From d2fc285af58c40b0a5cdd2fa02175d838f63b37a Mon Sep 17 00:00:00 2001
From: Eryn Wells <eryn@erynwells.me>
Date: Sat, 7 Jun 2025 08:55:34 -0700
Subject: [PATCH] [position] Track seen board positions in a HashSet

As moves are made, keep track of the hashes of those board position. When moves
are unmade, pop seen hashes from the HashSet.

Reformat the imports in position.rs.
---
 position/src/position.rs | 35 +++++++++++++++++++++++++----------
 1 file changed, 25 insertions(+), 10 deletions(-)

diff --git a/position/src/position.rs b/position/src/position.rs
index e674b81..17f36eb 100644
--- a/position/src/position.rs
+++ b/position/src/position.rs
@@ -2,15 +2,6 @@
 
 mod captures;
 
-use chessfriend_moves::{
-    generators::{
-        AllPiecesMoveGenerator, BishopMoveGenerator, KingMoveGenerator, KnightMoveGenerator,
-        PawnMoveGenerator, QueenMoveGenerator, RookMoveGenerator,
-    },
-    GeneratedMove, MakeMove, MakeMoveError, Move, MoveRecord, UnmakeMove, UnmakeMoveError,
-    UnmakeMoveResult, ValidateMove,
-};
-
 use captures::CapturesList;
 use chessfriend_bitboard::BitBoard;
 use chessfriend_board::{
@@ -18,7 +9,15 @@ use chessfriend_board::{
     ZobristState,
 };
 use chessfriend_core::{Color, Piece, Shape, Square};
-use std::{fmt, sync::Arc};
+use chessfriend_moves::{
+    generators::{
+        AllPiecesMoveGenerator, BishopMoveGenerator, KingMoveGenerator, KnightMoveGenerator,
+        PawnMoveGenerator, QueenMoveGenerator, RookMoveGenerator,
+    },
+    GeneratedMove, MakeMove, MakeMoveError, Move, MoveRecord, UnmakeMove, UnmakeMoveError,
+    UnmakeMoveResult, ValidateMove,
+};
+use std::{collections::HashSet, fmt, sync::Arc};
 
 #[must_use]
 #[derive(Clone, Debug, Default, Eq)]
@@ -26,6 +25,9 @@ pub struct Position {
     pub board: Board,
     pub(crate) moves: Vec<MoveRecord>,
     pub(crate) captures: CapturesList,
+
+    /// A set of hashes of board positions seen throughout the move record.
+    boards_seen: HashSet<u64>,
 }
 
 impl Position {
@@ -172,6 +174,13 @@ impl Position {
             self.captures.push(record.color, captured_piece);
         }
 
+        if let Some(hash) = self.board.zobrist_hash() {
+            // TODO: If the hash already exists here, it's a duplicate position
+            // and this move results in a draw. Find a way to indicate that,
+            // in either Board or Position.
+            self.boards_seen.insert(hash);
+        }
+
         self.moves.push(record.clone());
 
         Ok(())
@@ -187,6 +196,8 @@ impl Position {
     pub fn unmake_last_move(&mut self) -> UnmakeMoveResult {
         let last_move_record = self.moves.pop().ok_or(UnmakeMoveError::NoMove)?;
 
+        let hash_before_unmake = self.board.zobrist_hash();
+
         let unmake_result = self.board.unmake_move(&last_move_record);
 
         if unmake_result.is_ok() {
@@ -194,6 +205,10 @@ impl Position {
                 let popped_piece = self.captures.pop(last_move_record.color);
                 debug_assert_eq!(Some(capture), popped_piece);
             }
+
+            if let Some(hash_before_unmake) = hash_before_unmake {
+                self.boards_seen.remove(&hash_before_unmake);
+            }
         } else {
             self.moves.push(last_move_record);
         }