OpenChess

CDaley11 wrote:So I've discovered a major bug in my transposition table. Right now I'm only considering nodes that returned an exact value, not nodes that produced cutoffs. Here's the steps I'm taking for it:

1. In the search method, before looping through each move, see if there's an entry in the table for the current hash key. If there is, simply return the value stored in the table.

2. After looping through each move, save the current hash key along with the determined value in the transposition table. Only save the hash key and value if the value is an exact value for the position.

I am under the impression that a node value is exact if and only if: A move was found that beat the current alpha value, and no move was found that exceeded the beta value.

However, my engine is not working with this. It allows its queen to be taken by a pawn. It doesn't do this if I take out the transposition table.

EDIT: I should be more clear. I get it to a point in the game where I move one of my pawns up, threatening my engine's queen. It makes a mindless rook move and allows me to capture the queen. If, however, I clear the Ttable before I make my pawn move, it doesn't allow it's queen to be taken. It makes a perfectly acceptable retreat of its queen.

CDaley11 wrote: 1. In the search method, before looping through each move, see if there's an entry in the table for the current hash key. If there is, simply return the value stored in the table.

CDaley11 wrote: 2. After looping through each move, save the current hash key along with the determined value in the transposition table. Only save the hash key and value if the value is an exact value for the position.

CDaley11 wrote: I am under the impression that a node value is exact if and only if: A move was found that beat the current alpha value, and no move was found that exceeded the beta value.
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Yes, that is correct. In other words, when the best score verifies old_alpha < best_score < beta, then it is an exact score. If you use PVS, this can only happen at PV nodes: at all or cut nodes, all you have are upper and lower bound scores.

CDaley11 wrote: However, my engine is not working with this. It allows its queen to be taken by a pawn. It doesn't do this if I take out the transposition table.

EDIT: I should be more clear. I get it to a point in the game where I move one of my pawns up, threatening my engine's queen. It makes a mindless rook move and allows me to capture the queen. If, however, I clear the Ttable before I make my pawn move, it doesn't allow it's queen to be taken. It makes a perfectly acceptable retreat of its queen

CDaley11 wrote:First of all, sorry for not stating this but I do use also save the depth that the position was searched to. Before I go any further here's the psudo code I'm using right now:

Code: Select all

//Before looping through each move
int index = currentHash % HASH_TABLE_SIZE;
if ( savedPositions[index] == currentHash && savedDepths[index] >= currentDepth ) {
     //savedPositions being the saved hash-keys, and savedDepths being the depth the positions were searched to
     if ( savedFlags[index] == HASH_EXACT) {
          //saved flags being what kind of node it was, exact, upper bound, etc. Right now I'm only using exact scores
          return savedValues[index];
    }
}

// Loop through each move. Through the course of the move loop if the value gets larger than beta it immediately cuts off
//...
// After the move loop. It will only get to this point if it hasn't exceeded the beta

if (value >= alpha) { // Alpha was raised
     savedPositions[index] = currentHash;
     savedDepths[index] = currentDepth;
     savedFlags[index] = HASH_EXACT;
     savedValues[index] = value;
}

return value;

I think I understand what you are saying, right now I do not store the best moves for positions but I hope to do that in the future. You say not to use exact scores for pruning. Why not? If we have the exact score for a node, then what is the point in searching it, assuming that the exact score we have is a score from an acceptable search depth.

if (value >= alpha) { // Alpha was raised

TTEntry tte;
tte.type = score <= old_alpha ? HASH_UBOUND : (score >= beta ? HASH_LBOUND : HASH_EXACT);
tte.move = best_move;
tte.score = best_score;
tte->depth = depth;
// Now save tte into the transposition table