perf(data): support exact reservation for known numeric field additions

crates/fidc-core/src/numeric_factors.rs

@@ -38,6 +38,11 @@ impl NumericFactorMap {
         self.entries.clear();
     }
 
+    /// Reserve known new fields without geometric spare capacity per snapshot.
+    pub fn reserve_exact(&mut self, additional: usize) {
+        self.entries.reserve_exact(additional);
+    }
+
     pub fn get(&self, key: &str) -> Option<&f64> {
         self.entries
             .binary_search_by(|(name, _)| name.as_str().cmp(key))
@@ -253,6 +258,28 @@ impl<'de> Deserialize<'de> for NumericFactorMap {
 mod tests {
     use super::*;
 
+    #[test]
+    fn exact_reservation_preserves_values_and_avoids_growth_during_known_inserts() {
+        let mut map = NumericFactorMap::from([
+            (Cow::Borrowed("amount"), 125.25),
+            (Cow::Borrowed("nullable_value"), f64::from_bits(0x7ff8000000000042)),
+            (Cow::Borrowed("signal"), -0.0),
+        ]);
+        let original = map.iter().map(|(key, value)| (key.to_string(), value.to_bits())).collect::<Vec<_>>();
+        map.reserve_exact(2);
+        assert_eq!(map.iter().map(|(key, value)| (key.to_string(), value.to_bits())).collect::<Vec<_>>(), original);
+        let buffer = map.entries.as_ptr();
+        map.insert(Cow::Borrowed("pre_close"), 12.5);
+        map.insert(Cow::Borrowed("no_limit"), 0.0);
+        assert_eq!(map.entries.as_ptr(), buffer);
+        assert_eq!(map.len(), 5);
+        assert_eq!(map["signal"].to_bits(), (-0.0_f64).to_bits());
+        assert_eq!(map["nullable_value"].to_bits(), 0x7ff8000000000042);
+        let before = map.entries.as_ptr();
+        map.reserve_exact(0);
+        assert_eq!(map.entries.as_ptr(), before);
+    }
+
     #[test]
     fn compact_keys_inline_dynamic_names_and_keep_long_static_storage() {
         const LONG: &str = "a_long_static_factor_identifier_that_must_remain_borrowed";