Value Connect4::minimax(const GameState& board) {
Depth depth = board.getDepth();
Value value = board.evaluate();
#ifdef DEBUG
assert(board.isValid());
assert(depth <= DEPTH_MAX);
#endif
if(value != VALUE_UNKNOWN) {
return value;
} else if (depth == config.width*config.height) {
return VALUE_DRAW;
} else {
GameState buffer[WIDTH];
board.children(buffer, config.width, config.height);
value = VALUE_UNKNOWN;
for(int i=0; i<WIDTH; i++) {
if(buffer[i].isValid()) {
Value val = minimax(buffer[i]);
if(value == VALUE_UNKNOWN ||
(board.getPlayer() == PLAYER_MAX && val > value) ||
(board.getPlayer() == PLAYER_MIN && val < value)) {
value = val;
}
}
}
return value;
}
}
void Connect4::principleVariation(const GameState& board, Move buffer[DEPTH_MAX]) {
for(unsigned i=0; i<DEPTH_MAX; i++) {
buffer[i] = MOVE_INVALID;
}
GameState state = board;
Move bestMove;
Entry cacheEntry;
for(Depth depth=board.getDepth(); depth<DEPTH_MAX; depth++) {
// std::cout << "Depth=" << (int)depth << std::endl;
GameState children[WIDTH];
state.children(children, config.width, config.height);
Value value = state.evaluate();
if(value != VALUE_UNKNOWN) {
buffer[depth] = MOVE_INVALID;
} else if(depth >= config.width * config.height) {
buffer[depth] = MOVE_INVALID;
} else {
if(cache->get(state, cacheEntry) && cacheEntry.bestMove != MOVE_INVALID) {
// std::cout << "Cache hit" << std::endl;
// std::cout << "Best move=" << (int)cacheEntry.bestMove << std::endl;
buffer[depth] = cacheEntry.bestMove;
state = children[cacheEntry.bestMove];
} else {
// std::cout << "Cache miss" << std::endl;
alphaBeta(state, VALUE_MIN, VALUE_MAX, bestMove);
// std::cout << "Best move=" << (int)bestMove << std::endl;
buffer[depth] = bestMove;
state = children[bestMove];
}
}
}
}
Value Connect4::alphaBeta(const GameState& state, const Value alpha, const Value beta, Move& bestMove) {
stats->nodesExplored++;
const Depth depth = state.getDepth();
const Player player = state.getPlayer();
#ifdef DEBUG
assert(state.isValid());
assert(depth >= 0 && depth <= DEPTH_MAX);
assert(alpha < beta);
bestMove = MOVE_INVALID;
#endif
if(config.cachePrefetch) {
cache->prefetch(state);
}
Value value = state.evaluate();
if(value != VALUE_UNKNOWN) {
return value;
} else if(depth == config.height * config.width) {
return VALUE_DRAW;
}
Value a = alpha;
Value b = beta;
// Look up state in cache
Entry cacheEntry;
if(config.useCache && depth <= config.cacheMaxDepth &&
cache->get(state, cacheEntry)) {
stats->cacheHits++;
if(cacheEntry.lower == cacheEntry.upper) {
bestMove = cacheEntry.bestMove;
return cacheEntry.lower;
} else if(cacheEntry.lower >= beta) {
bestMove = MOVE_INVALID;
return beta;
} else if(cacheEntry.upper <= alpha) {
bestMove = MOVE_INVALID;
return alpha;
}
a = std::max(a, cacheEntry.lower);
b = std::min(b, cacheEntry.upper);
} else {
stats->cacheMisses++;
cacheEntry.lower = VALUE_MIN;
cacheEntry.upper = VALUE_MAX;
}
// Generate children
Move moves[WIDTH];
GameState* children;
PoolEntry<GameState> childArray = statePool.get(WIDTH);
children = childArray.data;
state.children(children, config.width, config.height);
// Check for obvious winners
if(config.preFilter) {
for(Move move=0; move<WIDTH; move++) {
const GameState& child = children[move];
if(!child.isValid()) continue;
value = child.evaluate();
if(player == PLAYER_MAX && value == VALUE_MAX) {
bestMove = move;
return VALUE_MAX;
}
if(player == PLAYER_MIN && value == VALUE_MIN) {
bestMove = move;
return VALUE_MIN;
}
}
}
// Move ordering step
if(config.reorderMoves) {
moveOrder.reorder(depth+1, MOVE_INVALID, moves);
} else {
moveOrder.getStaticOrder(moves);
}
for(unsigned i=0; i<WIDTH; i++) {
const Move move = moves[i];
const GameState& child = children[move];
if(!child.isValid()) continue;
Move childBestMove;
Value childVal = alphaBeta(child, a, b, childBestMove);
if(player == PLAYER_MAX) {
if(value == VALUE_UNKNOWN || childVal > value) {
value = childVal;
bestMove = move;
moveOrder.recordCutoffMove(depth+1, bestMove);
a = std::max(a, value);
}
} else {
if(value == VALUE_UNKNOWN || childVal < value) {
value = childVal;
bestMove = move;
moveOrder.recordCutoffMove(depth+1, bestMove);
b = std::min(b, value);
}
}
if(a >= b) {
stats->cutoffs++;
stats->cutoffTotal++;
stats->averageCutoff += (i - stats->averageCutoff) / stats->cutoffTotal;
break;
}
}
// Store new bounds to cache
if(config.useCache && depth <= config.cacheMaxDepth) {
// We need beta == VALUE_MAX because we may have cutoff on a
// draw because of our bounds, which does not mean a draw is
// the best possible score
if(player == PLAYER_MAX && beta == VALUE_MAX) {
cacheEntry.upper = a;
} else if(player == PLAYER_MIN && alpha == VALUE_MIN){
cacheEntry.lower = b;
}
if(cacheEntry.lower == cacheEntry.upper) {
cacheEntry.bestMove = bestMove;
} else {
cacheEntry.bestMove = MOVE_INVALID;
}
cacheEntry.depth = depth;
cache->put(state, cacheEntry);
}
#ifdef DEBUG
assert(bestMove != MOVE_INVALID);
#endif
return value;
}