fix(core): an expanded fold's depth is non-zero by type, so every fold the API builds reads back (improvise-felo.6)

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VdfEGXA7aJ6ftMsCc9PJze

context/test-evidence/formula-and-io.md

@@ -1648,3 +1648,14 @@ category named `expanded`.
 closed, the one its `collapsed:` line names and the three it does not, and the
 line is written back. Rival: the old open default; this test fails with the
 other eleven in that rival's list.
+
+### every_fold_the_api_can_build_reads_back_from_the_file_it_writes
+
+ChatGPT Codex, P2 on #317 at `e09ec752`: `GroupFold::Expanded(usize)` and
+`View::set_group_fold` are public, so `Expanded(0)` could be built, written as
+`expanded(0): …` and then refused by the grammar that wrote it. Confirmed by
+reading: the writer formatted any `usize`, and `depth_number` starts at `1`.
+The fix makes the state unrepresentable (`Expanded(NonZeroUsize)`), so the
+regression guard is the type, and the old failing input no longer compiles.
+This test covers every fold the API CAN build (`Collapsed` and depths 1 to 3,
+beside a category depth), each saved and read back.

crates/improvise-core/src/view/types.rs

@@ -1,5 +1,6 @@
 use indexmap::IndexMap;
 use std::collections::{HashMap, HashSet};
+use std::num::NonZeroUsize;
 
 use super::axis::Axis;
 use crate::model::category::CategoryKind;
@@ -32,8 +33,9 @@ pub fn unused_view_name<'a>(existing: impl IntoIterator<Item = &'a str>) -> Stri
 pub enum GroupFold {
     /// `collapsed: Cat/Group` — closed, whatever reaches it from above.
     Collapsed,
-    /// `expanded(N): Cat/Group` — open, with N-1 levels beneath it; N >= 1.
-    Expanded(usize),
+    /// `expanded(N): Cat/Group` — open, with N-1 levels beneath it. Non-zero by
+    /// type, since `expanded(0)` is no spelling the grammar reads back.
+    Expanded(NonZeroUsize),
 }
 
 #[derive(Clone)]
@@ -273,7 +275,7 @@ impl View {
         let depth = chain.len().saturating_sub(1);
         let by_ancestor = chain[..depth].iter().enumerate().any(|(at, ancestor)| {
             matches!(self.group_fold(cat_name, ancestor),
-                Some(GroupFold::Expanded(n)) if depth - at < n)
+                Some(GroupFold::Expanded(n)) if depth - at < n.get())
         });
         !by_ancestor && depth >= self.expand_depth(cat_name)
     }
@@ -310,7 +312,7 @@ impl View {
             false => {
                 let fold = match collapse {
                     true => GroupFold::Collapsed,
-                    false => GroupFold::Expanded(1),
+                    false => GroupFold::Expanded(NonZeroUsize::MIN),
                 };
                 self.set_group_fold(cat_name, group, fold);
             }
@@ -371,6 +373,7 @@ mod tests {
     use super::{GroupFold, View, unused_view_name};
     use crate::model::category::CategoryKind;
     use crate::view::Axis;
+    use std::num::NonZeroUsize;
 
     /// improvise-vw47: the count is the starting guess, never the answer.
     /// What a taken name costs: `context/test-evidence/core.md`.
@@ -664,7 +667,11 @@ mod tests {
     #[test]
     fn a_groups_depth_opens_it_and_n_minus_one_levels_beneath() {
         let mut v = View::new("Test");
-        v.set_group_fold("Time", "2024", GroupFold::Expanded(2));
+        v.set_group_fold(
+            "Time",
+            "2024",
+            GroupFold::Expanded(NonZeroUsize::new(2).unwrap()),
+        );
         assert!(!v.is_group_collapsed("Time", &["2024"]));
         assert!(!v.is_group_collapsed("Time", &["2024", "Q1"]));
         assert!(v.is_group_collapsed("Time", &["2024", "Q1", "Jan"]));
@@ -690,7 +697,7 @@ mod tests {
         v.toggle_group_collapse("Time", &["2024", "Q1-2024"]);
         assert_eq!(
             v.group_fold("Time", "Q1-2024"),
-            Some(GroupFold::Expanded(1))
+            Some(GroupFold::Expanded(NonZeroUsize::MIN))
         );
         v.toggle_group_collapse("Time", &["2024"]);
         assert_eq!(v.group_fold("Time", "2024"), Some(GroupFold::Collapsed));

crates/improvise-io/src/persistence/format.rs

@@ -148,7 +148,7 @@ pub fn format_md(workbook: &Workbook) -> String {
             let target = format!("{}/{}", quote_name(cat), quote_name(group));
             match fold {
                 GroupFold::Collapsed => w!(out, "collapsed: {target}"),
-                GroupFold::Expanded(1) => w!(out, "expanded: {target}"),
+                GroupFold::Expanded(n) if n.get() == 1 => w!(out, "expanded: {target}"),
                 GroupFold::Expanded(n) => w!(out, "expanded({n}): {target}"),
             }
         }

crates/improvise-io/src/persistence/parse.rs

@@ -399,7 +399,7 @@ fn walk_axis_line(entry: Pair<'_, Rule>, pv: &mut PView) -> Result<()> {
 
 /// `expanded(N): Cat` or `expanded(N): Cat/Group`; a bare `expanded:` is N = 1.
 fn walk_expanded_line(entry: Pair<'_, Rule>) -> Result<PFold> {
-    let mut depth = 1usize;
+    let mut depth = std::num::NonZeroUsize::MIN;
     let mut names = Vec::new();
     for part in entry.into_inner() {
         match part.as_rule() {
@@ -557,8 +557,8 @@ fn build_view(wb: &mut Workbook, pv: &PView) -> Result<()> {
                 _ => view.set_group_fold(category, g, *fold),
             },
             (None, GroupFold::Expanded(n)) => match view.expand_depth(category) {
-                0 => view.set_expand_depth(category, *n),
-                held if held != *n => return Err(conflict().into()),
+                0 => view.set_expand_depth(category, n.get()),
+                held if held != n.get() => return Err(conflict().into()),
                 _ => {}
             },
             (None, GroupFold::Collapsed) => bail!("a category-wide collapse has no spelling"),

crates/improvise-io/src/persistence/roundtrip_tests.rs

@@ -305,9 +305,12 @@ fn expanded_and_collapsed_lines_round_trip_together() {
     assert_eq!(v.group_fold("Date", "Q1-2024"), Some(GroupFold::Collapsed));
     assert_eq!(
         v.group_fold("Date", "Q2-2024"),
-        Some(GroupFold::Expanded(1))
+        Some(GroupFold::Expanded(std::num::NonZeroUsize::MIN))
+    );
+    assert_eq!(
+        v.group_fold("Date", "2025"),
+        Some(GroupFold::Expanded(std::num::NonZeroUsize::new(3).unwrap()))
     );
-    assert_eq!(v.group_fold("Date", "2025"), Some(GroupFold::Expanded(3)));
     let written = format_md(&loaded);
     for line in [
         "expanded(2): Date",
@@ -1423,3 +1426,28 @@ fn a_loaded_label_category_is_not_on_a_pivot_axis() {
         );
     }
 }
+
+/// ChatGPT Codex on #317: a fold built through the public API must survive its
+/// own save. `Expanded(0)` is now unconstructible; every depth that is survives.
+#[test]
+fn every_fold_the_api_can_build_reads_back_from_the_file_it_writes() {
+    use crate::view::GroupFold;
+    use std::num::NonZeroUsize;
+    let base = parse_md(&dates_with("")).unwrap();
+    for n in 1..=3 {
+        let depth = NonZeroUsize::new(n).unwrap();
+        for fold in [GroupFold::Collapsed, GroupFold::Expanded(depth)] {
+            let mut wb = base.clone();
+            wb.active_view_mut().set_group_fold("Date", "2024", fold);
+            wb.active_view_mut().set_expand_depth("Date", n);
+            let text = format_md(&wb);
+            let back = parse_md(&text).unwrap_or_else(|e| panic!("{e}\n{text}"));
+            assert_eq!(
+                back.active_view().group_fold("Date", "2024"),
+                Some(fold),
+                "{text}"
+            );
+            assert_eq!(back.active_view().expand_depth("Date"), n, "{text}");
+        }
+    }
+}

tests/grid_frame_agreement.rs

@@ -215,7 +215,11 @@ fn a_date_hierarchy() -> Workbook {
     view.set_axis("Date", Axis::Row);
     view.set_axis("_Measure", Axis::Column);
     view.set_expand_depth("Recipient", 1);
-    view.set_group_fold("Date", "Y24", GroupFold::Expanded(2));
+    view.set_group_fold(
+        "Date",
+        "Y24",
+        GroupFold::Expanded(std::num::NonZeroUsize::new(2).unwrap()),
+    );
     view.set_group_fold("Date", "Q2", GroupFold::Collapsed);
     for r in ["Alice", "Carol"] {
         for d in ["Jan", "Feb", "Apr", "Jan25", "Undated"] {