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POAlpha Beta pruning problems in Othello
 

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  1. POAlpha Beta pruning problems in Othello
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    <p>I'm creating a simple engine that plays Othello, using minimax with alpha beta cuts. It's playing well, but sometimes i get a weird index out of bounds exception (near the endgame, always).</p> <p>Here' my algorithm</p> <pre><code>private float minimax(OthelloBoard board, OthelloMove best, float alpha, float beta, int depth) { calls++; float bestResult = -Float.MAX_VALUE; OthelloMove garbage = new OthelloMove(); int state = board.getState(); int currentPlayer = board.getCurrentPlayer(); if (state == OthelloBoard.STATE_DRAW) return 0.0f; if ((state == OthelloBoard.STATE_BLACK_WINS) &amp;&amp; (currentPlayer == OthelloBoard.BLACK)) return Float.MAX_VALUE; if ((state == OthelloBoard.STATE_WHITE_WINS) &amp;&amp; (currentPlayer == OthelloBoard.WHITE)) return Float.MAX_VALUE; if ((state == OthelloBoard.STATE_BLACK_WINS) &amp;&amp; (currentPlayer == OthelloBoard.WHITE)) return -Float.MAX_VALUE; if ((state == OthelloBoard.STATE_WHITE_WINS) &amp;&amp; (currentPlayer == OthelloBoard.BLACK)) return -Float.MAX_VALUE; if (depth == maxDepth) return OthelloHeuristics.eval(currentPlayer, board); ArrayList&lt;OthelloMove&gt; moves = board.getAllMoves(currentPlayer); for (OthelloMove mv : moves) { board.makeMove(mv); alpha = - minimax(board, garbage, -beta, -alpha, depth + 1); board.undoMove(mv); if (beta &lt;= alpha) return alpha; if (alpha &gt; bestResult) { best.setFlipSquares(mv.getFlipSquares()); best.setIdx(mv.getIdx()); best.setPlayer(mv.getPlayer()); bestResult = alpha; } } return bestResult; } </code></pre> <p>Inside makeMove and undoMove i update the game state(black wins, white wins, draw). I also toggle the players inside these methods. When a player has no moves i make a dummy move without changing the board, and toggle the players.</p> <p>There's a lot more of code, but i think the problem happens when the algorithm hits the game over position. This problem doesn't happen when i set the engine to play random moves, so the problem should be the alpha beta algorithm.</p> <p>Here is getAllMoves, this call getFlips:</p> <pre><code> public ArrayList&lt;OthelloMove&gt; getAllMoves(int player) { ArrayList&lt;OthelloMove&gt; moves = new ArrayList&lt;OthelloMove&gt;(); for (int i = 10; i &lt; 90; i++) { int col = i % 10; if (col != 0 &amp;&amp; col != 9) { if (cells[i] == EMPTY) { ArrayList&lt;Integer&gt; flips = getFlips(i, player); if (flips.size() &gt; 0) { OthelloMove mv = new OthelloMove(); mv.setFlipSquares(flips); mv.setIdx(i); mv.setPlayer(player); moves.add(mv); } } } } return moves; } </code></pre> <p>Here is getFlips.</p> <pre><code> public ArrayList&lt;Integer&gt; getFlips(int idx, int player) { int opponent = getOpponent(player); ArrayList&lt;Integer&gt; flips = new ArrayList&lt;Integer&gt;(); if (cells[idx] != EMPTY) return flips; for (Integer dir : DIRECTIONS) { int distance = 1; int tempIdx = idx; while (cells[tempIdx += dir] == opponent) distance++; if ((cells[tempIdx] == player) &amp;&amp; (distance &gt; 1)) { while (distance-- &gt; 1) { tempIdx -= dir; flips.add(tempIdx); } } } return flips; } </code></pre> <p>Here is updateState:</p> <pre><code> public void updateState() { int opponent = getOpponent(currentPlayer); int playerMoves = getAllMoves(currentPlayer).size(); int opponentMoves = getAllMoves(opponent).size(); if ( ((playerMoves == 0) &amp;&amp; (opponentMoves == 0)) || (emptyCells == 0)) { int blackDiscs = countDiscs(BLACK); int whiteDiscs = countDiscs(WHITE); if (blackDiscs &gt; whiteDiscs) state = STATE_BLACK_WINS; else if (blackDiscs &lt; whiteDiscs) state = STATE_WHITE_WINS; else state = STATE_DRAW; } } </code></pre> <p>Thanks!</p>
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    1. PTQuestion
    1. USFernando
    1. USFernando
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    1. PO
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    1. VO
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      1. POAlpha Beta pruning problems in Othello
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      1. VTUpMod
    2. VO
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      1. VTUpMod
    1. COwhat is your stack trace?
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      1. USamit
    2. COWo the guys are fast here, thanks. Exception in thread "Thread-3" java.lang.ArrayIndexOutOfBoundsException: 100 at OthelloBoard.getFlips(OthelloBoard.java:119) at OthelloBoard.getAllMoves(OthelloBoard.java:85) at MinimaxOthello.minimax(MinimaxOthello.java:53) at MinimaxOthello.doMove(MinimaxOthello.java:23) at OthelloPanel.doLogic(OthelloPanel.java:118) at OthelloPanel.run(OthelloPanel.java:162) at java.lang.Thread.run(Thread.java:680)
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      1. POAlpha Beta pruning problems in Othello
      1. USFernando
    3. COAs you can see, it is in `getFlips()`. Please add the relevant code of this method. `getAllMoves()` might also be needed.
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      1. POAlpha Beta pruning problems in Othello
      1. USamit
 

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