/*
* Main function for getting all of the moves currently possible for a player.
*
* @params: (player) - the player whose moves are to be put in the list
* @return: a list of all moves currently possible for the player, or NULL if any allocation errors occurred
*/
struct LinkedList* Board_getPossibleMoves(char** board, int player){
struct LinkedList* possibleJumpMoves = getPossibleJumps(board, player);
if (possibleJumpMoves == NULL){ // allocation failed
return NULL;
}
if (LinkedList_length(possibleJumpMoves) != 0){ /* if jumps are possible, they are the only type of move legally possible */
struct LinkedList* trimmedJumpMoves = trimJumpMovesList(possibleJumpMoves);
if(trimmedJumpMoves == NULL){ // allocation failed
LinkedList_free(possibleJumpMoves);
return NULL;
}
return trimmedJumpMoves;
}
LinkedList_free(possibleJumpMoves);
struct LinkedList* singleMoves = getPossibleSingleMoves(board, player);
if (singleMoves == NULL){ // allocation failed
return NULL;
}
return singleMoves;
}
unsigned int RenderBatch_getDrawableCount(RenderBatch* batch)
{
return LinkedList_length(batch->drawables);
}
/*
* The minimax AI algorithm.
*/
int alphabeta(PossibleMove* possibleMove, int depth, int player, int alpha, int beta){
int (*evaluationFunction)(Board*, int, int) = (maxRecursionDepth == BEST)?
&Board_getBetterScore:
&Board_getScore;
int thisBoardScore = evaluationFunction(possibleMove->board, turn, player);
// maximum depth reached or game is over or allocation error occurred in Board_getScore
if (depth == 1 || thisBoardScore == 10000 || thisBoardScore == -10000 || thisBoardScore == -10001){
return thisBoardScore;
}
Board* board = possibleMove->board;
LinkedList* possibleMoves = Board_getPossibleMoves(board, player);
if (!possibleMoves){
return -10001;
}
//terminal node
if (LinkedList_length(possibleMoves) == 0){
LinkedList_free(possibleMoves);
return thisBoardScore;
}
//single child node
if (LinkedList_length(possibleMoves) == 1){
PossibleMove* onlyMove = PossibleMoveList_first(possibleMoves);
int score = evaluationFunction(onlyMove->board, turn, player);
LinkedList_free(possibleMoves);
return score;
}
int extremum = (player == turn)? INT_MIN : INT_MAX;
Iterator iterator;
Iterator_init(&iterator, possibleMoves);
while (Iterator_hasNext(&iterator)) {
PossibleMove* currentPossibleMove = (PossibleMove*)Iterator_next(&iterator);
int score = alphabeta(currentPossibleMove, depth-1, !player, alpha, beta);
if (score == -10001){ //allocation error occured
extremum = score;
break;
}
if ( (player != turn && score < extremum) ||
(player == turn && score > extremum) ||
(rand()%2 && score == extremum)
){
extremum = score;
}
//game over - no need to evaluate further moves
// if (extremum == 10000 || extremum == -10000){
// break;
// }
//alpha-beta pruning
if (turn == player){
alpha = (score > alpha)? score: alpha;
if (alpha >= beta){
break;
}
}
else{
beta = (score < beta)? score: beta;
if (beta <= alpha){
break;
}
}
}
LinkedList_free(possibleMoves);
return extremum;
}