修正部分卖出持仓成本

crates/fidc-core/src/portfolio.rs

@@ -117,6 +117,7 @@ impl Position {
         let mut remaining = quantity;
         let mut realized = 0.0;
         let mut realized_entry = 0.0;
+        let average_cost_before_sell = self.average_cost;
 
         while remaining > 0 {
             let Some(first_lot) = self.lots.first_mut() else {
@@ -141,7 +142,13 @@ impl Position {
         self.day_trade_quantity_delta -= quantity as i32;
         self.day_sell_quantity += quantity;
         self.day_sell_value += execution_price * quantity as f64;
-        self.recalculate_average_cost();
+        if self.quantity == 0 {
+            self.recalculate_average_cost();
+        } else if average_cost_before_sell.is_finite() && average_cost_before_sell > 0.0 {
+            self.average_cost = average_cost_before_sell;
+        } else {
+            self.recalculate_average_cost();
+        }
         self.refresh_day_pnl();
         Ok(realized)
     }
@@ -955,6 +962,26 @@ mod tests {
         assert!((position.holding_return(6.06).unwrap() - (6.06 / 5.66 - 1.0)).abs() < 1e-12);
     }
 
+    #[test]
+    fn partial_sell_preserves_remaining_average_cost() {
+        let date = NaiveDate::from_ymd_opt(2025, 1, 2).unwrap();
+        let mut position = Position::new("603958.SH");
+        position.buy(date, 800, 18.0981);
+        position.record_buy_trade_cost(800, 5.0);
+        position.buy(date, 1700, 19.4694);
+        position.record_buy_trade_cost(1700, 8.27451625);
+        position.buy(date, 200, 18.4584);
+        position.record_buy_trade_cost(200, 5.0);
+        position.buy(date, 100, 17.8378);
+        position.record_buy_trade_cost(100, 5.0);
+        let average_cost_before = position.average_cost;
+
+        position.sell(2700, 16.8331).expect("partial sell");
+
+        assert_eq!(position.quantity, 100);
+        assert!((position.average_cost - average_cost_before).abs() < 1e-12);
+    }
+
     #[test]
     fn holdings_summary_reports_entry_price_pnl_excluding_buy_commission() {
         let date = NaiveDate::from_ymd_opt(2025, 1, 2).unwrap();