diff --git a/src/formula/parser.rs b/src/formula/parser.rs
index d4628d8..7d2d9fb 100644
--- a/src/formula/parser.rs
+++ b/src/formula/parser.rs
@@ -304,3 +304,60 @@ fn parse_comparison(tokens: &[Token], pos: &mut usize) -> Result<Expr> {
     let right = parse_add_sub(tokens, pos)?;
     Ok(Expr::BinOp(op.to_string(), Box::new(left), Box::new(right)))
 }
+
+#[cfg(test)]
+mod tests {
+    use super::parse_formula;
+    use crate::formula::{Expr, AggFunc};
+
+    #[test]
+    fn parse_simple_subtraction() {
+        let f = parse_formula("Profit = Revenue - Cost", "Measure").unwrap();
+        assert_eq!(f.target, "Profit");
+        assert_eq!(f.target_category, "Measure");
+        assert!(matches!(f.expr, Expr::BinOp(ref op, _, _) if op == "-"));
+    }
+
+    #[test]
+    fn parse_where_clause() {
+        let f = parse_formula("EastRev = Revenue WHERE Region = \"East\"", "Measure").unwrap();
+        assert_eq!(f.target, "EastRev");
+        let filter = f.filter.as_ref().unwrap();
+        assert_eq!(filter.category, "Region");
+        assert_eq!(filter.item, "East");
+    }
+
+    #[test]
+    fn parse_sum_aggregation() {
+        let f = parse_formula("Total = SUM(Revenue)", "Measure").unwrap();
+        assert!(matches!(f.expr, Expr::Agg(AggFunc::Sum, _, _)));
+    }
+
+    #[test]
+    fn parse_avg_aggregation() {
+        let f = parse_formula("Avg = AVG(Revenue)", "Measure").unwrap();
+        assert!(matches!(f.expr, Expr::Agg(AggFunc::Avg, _, _)));
+    }
+
+    #[test]
+    fn parse_if_expression() {
+        let f = parse_formula("Capped = IF(Revenue > 1000, 1000, Revenue)", "Measure").unwrap();
+        assert!(matches!(f.expr, Expr::If(_, _, _)));
+    }
+
+    #[test]
+    fn parse_numeric_literal() {
+        let f = parse_formula("Fixed = 42", "Measure").unwrap();
+        assert!(matches!(f.expr, Expr::Number(n) if (n - 42.0).abs() < 1e-10));
+    }
+
+    #[test]
+    fn parse_chained_arithmetic() {
+        parse_formula("X = (A + B) * (C - D)", "Cat").unwrap();
+    }
+
+    #[test]
+    fn parse_missing_equals_returns_error() {
+        assert!(parse_formula("BadFormula Revenue Cost", "Cat").is_err());
+    }
+}
diff --git a/src/model/category.rs b/src/model/category.rs
index 9f73f25..4b3235d 100644
--- a/src/model/category.rs
+++ b/src/model/category.rs
@@ -119,3 +119,76 @@ impl Category {
         seen
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::{Category, Group};
+
+    fn cat() -> Category {
+        Category::new(0, "Region")
+    }
+
+    #[test]
+    fn add_item_returns_sequential_ids() {
+        let mut c = cat();
+        let id0 = c.add_item("East");
+        let id1 = c.add_item("West");
+        assert_eq!(id0, 0);
+        assert_eq!(id1, 1);
+    }
+
+    #[test]
+    fn add_item_duplicate_returns_same_id() {
+        let mut c = cat();
+        let id1 = c.add_item("East");
+        let id2 = c.add_item("East");
+        assert_eq!(id1, id2);
+        assert_eq!(c.items.len(), 1);
+    }
+
+    #[test]
+    fn add_item_in_group_sets_group() {
+        let mut c = cat();
+        c.add_item_in_group("Jan", "Q1");
+        let item = c.item_by_name("Jan").unwrap();
+        assert_eq!(item.group.as_deref(), Some("Q1"));
+    }
+
+    #[test]
+    fn add_item_in_group_duplicate_returns_same_id() {
+        let mut c = cat();
+        let id1 = c.add_item_in_group("Jan", "Q1");
+        let id2 = c.add_item_in_group("Jan", "Q1");
+        assert_eq!(id1, id2);
+        assert_eq!(c.items.len(), 1);
+    }
+
+    #[test]
+    fn add_group_deduplicates() {
+        let mut c = cat();
+        c.add_group(Group::new("Q1"));
+        c.add_group(Group::new("Q1"));
+        assert_eq!(c.groups.len(), 1);
+    }
+
+    #[test]
+    fn top_level_groups_returns_unique_groups_in_insertion_order() {
+        let mut c = cat();
+        c.add_item_in_group("Jan", "Q1");
+        c.add_item_in_group("Feb", "Q1");
+        c.add_item_in_group("Apr", "Q2");
+        let groups = c.top_level_groups();
+        assert_eq!(groups, vec!["Q1", "Q2"]);
+    }
+
+    #[test]
+    fn item_index_reflects_insertion_order() {
+        let mut c = cat();
+        c.add_item("East");
+        c.add_item("West");
+        c.add_item("North");
+        assert_eq!(c.item_index("East"),  Some(0));
+        assert_eq!(c.item_index("West"),  Some(1));
+        assert_eq!(c.item_index("North"), Some(2));
+    }
+}
diff --git a/src/model/cell.rs b/src/model/cell.rs
index 917f152..d36285f 100644
--- a/src/model/cell.rs
+++ b/src/model/cell.rs
@@ -156,3 +156,177 @@ impl DataStore {
             .collect()
     }
 }
+
+#[cfg(test)]
+mod cell_key {
+    use super::{CellKey, CellValue, DataStore};
+
+    fn key(pairs: &[(&str, &str)]) -> CellKey {
+        CellKey::new(pairs.iter().map(|(c, i)| (c.to_string(), i.to_string())).collect())
+    }
+
+    #[test]
+    fn coords_are_sorted_by_category_name() {
+        let k = key(&[("Region", "East"), ("Measure", "Revenue"), ("Product", "Shirts")]);
+        assert_eq!(k.0[0].0, "Measure");
+        assert_eq!(k.0[1].0, "Product");
+        assert_eq!(k.0[2].0, "Region");
+    }
+
+    #[test]
+    fn get_returns_item_for_known_category() {
+        let k = key(&[("Region", "East"), ("Product", "Shirts")]);
+        assert_eq!(k.get("Region"), Some("East"));
+        assert_eq!(k.get("Product"), Some("Shirts"));
+    }
+
+    #[test]
+    fn get_returns_none_for_unknown_category() {
+        let k = key(&[("Region", "East")]);
+        assert_eq!(k.get("Measure"), None);
+    }
+
+    #[test]
+    fn with_adds_new_coordinate_in_sorted_order() {
+        let k = key(&[("Region", "East")]).with("Measure", "Revenue");
+        assert_eq!(k.get("Measure"), Some("Revenue"));
+        assert_eq!(k.get("Region"), Some("East"));
+        assert_eq!(k.0[0].0, "Measure");
+        assert_eq!(k.0[1].0, "Region");
+    }
+
+    #[test]
+    fn with_replaces_existing_coordinate() {
+        let k = key(&[("Region", "East"), ("Product", "Shirts")]).with("Region", "West");
+        assert_eq!(k.get("Region"), Some("West"));
+        assert_eq!(k.0.len(), 2);
+    }
+
+    #[test]
+    fn without_removes_coordinate() {
+        let k = key(&[("Region", "East"), ("Product", "Shirts")]).without("Region");
+        assert_eq!(k.get("Region"), None);
+        assert_eq!(k.get("Product"), Some("Shirts"));
+        assert_eq!(k.0.len(), 1);
+    }
+
+    #[test]
+    fn without_missing_category_is_noop() {
+        let k = key(&[("Region", "East")]).without("Measure");
+        assert_eq!(k.0.len(), 1);
+    }
+
+    #[test]
+    fn matches_partial_full_match() {
+        let k = key(&[("Region", "East"), ("Product", "Shirts")]);
+        let partial = vec![("Region".to_string(), "East".to_string())];
+        assert!(k.matches_partial(&partial));
+    }
+
+    #[test]
+    fn matches_partial_empty_matches_all() {
+        let k = key(&[("Region", "East"), ("Product", "Shirts")]);
+        assert!(k.matches_partial(&[]));
+    }
+
+    #[test]
+    fn matches_partial_wrong_item_no_match() {
+        let k = key(&[("Region", "East"), ("Product", "Shirts")]);
+        let partial = vec![("Region".to_string(), "West".to_string())];
+        assert!(!k.matches_partial(&partial));
+    }
+
+    #[test]
+    fn matches_partial_missing_category_no_match() {
+        let k = key(&[("Region", "East")]);
+        let partial = vec![("Product".to_string(), "Shirts".to_string())];
+        assert!(!k.matches_partial(&partial));
+    }
+
+    #[test]
+    fn display_format() {
+        let k = key(&[("Region", "East")]);
+        assert_eq!(k.to_string(), "{Region=East}");
+    }
+}
+
+#[cfg(test)]
+mod data_store {
+    use super::{CellKey, CellValue, DataStore};
+
+    fn key(pairs: &[(&str, &str)]) -> CellKey {
+        CellKey::new(pairs.iter().map(|(c, i)| (c.to_string(), i.to_string())).collect())
+    }
+
+    #[test]
+    fn get_missing_returns_empty() {
+        let store = DataStore::new();
+        assert_eq!(store.get(&key(&[("Region", "East")])), &CellValue::Empty);
+    }
+
+    #[test]
+    fn set_and_get_roundtrip() {
+        let mut store = DataStore::new();
+        let k = key(&[("Region", "East"), ("Product", "Shirts")]);
+        store.set(k.clone(), CellValue::Number(42.0));
+        assert_eq!(store.get(&k), &CellValue::Number(42.0));
+    }
+
+    #[test]
+    fn overwrite_value() {
+        let mut store = DataStore::new();
+        let k = key(&[("Region", "East")]);
+        store.set(k.clone(), CellValue::Number(1.0));
+        store.set(k.clone(), CellValue::Number(99.0));
+        assert_eq!(store.get(&k), &CellValue::Number(99.0));
+    }
+
+    #[test]
+    fn setting_empty_removes_key() {
+        let mut store = DataStore::new();
+        let k = key(&[("Region", "East")]);
+        store.set(k.clone(), CellValue::Number(5.0));
+        store.set(k.clone(), CellValue::Empty);
+        assert!(store.cells().is_empty());
+    }
+
+    #[test]
+    fn sum_matching_sums_across_dimension() {
+        let mut store = DataStore::new();
+        store.set(key(&[("Measure", "Revenue"), ("Region", "East")]), CellValue::Number(100.0));
+        store.set(key(&[("Measure", "Revenue"), ("Region", "West")]), CellValue::Number(200.0));
+        store.set(key(&[("Measure", "Cost"),    ("Region", "East")]), CellValue::Number(50.0));
+        let partial = vec![("Measure".to_string(), "Revenue".to_string())];
+        assert_eq!(store.sum_matching(&partial), 300.0);
+    }
+
+    #[test]
+    fn sum_matching_empty_partial_sums_everything() {
+        let mut store = DataStore::new();
+        store.set(key(&[("Region", "East")]), CellValue::Number(10.0));
+        store.set(key(&[("Region", "West")]), CellValue::Number(20.0));
+        assert_eq!(store.sum_matching(&[]), 30.0);
+    }
+
+    #[test]
+    fn matching_cells_returns_correct_subset() {
+        let mut store = DataStore::new();
+        store.set(key(&[("Measure", "Revenue"), ("Region", "East")]), CellValue::Number(100.0));
+        store.set(key(&[("Measure", "Revenue"), ("Region", "West")]), CellValue::Number(200.0));
+        store.set(key(&[("Measure", "Cost"),    ("Region", "East")]), CellValue::Number(50.0));
+        let partial = vec![("Measure".to_string(), "Revenue".to_string())];
+        let cells = store.matching_cells(&partial);
+        assert_eq!(cells.len(), 2);
+        let values: Vec<f64> = cells.iter().filter_map(|(_, v)| v.as_f64()).collect();
+        assert!(values.contains(&100.0));
+        assert!(values.contains(&200.0));
+    }
+
+    #[test]
+    fn text_values_excluded_from_sum() {
+        let mut store = DataStore::new();
+        store.set(key(&[("Cat", "A")]), CellValue::Number(10.0));
+        store.set(key(&[("Cat", "B")]), CellValue::Text("hello".into()));
+        assert_eq!(store.sum_matching(&[]), 10.0);
+    }
+}
diff --git a/src/view/view.rs b/src/view/view.rs
index ec5f6b6..c3385f2 100644
--- a/src/view/view.rs
+++ b/src/view/view.rs
@@ -125,3 +125,117 @@ impl View {
         self.col_offset = 0;
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::View;
+    use crate::view::Axis;
+
+    fn view_with_cats(cats: &[&str]) -> View {
+        let mut v = View::new("Test");
+        for &c in cats { v.on_category_added(c); }
+        v
+    }
+
+    #[test]
+    fn first_category_assigned_to_row() {
+        let v = view_with_cats(&["Region"]);
+        assert_eq!(v.axis_of("Region"), Axis::Row);
+    }
+
+    #[test]
+    fn second_category_assigned_to_column() {
+        let v = view_with_cats(&["Region", "Product"]);
+        assert_eq!(v.axis_of("Product"), Axis::Column);
+    }
+
+    #[test]
+    fn third_and_later_categories_assigned_to_page() {
+        let v = view_with_cats(&["Region", "Product", "Time", "Scenario"]);
+        assert_eq!(v.axis_of("Time"),     Axis::Page);
+        assert_eq!(v.axis_of("Scenario"), Axis::Page);
+    }
+
+    #[test]
+    fn set_axis_changes_assignment() {
+        let mut v = view_with_cats(&["Region", "Product"]);
+        v.set_axis("Region", Axis::Column);
+        assert_eq!(v.axis_of("Region"), Axis::Column);
+    }
+
+    #[test]
+    fn categories_on_returns_correct_list() {
+        let v = view_with_cats(&["Region", "Product", "Time"]);
+        assert_eq!(v.categories_on(Axis::Row),    vec!["Region"]);
+        assert_eq!(v.categories_on(Axis::Column), vec!["Product"]);
+        assert_eq!(v.categories_on(Axis::Page),   vec!["Time"]);
+    }
+
+    #[test]
+    fn axis_of_unknown_category_returns_unassigned() {
+        let v = View::new("Test");
+        assert_eq!(v.axis_of("Ghost"), Axis::Unassigned);
+    }
+
+    #[test]
+    fn page_selection_set_and_get() {
+        let mut v = view_with_cats(&["Region", "Product", "Time"]);
+        v.set_page_selection("Time", "Q1");
+        assert_eq!(v.page_selection("Time"),   Some("Q1"));
+        assert_eq!(v.page_selection("Region"), None);
+    }
+
+    #[test]
+    fn toggle_group_collapse_toggles_twice() {
+        let mut v = View::new("Test");
+        assert!(!v.is_group_collapsed("Time", "Q1"));
+        v.toggle_group_collapse("Time", "Q1");
+        assert!(v.is_group_collapsed("Time", "Q1"));
+        v.toggle_group_collapse("Time", "Q1");
+        assert!(!v.is_group_collapsed("Time", "Q1"));
+    }
+
+    #[test]
+    fn hide_and_show_item() {
+        let mut v = View::new("Test");
+        assert!(!v.is_hidden("Region", "East"));
+        v.hide_item("Region", "East");
+        assert!(v.is_hidden("Region", "East"));
+        v.show_item("Region", "East");
+        assert!(!v.is_hidden("Region", "East"));
+    }
+
+    #[test]
+    fn cycle_axis_row_to_column() {
+        let mut v = view_with_cats(&["Region", "Product"]);
+        v.cycle_axis("Region");
+        assert_eq!(v.axis_of("Region"), Axis::Column);
+    }
+
+    #[test]
+    fn cycle_axis_column_to_page() {
+        let mut v = view_with_cats(&["Region", "Product"]);
+        v.set_axis("Product", Axis::Column);
+        v.cycle_axis("Product");
+        assert_eq!(v.axis_of("Product"), Axis::Page);
+    }
+
+    #[test]
+    fn cycle_axis_page_to_row() {
+        let mut v = view_with_cats(&["Region", "Product", "Time"]);
+        v.cycle_axis("Time");
+        assert_eq!(v.axis_of("Time"), Axis::Row);
+    }
+
+    #[test]
+    fn cycle_axis_resets_scroll_and_selection() {
+        let mut v = view_with_cats(&["Region"]);
+        v.row_offset = 5;
+        v.col_offset = 3;
+        v.selected = (2, 2);
+        v.cycle_axis("Region");
+        assert_eq!(v.row_offset, 0);
+        assert_eq!(v.col_offset, 0);
+        assert_eq!(v.selected,   (0, 0));
+    }
+}