Question

I am working on a homework project in which we need to generate a state tree for the game of Golf Solitaire. To do this I decided to create a Move class which represents a move from one stack of cards to another – simply put, it holds a reference to the two stacks involved in a single move. Each node in my tree knows what it’s move is.

The problem I am currently having is that my trees are much too large – like OutOfMemoryError too large. Edit – I should note that when I make a tree for a very small game I do not get an error, but my tree is still much larger than it should be!

For those unfamiliar with the game: Golf Solitaire Rules. Note – We did not need to implement the restriction on playing a Queen after a King.

public void makeTree()
{
  _root = makeNode( null, null, 0 );
}

private Node makeNode( Node parent, Move m, int depth )
{
  Node node = new Node( parent, m, depth );
  ArrayList<Move> moves = findAllMoves();
  if( moves.size() == 0 )
    node.setScore( getScore() );
  else {
    for( Move mv : moves ) {
      mv.makeMove();
      Node child = makeNode( node, mv, depth++ );
      node.addChild( child );
      mv.undoMove();
    }
  }
  return node;
}

private ArrayList<Move> findAllMoves()
{
  ArrayList<Move> moves = new ArrayList<Move>();

  for( int i = 0; i < numPlayPiles; i++ ) {
    if( _play[i].size() != 0 ) {
       if( Math.abs( _play[i].top().getRank().ordinal() - 
          discard.getRank().ordinal() ) == 1 ) {
          moves.add( new Move( _play[i], _discard ) );
       }
    }
  }

  if( _draw.size() != 0 )
    moves.add( new Move( _draw, _discard ) );

  return moves;
}

_play[i] is simply referencing a pile of cards in the game.

I realize this is a bit much of code to be posting in a question, but my question is simply about my logic. I have a suspicion that my error is in how I am returning makeNode yet I am totally stuck here. Could anybody confirm my suspicion and/or possibly give me some guidance?

Answer 1

For anybody who is curious, my issue was one of not reading the assignment properly. Specifically, my findAllMoves method was the issue. A node should have either a move from the draw pile or all possible moves from play piles, but not both. My code above allows each child, even at the same depth, to each create a Move from the draw pile to discard pile – resulting in a huge amount of duplicate nodes.

It also never checks the end-game conditions. As a result, each leaf node was either a losing score, or 0 (the method would recurse until the draw pile was empty even if the player already won).