From c1008ef672bff5ca9e9392763cfc67d4fb3125a0 Mon Sep 17 00:00:00 2001
From: Eryn Wells <eryn@erynwells.me>
Date: Mon, 22 Jan 2024 08:12:28 -0800
Subject: [PATCH] [board] Put a cached Moves object into a OnceCell on Position

Cache move generation so we don't have to remake it every time.
---
 board/src/position/position.rs | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/board/src/position/position.rs b/board/src/position/position.rs
index abdfbe1..484293e 100644
--- a/board/src/position/position.rs
+++ b/board/src/position/position.rs
@@ -18,6 +18,7 @@ pub struct Position {
     pieces: PieceBitBoards,
     en_passant_square: Option<Square>,
     sight: [OnceCell<BitBoard>; 2],
+    moves: OnceCell<Moves>,
 
     half_move_counter: u16,
     full_move_number: u16,
@@ -31,6 +32,7 @@ impl Position {
             pieces: PieceBitBoards::default(),
             en_passant_square: None,
             sight: [OnceCell::new(), OnceCell::new()],
+            moves: OnceCell::new(),
             half_move_counter: 0,
             full_move_number: 1,
         }
@@ -62,6 +64,7 @@ impl Position {
             pieces: PieceBitBoards::new([white_pieces, black_pieces]),
             en_passant_square: None,
             sight: [OnceCell::new(), OnceCell::new()],
+            moves: OnceCell::new(),
             half_move_counter: 0,
             full_move_number: 1,
         }
@@ -121,8 +124,9 @@ impl Position {
         self.piece_on_square(square)
     }
 
-    pub fn moves(&self) -> Moves {
-        Moves::new(self, self.color_to_move)
+    pub fn moves(&self) -> &Moves {
+        self.moves
+            .get_or_init(|| Moves::new(self, self.color_to_move))
     }
 
     /// Return a BitBoard representing the set of squares containing a piece.
@@ -215,6 +219,7 @@ impl Position {
             en_passant_square,
             pieces,
             sight: [OnceCell::new(), OnceCell::new()],
+            moves: OnceCell::new(),
             half_move_counter: 0,
             full_move_number: 1,
         }