test(engine): retain static schema names across shared factor rows

crates/fidc-core/src/numeric_factors.rs

@@ -436,6 +436,19 @@ mod tests {
         assert!(rows.iter().all(|row| matches!(row.storage, Storage::Owned(_))));
     }
 
+    #[test]
+    fn shared_schema_borrows_a_static_key_even_after_a_dynamic_equal_key() {
+        const KEY: &str = "a_shared_factor_name_longer_than_inline_storage";
+        let mut rows = (0..64).map(|index| NumericFactorMap::from([
+            (if index == 0 { Cow::Owned(KEY.to_string()) } else { Cow::Borrowed(KEY) }, index as f64),
+            (Cow::Borrowed("another"), 2.0),
+        ])).collect::<Vec<_>>();
+        assert_eq!(NumericFactorMap::share_rows(rows.iter_mut()), rows.len());
+        for row in &rows {
+            assert_eq!(row.keys().find(|key| key.as_str() == KEY).unwrap().as_static_str(), Some(KEY));
+        }
+    }
+
     #[test]
     fn compact_keys_inline_dynamic_names_and_keep_long_static_storage() {
         const LONG: &str = "a_long_static_factor_identifier_that_must_remain_borrowed";

crates/fidc-core/src/numeric_factors/shared_rows.rs

@@ -1,4 +1,4 @@
-use std::collections::BTreeSet;
+use std::collections::BTreeMap;
 use std::sync::Arc;
 
 use compact_str::CompactString;
@@ -103,8 +103,15 @@ pub(super) fn share<'a>(rows: impl IntoIterator<Item = &'a mut NumericFactorMap>
         let Storage::Owned(entries) = &row.storage else { return None };
         sum.checked_add(entries.capacity().checked_mul(std::mem::size_of::<(CompactString, f64)>())?)
     }) else { return 0 };
-    let fields = rows.iter().flat_map(|row| row.keys()).collect::<BTreeSet<_>>()
+    let mut names = BTreeMap::<&str, &CompactString>::new();
-        .into_iter().cloned().collect::<Vec<_>>();
+    for name in rows.iter().flat_map(|row| row.keys()) {
+        names.entry(name.as_str()).and_modify(|existing| {
+            if existing.as_static_str().is_none() && name.as_static_str().is_some() {
+                *existing = name;
+            }
+        }).or_insert(name);
+    }
+    let fields = names.into_values().cloned().collect::<Vec<_>>();
     let Some(cells) = rows.len().checked_mul(fields.len()) else { return 0 };
     if cells == 0 {
         return 0;