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minimax.h
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minimax.h
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#pragma once
#include <limits>
#include "util.h"
#include "board.h"
int min_move(BoardState *state, int d, int player, int alpha, eval_func eval);
int max_move(BoardState *state, int d, int player, int beta, eval_func eval) {
auto valid_moves = state->moves();
if (d == 0) {
return eval(state, player);
}
if (valid_moves.size() == 0) {
BoardState next_state(*state);
next_state.apply(PASS);
return min_move(&next_state, d - 1, player, std::numeric_limits<int>::min(), eval);
}
int best_score = 0;
Point best_move;
for (auto move : valid_moves) {
BoardState next_state(*state);
next_state.apply(move);
int score = min_move(&next_state, d - 1, player, best_score, eval);
if (score > beta) {
best_score = beta;
break;
}
if (score > best_score) {
best_move = move;
best_score = score;
}
}
return best_score;
}
int min_move(BoardState *state, int d, int player, int alpha, eval_func eval) {
auto valid_moves = state->moves();
if (d == 0) {
return eval(state, player);
}
if (valid_moves.size() == 0) {
BoardState next_state(*state);
next_state.apply(PASS);
return max_move(&next_state, d - 1, player, std::numeric_limits<int>::max(), eval);
}
int best_score = std::numeric_limits<int>::max();
Point best_move;
for (auto move : valid_moves) {
BoardState next_state(*state);
next_state.apply(move);
int score = max_move(&next_state, d - 1, player, best_score, eval);
if (score < alpha) {
best_score = alpha;
break;
}
if (score < best_score) {
best_move = move;
best_score = score;
}
}
return best_score;
}
bool minimax_move(BoardState *state, eval_func eval, int max_depth) {
auto valid_moves = state->moves();
if (valid_moves.size() == 0) {
printf("pass\n");
state->apply(PASS);
return false;
}
int player = state->active_player;
int best_score = -1;
Point best_move;
for (auto move : valid_moves) {
BoardState next_state(*state);
next_state.apply(move);
int score = min_move(&next_state, max_depth, player, best_score, eval);
if (score > best_score) {
best_move = move;
best_score = score;
}
}
assert(best_score != -1);
state->apply(best_move);
return true;
}