perf: avoid stable universe sort allocation

crates/fidc-core/src/platform_expr_strategy.rs

@@ -9214,6 +9214,17 @@ impl PlatformExprStrategy {
         .0
     }
 
+    #[inline]
+    fn eligible_universe_market_cap_order(
+        left: &EligibleUniverseSnapshot,
+        right: &EligibleUniverseSnapshot,
+    ) -> std::cmp::Ordering {
+        left.market_cap_bn
+            .partial_cmp(&right.market_cap_bn)
+            .unwrap_or(std::cmp::Ordering::Equal)
+            .then_with(|| left.symbol.cmp(&right.symbol))
+    }
+
     fn selection_universe_and_risk_decisions_with_options(
         &self,
         ctx: &StrategyContext<'_>,
@@ -9296,12 +9307,7 @@ impl PlatformExprStrategy {
                 free_float_cap_bn,
             });
         }
-        rows.sort_by(|left, right| {
-            left.market_cap_bn
-                .partial_cmp(&right.market_cap_bn)
-                .unwrap_or(std::cmp::Ordering::Equal)
-                .then_with(|| left.symbol.cmp(&right.symbol))
-        });
+        rows.sort_unstable_by(Self::eligible_universe_market_cap_order);
         (rows, decisions)
     }
 
@@ -14256,6 +14262,36 @@ mod tests {
         assert!(!PlatformExprStrategy::new(cfg).rank_reuses_market_cap_order());
     }
 
+    #[test]
+    fn eligible_universe_unstable_sort_preserves_market_cap_symbol_total_order() {
+        let mut rows = vec![
+            EligibleUniverseSnapshot {
+                symbol: "000003.SZ".to_string(),
+                market_cap_bn: 2.0,
+                free_float_cap_bn: 1.0,
+            },
+            EligibleUniverseSnapshot {
+                symbol: "000002.SZ".to_string(),
+                market_cap_bn: 1.0,
+                free_float_cap_bn: 1.0,
+            },
+            EligibleUniverseSnapshot {
+                symbol: "000001.SZ".to_string(),
+                market_cap_bn: 1.0,
+                free_float_cap_bn: 1.0,
+            },
+        ];
+
+        rows.sort_unstable_by(PlatformExprStrategy::eligible_universe_market_cap_order);
+
+        assert_eq!(
+            rows.iter()
+                .map(|row| (row.market_cap_bn, row.symbol.as_str()))
+                .collect::<Vec<_>>(),
+            vec![(1.0, "000001.SZ"), (1.0, "000002.SZ"), (2.0, "000003.SZ"),]
+        );
+    }
+
     #[test]
     fn current_rolling_helpers_do_not_load_factor_maps_or_decision_rollings() {
         let mut cfg = PlatformExprStrategyConfig::microcap_rotation();