[gnome-games] chess: Detect draw by three fold repetition

[Robert Ancell <rancell src gnome org>]

commit 5f67ffe1d690b054083c3ad78fbff0da95ac344e
Author: Robert Ancell <robert ancell canonical com>
Date:   Tue Jan 25 20:44:16 2011 +1000

    chess: Detect draw by three fold repetition

 glchess/src/chess-game.vala      |   52 ++++++++++++++++++++++++++++++++++++-
 glchess/src/test-chess-game.vala |    3 ++
 2 files changed, 53 insertions(+), 2 deletions(-)
---
diff --git a/glchess/src/chess-game.vala b/glchess/src/chess-game.vala
index 340471b..d5ef9c0 100644
--- a/glchess/src/chess-game.vala
+++ b/glchess/src/chess-game.vala
@@ -371,6 +371,36 @@ public class ChessState
         return state;
     }
 
+    public bool equals (ChessState state)
+    {
+        /* Check first if there is the same layout of pieces (unlikely),
+         * then the move castling and en-passant state are the same,
+         * then finally that it is the same move */
+        if (piece_masks[Color.WHITE] != state.piece_masks[Color.WHITE] ||
+            piece_masks[Color.BLACK] != state.piece_masks[Color.BLACK] || 
+            can_castle_kingside[Color.WHITE] != state.can_castle_kingside[Color.WHITE] ||
+            can_castle_queenside[Color.WHITE] != state.can_castle_queenside[Color.WHITE] ||
+            can_castle_kingside[Color.BLACK] != state.can_castle_kingside[Color.BLACK] ||
+            can_castle_queenside[Color.BLACK] != state.can_castle_queenside[Color.BLACK] ||
+            en_passant_index != state.en_passant_index ||
+            (last_move != null) != (state.last_move != null) ||
+            last_move.piece.type != state.last_move.piece.type ||
+            last_move.r0 != state.last_move.r0 ||
+            last_move.f0 != state.last_move.f0 ||
+            last_move.r1 != state.last_move.r1 ||
+            last_move.f1 != state.last_move.f1)
+            return false;
+
+        /* Finally check the same piece types are present */
+        for (int i = 0; i < 64; i++)
+        {
+            if (board[i] != null && board[i].type != state.board[i].type)
+                return false;
+        }
+
+        return true;
+    }
+
     public string get_fen ()
     {
         var value = new StringBuilder ();
@@ -1219,8 +1249,8 @@ public class ChessGame
 
         if (move_stack.data.halfmove_clock >= 50)
             stop (ChessResult.DRAW, ChessRule.FIFTY_MOVES);
-        //else if () // FIXME: Check for three-fold-repetition
-        //    ;
+        else if (is_three_fold_repeat ())
+            stop (ChessResult.DRAW, ChessRule.THREE_FOLD_REPETITION);
         else
             return false;
 
@@ -1285,4 +1315,22 @@ public class ChessGame
         is_started = false;
         ended ();
     }
+
+    private bool is_three_fold_repeat ()
+    {
+        var count = 1;
+
+        var current_state = move_stack.data;
+        foreach (var state in move_stack.next)
+        {
+            if (current_state.equals (state))
+            {
+                count++;
+                if (count >= 3)
+                    return true;
+            }
+        }
+
+        return false;
+    }
 }
diff --git a/glchess/src/test-chess-game.vala b/glchess/src/test-chess-game.vala
index f0cfedc..1071531 100644
--- a/glchess/src/test-chess-game.vala
+++ b/glchess/src/test-chess-game.vala
@@ -128,6 +128,9 @@ class GlChess
         /* Need 50 moves for 50 move rule */
         //test_bad_move ("p7/8/8/8/8/8/8/P7 w - - 49 1", "draw");
 
+        /* Three fold repetition */
+        // FIXME: Need a test for three fold repetition
+
         stdout.printf ("%d/%d tests successful\n", test_count - failure_count, test_count);
 
         return failure_count;