From cae93cb090349f7c6232ce96d04c78d3c68cbe2a Mon Sep 17 00:00:00 2001
From: Eryn Wells <eryn@erynwells.me>
Date: Mon, 2 Jun 2025 15:54:00 -0700
Subject: [PATCH] [board, position] Return replaced piece when placing a piece
 with PlacePieceStrategy::Replace

When place_piece() is called with PlacePieceStrategy::Replace, return the replaced
piece in the Ok() variant of the Result as an Option<Piece>.

Plumb this new return type through Board and Position.
---
 board/src/board.rs       | 2 +-
 board/src/piece_sets.rs  | 8 +++++---
 position/src/position.rs | 2 +-
 3 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/board/src/board.rs b/board/src/board.rs
index 3e866e0..63c0ad5 100644
--- a/board/src/board.rs
+++ b/board/src/board.rs
@@ -80,7 +80,7 @@ impl Board {
         piece: Piece,
         square: Square,
         strategy: PlacePieceStrategy,
-    ) -> Result<(), PlacePieceError> {
+    ) -> Result<Option<Piece>, PlacePieceError> {
         self.pieces.place(piece, square, strategy)
     }
 
diff --git a/board/src/piece_sets.rs b/board/src/piece_sets.rs
index b6de5c8..bb6d712 100644
--- a/board/src/piece_sets.rs
+++ b/board/src/piece_sets.rs
@@ -106,9 +106,11 @@ impl PieceSet {
         piece: Piece,
         square: Square,
         strategy: PlacePieceStrategy,
-    ) -> Result<(), PlacePieceError> {
+    ) -> Result<Option<Piece>, PlacePieceError> {
+        let existing_piece = self.mailbox.get(square);
+
         if strategy == PlacePieceStrategy::PreserveExisting {
-            if let Some(piece) = self.mailbox.get(square) {
+            if let Some(piece) = existing_piece {
                 return Err(PlacePieceError::ExisitingPiece { piece, square });
             }
         }
@@ -120,7 +122,7 @@ impl PieceSet {
         self.shape_occupancy[shape as usize].set(square);
         self.mailbox.set(piece, square);
 
-        Ok(())
+        Ok(existing_piece)
     }
 
     pub(crate) fn remove(&mut self, square: Square) -> Option<Piece> {
diff --git a/position/src/position.rs b/position/src/position.rs
index 4839095..8b08bce 100644
--- a/position/src/position.rs
+++ b/position/src/position.rs
@@ -59,7 +59,7 @@ impl Position {
         piece: Piece,
         square: Square,
         strategy: PlacePieceStrategy,
-    ) -> Result<(), PlacePieceError> {
+    ) -> Result<Option<Piece>, PlacePieceError> {
         self.board.place_piece(piece, square, strategy)
     }