(items.last(), Some(AxisEntry::GroupEnd { .. })),
                "{name}: trailing marker in {items:?}"
            );
        }
    }

    /// `prune_empty` reaches the same pruner from funded cells: the ticket's own
    /// shape, only `North/Mar` funded, keeps `North/Mar` in `Upper`.
    #[test]
    fn prune_empty_keeps_a_row_past_an_inner_run_in_the_outer_group() {
        let mut wb = book(TWO, &[]);
        let cell = crate::model::cell::CellKey::new(vec![
            ("Region".into(), "North".into()),
            ("Month".into(), "Mar".into()),
            ("Col".into(), "X".into()),
        ]);
        wb.model
            .set_cell(cell, crate::model::cell::CellValue::Number(1.0));
        wb.active_view_mut().prune_empty = true;
        let l = layout(&wb);
        assert_eq!(l.row_count(), 1);
        assert_eq!(l.row_group_for(0), Some(("Region".into(), "Upper".into())));
    }

    /// A lookup reads only up to its own row (improvise-0r01). The header after
    /// row 0 is unreadable on purpose: reaching it would try to allocate usize::MAX.
    #[test]
    fn a_lookup_does_not_read_past_its_row() {
        let items = vec![
            AxisEntry::GroupHeader {
                level: 0,
                depth: 0,
                cat_name: "Region".into(),
                group_name: "Upper".into(),
                collapsed: false,
            },
            AxisEntry::item(vec!["North".into()]),
            AxisEntry::GroupHeader {
                level: usize::MAX,
                depth: 0,
                cat_name: "Nowhere".into(),
                group_name: "Unreachable".into(),
                collapsed: false,
            },
            AxisEntry::item(vec!["South".into()]),
        ];
        assert_eq!(
            super::innermost_group(&items, 0),
            Some(("Region".into(), "Upper".into()))
        );
    }

    /// A lookup past the first run answers for its own row, not a neighbour's
    /// (improvise-0r01's late index). Evidence file.
    #[test]
    fn a_late_lookup_answers_for_its_own_row() {
        let header = |group: &str| AxisEntry::GroupHeader {
            level: 0,
            depth: 0,
            cat_name: "Month".into(),
            group_name: group.into(),
            collapsed: false,
        };
        let items = vec![
            header("Q1"),
            AxisEntry::item(vec!["Jan".into()]),
            AxisEntry::item(vec!["Feb".into()]),
            AxisEntry::GroupEnd { level: 0, depth: 0 },
            AxisEntry::item(vec!["Mar".into()]),
            header("Q2"),
            AxisEntry::item(vec!["Apr".into()]),
            AxisEntry::item(vec!["May".into()]),
        ];
        let q = |group: &str| Some(("Month".to_string(), group.to_string()));
        let found: Vec<_> = (0..6).map(|i| super::innermost_group(&items, i)).collect();
        assert_eq!(found, vec![q("Q1"), q("Q1"), None, q("Q2"), q("Q2"), None]);
    }

    /// The user's ruling on improvise-e265: membership is per level, and `z`
    /// names the innermost open group — the one nearest the cursor.
    #[test]
    fn a_row_after_an_inner_groups_end_is_still_in_the_outer_group() {
        let l = layout(&book(TWO, &[]));
        let at = |tuple: [&str; 2]| {
            (0..l.row_count())
                .find(|&r| l.row_tuple(r) == Some(&[tuple[0].into(), tuple[1].into()][..]))
                .unwrap()
        };
        assert_eq!(
            l.row_group_for(at(["North", "Jan"])),
            Some(("Month".into(), "Q1".into())),
            "the innermost open group"
        );
        assert_eq!(
            l.row_group_for(at(["North", "Mar"])),
            Some(("Region".into(), "Upper".into())),
            "Q1 ended; Upper did not"
        );
        assert_eq!(l.row_group_for(at(["South", "Mar"])), None);
    }

    /// Precondition for the enumeration above: NESTED must really produce runs
    /// below depth 0, or it enumerates nothing new.
    #[test]
    fn the_nested_fixture_draws_runs_inside_one_category() {
        let wb = nested_book(NESTED, NESTED_PARENTS, &[]);
        let deep = layout(&wb)
            .row_items
            .iter()
            .filter(|e| matches!(e, AxisEntry::GroupHeader { depth: 1, .. }))
            .count();
        assert!(
            deep >= 4,
            "Q1, Q2, Q1 again and Q3 under each region: {deep}"
        );
    }
}

/// improvise-felo.6: a hierarchy inside one category — Year > Quarter > Month.
#[cfg(test)]
mod hierarchy {
    use super::{AxisEntry, GridLayout};
    use crate::model::category::Group;
    use crate::model::cell::{CellKey, CellValue};
    use crate::view::Axis;
    use crate::workbook::Workbook;

    /// `2024-02` is added AFTER `2025-01` and `Undated`, so Q1-2024 and 2024 are
    /// non-contiguous. Values are powers of ten, so every fold is its own sum.
    fn dates() -> Workbook {
        let mut wb = Workbook::new("T");
        wb.add_category("Date").unwrap();
        for (month, quarter) in [
            ("2024-01", Some("Q1-2024")),
            ("2024-04", Some("Q2-2024")),
            ("2025-01", Some("Q1-2025")),
            ("Undated", None),
            ("2024-02", Some("Q1-2024")),
        ] {
            match quarter {
                Some(q) => wb.add_item_in_group("Date", month, q).unwrap(),
                None => wb.add_item("Date", month).unwrap(),
            };
        }
        let cat = wb.model.category_mut("Date").unwrap();
        for (quarter, year) in [
            ("Q1-2024", "2024"),
            ("Q2-2024", "2024"),
            ("Q1-2025", "2025"),
        ] {
            cat.add_group(Group::new(quarter).with_parent(year));
        }
        wb.add_item("_Measure", "Revenue").unwrap();
        wb.active_view_mut().set_axis("Date", Axis::Row);
        wb.active_view_mut().set_axis("_Measure", Axis::Column);
        for (month, v) in [
            ("2024-01", 1.0),
            ("2024-02", 10.0),
            ("2024-04", 100.0),
            ("2025-01", 1000.0),
            ("Undated", 5.0),
        ] {
            wb.model.set_cell(key(month), CellValue::Number(v));
        }
        wb
    }

    fn key(month: &str) -> CellKey {
        CellKey::new(vec![
            ("Date".into(), month.into()),
            ("_Measure".into(), "Revenue".into()),
        ])
    }

    fn layout(wb: &Workbook) -> GridLayout {
        GridLayout::with_frozen_records(&wb.model, wb.active_view(), None)
    }

    fn rows(l: &GridLayout) -> Vec<String> {
        (0..l.row_count()).map(|r| l.row_label(r)).collect()
    }

    fn value(wb: &Workbook, l: &GridLayout, label: &str) -> Option<f64> {
        let r = rows(l).iter().position(|x| x == label)?;
        l.evaluate(&wb.model, &l.cell_key(r, 0)?)?.as_f64()
    }

    fn header(depth: usize, group: &str, collapsed: bool) -> AxisEntry {
        AxisEntry::GroupHeader {
            level: 0,
            depth,
            cat_name: "Date".into(),
            group_name: group.into(),
            collapsed,
        }
    }

    fn end(depth: usize) -> AxisEntry {
        AxisEntry::GroupEnd { level: 0, depth }
    }

    fn item(name: &str) -> AxisEntry {
        AxisEntry::item(vec![name.into()])
    }

    /// Opened to depth 2, the whole sequence: runs nest by depth, a new year's
    /// header replaces the old, and a non-contiguous quarter reopens both.
    #[test]
    fn a_year_quarter_month_hierarchy_nests_its_runs() {
        let mut wb = dates();
        wb.active_view_mut().set_expand_depth("Date", 2);
        assert_eq!(
            layout(&wb).row_items,
            vec![
                header(0, "2024", false),
                header(1, "Q1-2024", false),
                item("2024-01"),
                header(1, "Q2-2024", false),
                item("2024-04"),
                end(1),
                header(0, "2025", false),
                header(1, "Q1-2025", false),
                item("2025-01"),
                end(1),
                end(0),
                item("Undated"),
                header(0, "2024", false),
                header(1, "Q1-2024", false),
                item("2024-02"),
            ]
        );
    }

    /// The user's ruling: a hierarchy starts closed, one row per root.
    #[test]
    fn a_hierarchy_opens_closed_as_one_fold_per_root() {
        let wb = dates();
        let l = layout(&wb);
        assert_eq!(rows(&l), vec!["2024", "2025", "Undated"]);
        assert_eq!(value(&wb, &l, "2024"), Some(111.0), "every month of 2024");
        assert_eq!(value(&wb, &l, "2025"), Some(1000.0));
        assert!(l.row_folds_a_group(0) && !l.row_folds_a_group(2));
    }

    /// Open the years only: each quarter folds its months, ONCE, even when its
    /// months are not contiguous.
    #[test]
    fn a_collapsed_group_at_an_inner_level_folds_its_leaves_once() {
        let mut wb = dates();
        wb.active_view_mut().set_expand_depth("Date", 1);
        let l = layout(&wb);
        assert_eq!(rows(&l), vec!["Q1-2024", "Q2-2024", "Q1-2025", "Undated"]);
        assert_eq!(
            value(&wb, &l, "Q1-2024"),
            Some(11.0),
            "Jan and the late Feb"
        );
        assert_eq!(value(&wb, &l, "Q2-2024"), Some(100.0));
    }

    /// Collapsing an outer group folds everything under it, whatever its
    /// children's own state.
    #[test]
    fn collapsing_a_parent_folds_its_children_whatever_their_state() {
        let mut wb = dates();
        wb.active_view_mut().set_expand_depth("Date", 2);
        wb.active_view_mut()
            .toggle_group_collapse("Date", &["2024"]);
        let l = layout(&wb);
        assert_eq!(rows(&l), vec!["2024", "2025-01", "Undated"]);
        assert_eq!(value(&wb, &l, "2024"), Some(111.0));
    }

    /// `z` names the innermost open group: a month's quarter, a quarter fold's
    /// own quarter, a year fold's year — at a nonzero row each time.
    #[test]
    fn the_group_under_the_cursor_is_the_innermost_at_every_depth() {
        let mut wb = dates();
        wb.active_view_mut().set_expand_depth("Date", 1);
        wb.active_view_mut()
            .toggle_group_collapse("Date", &["2024", "Q2-2024"]);
        let l = layout(&wb);
        let at = |label: &str| rows(&l).iter().position(|x| x == label).unwrap();
        let g = |name: &str| Some(("Date".to_string(), name.to_string()));
        assert_eq!(l.row_group_for(at("2024-04")), g("Q2-2024"));
        assert_eq!(l.row_group_for(at("Q1-2025")), g("Q1-2025"));
        assert_eq!(l.row_group_for(at("Undated")), None);
        wb.active_view_mut().set_expand_depth("Date", 0);
        let l = layout(&wb);
        assert_eq!(l.row_group_for(1), g("2025"));
    }

    /// A hidden month leaves every fold above it; a quarter whose every month
    /// is hidden has no row and no header.
    #[test]
    fn hidden_items_leave_every_fold_above_them() {
        let mut wb = dates();
        wb.active_view_mut().hide_item("Date", "2024-04");
        let l = layout(&wb);
        assert_eq!(value(&wb, &l, "2024"), Some(11.0), "2024-04 hidden");
        wb.active_view_mut().set_expand_depth("Date", 2);
        let l = layout(&wb);
        let q2 = |e: &AxisEntry| matches!(e, AxisEntry::GroupHeader { group_name, .. } if group_name == "Q2-2024");
        assert!(!l.row_items.iter().any(q2), "{:?}", l.row_items);
    }

    /// `prune_empty` at depth: an empty quarter goes, header and all, and the
    /// year above it stays for the quarter that has data.
    #[test]
    fn pruning_drops_an_empty_inner_group_and_keeps_its_parent() {
        let mut wb = dates();
        wb.model.clear_cell(&key("2024-04"));
        wb.active_view_mut().set_expand_depth("Date", 1);
        wb.active_view_mut().prune_empty = true;
        let l = layout(&wb);
        assert_eq!(rows(&l), vec!["Q1-2024", "Q1-2025", "Undated"]);
        assert_eq!(l.row_items[0], header(0, "2024", false));
    }
}

#[cfg(test)]
mod lookup_cost_tests {
    use super::{DATA_ITEM_STEPS, GridLayout};
    use crate::view::Axis;
    use crate::workbook::Workbook;

    fn tall(rows: usize) -> GridLayout {
        let mut wb = Workbook::new("T");
        wb.add_category("Row").unwrap();
        wb.add_category("Col").unwrap();
        for i in 0..rows {
            wb.add_item("Row", format!("r{i}")).unwrap();
        }
        wb.add_item("Col", "c0").unwrap();
        wb.add_item("Col", "c1").unwrap();
        wb.active_view_mut().set_axis("Row", Axis::Row);
        wb.active_view_mut().set_axis("Col", Axis::Column);
        GridLayout::new(&wb.model, wb.active_view())
    }

    fn steps(f: impl FnOnce()) -> usize {
        DATA_ITEM_STEPS.with(|n| n.set(0));
        f();
        DATA_ITEM_STEPS.with(|n| n.get())
    }

    /// improvise-felo.13: context/test-evidence/layout-cost.md.
    #[test]
    fn an_axis_lookup_does_work_independent_of_the_row_index() {
        let layout = tall(400);
        let last = layout.row_count() - 1;
        assert_eq!(last, 399, "the fixture is as tall as the test needs");
        let lookups: [(&str, &dyn Fn(usize)); 4] = [
            ("cell_key", &|r| {
                let _ = layout.cell_key(r, 1);
            }),
            ("row_tuple", &|r| {
                let _ = layout.row_tuple(r);
            }),
            ("row_label", &|r| {
                let _ = layout.row_label(r);
            }),
            ("column_key", &|r| {
                let _ = layout.column_key(r % 2);
            }),
        ];
        for (name, lookup) in lookups {
            let first = steps(|| lookup(0));
            let deep = steps(|| lookup(last));
            assert!(
                deep <= first + 2,
                "{name}: row {last} cost {deep} steps against {first} at row 0"
            );
        }
    }

    /// improvise-felo.13: the index answers what the walk it replaced did, through
    /// groups, a collapsed fold and a pruned axis. Evidence: layout-cost.md.
    #[test]
    fn the_position_index_answers_what_the_walk_answered() {
        use super::{AxisEntry, data_items};
        use crate::model::cell::{CellKey, CellValue};
        let mut wb = Workbook::new("T");
        for cat in ["Row", "Col"] {
            wb.add_category(cat).unwrap();
        }
        for (i, g) in ["Q1", "Q1", "Q2", "Q2", "Q3"].iter().enumerate() {
            wb.add_item_in_group("Row", format!("r{i}"), *g).unwrap();
        }
        wb.add_item("Row", "ghost").unwrap();
        wb.add_item("Col", "c0").unwrap();
        wb.active_view_mut().set_axis("Row", Axis::Row);
        wb.active_view_mut().set_axis("Col", Axis::Column);
        wb.active_view_mut().set_expand_depth("Row", 1);
        wb.active_view_mut().toggle_group_collapse("Row", &["Q2"]);
        wb.active_view_mut().prune_empty = true;
        for i in 0..5 {
            let key = CellKey::new(vec![
                ("Row".into(), format!("r{i}")),
                ("Col".into(), "c0".into()),
            ]);
            wb.model.set_cell(key, CellValue::Number(1.0));
        }
        let layout = GridLayout::with_frozen_records(&wb.model, wb.active_view(), None);
        assert!(
            layout
                .row_items
                .iter()
                .any(|e| !matches!(e, AxisEntry::DataItem { .. }))
        );
        assert!(layout.row_items.iter().any(|e| matches!(
            e,
            AxisEntry::DataItem {
                collapsed: true,
                ..
            }
        )));
        let walked: Vec<&Vec<String>> = data_items(&layout.row_items).collect();
        assert_eq!(layout.row_count(), walked.len());
        let visual: Vec<usize> = layout
            .row_items
            .iter()
            .enumerate()
            .filter(|(_, e)| matches!(e, AxisEntry::DataItem { .. }))
            .map(|(at, _)| at)
            .collect();
        for i in 0..walked.len() + 2 {
            assert_eq!(
                layout.row_tuple(i),
                walked.get(i).map(|t| t.as_slice()),
                "row {i}"
            );
            assert_eq!(
                layout.data_row_to_visual(i),
                visual.get(i).copied(),
                "visual {i}"
            );
            let folded = matches!(
                data_items_entry(&layout.row_items, i),
                Some(AxisEntry::DataItem {
                    collapsed: true,
                    ..
                })
            );
            assert_eq!(layout.row_folds_a_group(i), folded, "fold {i}");
        }
    }

    /// improvise-felo.13: a drill held open while records are removed elsewhere.
    /// Evidence: layout-cost.md.
    #[test]
    fn a_frozen_records_snapshot_replaces_the_live_row_index() {
        use crate::model::cell::{CellKey, CellValue};
        let mut wb = Workbook::new("T");
        wb.add_category("Region").unwrap();
        wb.add_item("_Measure", "Revenue").unwrap();
        let cell = |i: usize| {
            CellKey::new(vec![
                ("Region".into(), format!("r{i}")),
                ("_Measure".into(), "Revenue".into()),
            ])
        };
        for i in 0..3 {
            wb.add_item("Region", format!("r{i}")).unwrap();
            wb.model.set_cell(cell(i), CellValue::Number(1.0));
        }
        let v = wb.active_view_mut();
        v.set_axis("_Index", Axis::Row);
        v.set_axis("_Dim", Axis::Column);
        let frozen = GridLayout::new(&wb.model, wb.active_view()).records;
        wb.model.clear_cell(&cell(1));
        wb.model.clear_cell(&cell(2));
        let live = GridLayout::with_frozen_records(&wb.model, wb.active_view(), None);
        assert_eq!(live.row_count(), 1, "the control: one record is live");
        let held = GridLayout::with_frozen_records(&wb.model, wb.active_view(), frozen);
        assert_eq!(held.row_count(), 3);
        assert_eq!(held.row_tuple(2), Some(&["2".to_string()][..]));
        assert!(held.cell_key(2, 0).is_some());
    }

    fn data_items_entry(items: &[super::AxisEntry], n: usize) -> Option<&super::AxisEntry> {
        items
            .iter()
            .filter(|e| matches!(e, super::AxisEntry::DataItem { .. }))
            .nth(n)
    }
}