I got the following problem. I have implemented transposition tables, which seem to work well or at least I can not see they are not working.
Additionally, I wanted to implement some move-ordering.
Basically, I search the move first that's saved in my transposition table. For some reason, that doesn't improve the search at all. Either I haven't implemented move-ordering properly or my implementation of the alpha-beta algorithm along with transposition tables is just wrong.
To measure the performance I counted the number of nodes visited during search. My implementation of move-ordering didn't have any effect at all.
Here is the Java-Code I have come up with:
public int alphabeta(final CheckerBoard board,int alpha,int beta,int depth,boolean nullMoveAvailable){
nodeCount++;
int alphaOrig=alpha;
Transposition entry =table.find(board.key);
if(entry!=null && entry.depth>=depth){
if(entry.flag==Transposition.EXACT){
return entry.value;
}else if(entry.flag==Transposition.UPPER){
beta = Math.min(beta,entry.value);
}else if(entry.flag == Transposition.LOWER){
alpha =Math.max(alpha,entry.value);
}
if(alpha>=beta){
return entry.value;
}
}
if(depth==0 || board.isTerminalState()){
return quiesceneSearch2(board,alpha,beta);
}
ArrayList<Move>sucessors =MGenerator.getMoves(board);
Move currentBest =null;
for( int i=0;i<sucessors.size();i++){
if(entry!=null && entry.depth<depth && (entry.flag == Transposition.UPPER || entry.flag == Transposition.EXACT) && sucessors.get(i).equals(entry.best)){
Collections.swap(sucessors,0,i);
break;
}
}
int bestValue =Integer.MIN_VALUE;
for(Move move : sucessors){
board.makeMove(move);
int value =-alphabeta(board, -beta, -alpha, depth - 1, true);
board.undoMove(move);
if(value>bestValue){
bestValue =value;
currentBest = move;
}
alpha =Math.max(alpha,value);
if(alpha>=beta){
break;
}
}
Transposition next =new Transposition();
next.depth=depth;
next.value =bestValue;
next.zobrisKey=board.key;
next.best = currentBest;
if(bestValue<=alphaOrig){
next.flag =Transposition.UPPER;
}else if(bestValue>=beta){
next.flag = Transposition.LOWER ;
}else{
next.flag = Transposition.EXACT;
}
table.insert(next);
return alpha;
}
and the following code to start the search:
public int findBestMove(int depth){
if (table != null) {
table.clear();
}
ArrayList<Move> sucessors =MGenerator.getMoves(board);
int max = Integer.MIN_VALUE;
for(Move m : sucessors){
board.makeMove(m);
int value =-alphabeta(board, -1000000, 1000000, depth, true);
board.undoMove(m);
if(value>max){
max=value;
bestMove=m;
}
}
board.makeMove(bestMove);
return max;
}
Would appreciate it if someone looked at my code. Maybe there isn't anything wrong with this part of my code but I didn't want to post everything since it's already quite a lot of code to look at.