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#include "negamax.h"
// ===================================================================
// Globals
// ===================================================================
const float infty = 3.0;
char negamax_ptn[9];
uint8_t negamax_search_depth = 3;
static uint64_t *zobrist[15];
// ===================================================================
// Helpers
// ===================================================================
static void
zobrist_free(void);
static int
zobrist_init(void);
static uint64_t
zobrist_compute(void);
static float
negamax(const uint8_t cur_depth, float alpha, float beta,
const float colour);
// ===================================================================
// Zobrist hashing
// ===================================================================
static uint64_t
zobrist_compute(void) {
uint64_t hash = 0;
for (uint8_t l=0; l<board_size*board_size; l++) {
colour_stack_t c = colours[l];
const uint8_t count = COUNT_AT(l);
enum STONE_VARIANT s = STONE_AT(l);
for (uint8_t h=0; h<15; h++) {
if (h<count) {
hash ^= zobrist[h][l*(2*3+1)+(c&1)*3+s];
c >>= 1;
}
}
}
return hash;
}
static int
zobrist_init(void) {
for (int k=0; k<15; k++) {
if (zobrist[k] != NULL) return EXIT_FAILURE;
}
for (int j=0; j<15; j++) {
zobrist[j] = malloc(sizeof(uint64_t)*board_size*board_size*(2*3+1));
for (int k=0; k<board_size*board_size*(2*3+1); k++) {
XORSHIFT64;
zobrist[j][k] = RANDOM64;
}
}
return EXIT_SUCCESS;
}
static void
zobrist_free(void) {
for (int k=0; k<15; k++) {
if (zobrist[k] != NULL) {
free(zobrist[k]);
zobrist[k] = NULL;
}
}
}
// ===================================================================
// α-β negamax using the cnn1986 evaluation function and transposition
// tables using Zobrist hasing and a treap
// ===================================================================
void
negamax_init(const uint8_t new_board_size) {
board_size = new_board_size;
action_list_init();
zobrist_init();
tt_init();
}
void
negamax_free(void) {
zobrist_free();
}
static enum WIN_TYPE w;
static inline float
negamax_evaluate_terminal(const float colour) {
if (ply >= 2*board_size - 2 && (w = check_win()) < 0xFF) {
// Check win if far enough into the game
if (w == WIN_ROAD_BLACK || w == WIN_FLAT_BLACK) return colour*infty;
else if (w == WIN_DRAW) return 0; // Draw is fixed at neutral
else return -colour*infty;
} else {
return colour * cnn1986_evaluate_black_win();
}
}
float
negamax_generate(void) {
// We need to start with something outside of [-∞,∞] because those
// values are wins
const float safe_infty = infty + 1;
tt_init();
float result = negamax(negamax_search_depth,
-safe_infty, safe_infty,
(ply & 1) ? +1.0 : -1.0);
tt_free();
return result;
}
static enum TT_FLAG flag;
static float
negamax(const uint8_t cur_depth, float alpha, float beta,
const float colour) {
uint64_t hash = zobrist_compute();
tt_entry_t *entry = tt_seek(hash);
if (entry != NULL && entry->depth == cur_depth) {
if (entry->flag == TT_EXACT) {
return entry->value;
} else if (entry->flag == TT_LOWERBOUND && entry->value > alpha) {
alpha = entry->value;
} else if (entry->flag == TT_UPPERBOUND && entry->value < beta) {
beta = entry->value;
}
if (alpha >= beta) return entry->value;
}
action_list_t *list;
if ((list = action_list_generate()) == NULL)
return alpha; // should never happen!
/*
* action_node_t *best = NULL;
*/
float value = -infty;
for (action_node_t *node=list->head; node!=NULL; node=node->next) {
action_take(node);
// Compute the value of the node
float node_value;
if (ply >= 2*board_size - 2 && (w = check_win()) < 0xFF) {
node_value = -colour*infty;
// Check win if far enough into the game
if (w == WIN_ROAD_BLACK || w == WIN_FLAT_BLACK) node_value = colour*infty;
else if (w == WIN_DRAW) node_value = 0; // Draw is neutral
} else if (cur_depth > 1) {
// If nobody won, or too early and not leaf, recurse
node_value = -negamax(cur_depth - 1, -beta, -alpha, -colour);
} else {
node_value = colour * cnn1986_evaluate_black_win();
}
action_undo(node);
negamax_display_progress(cur_depth, list->length);
if (node_value > value) {
value = node_value;
if (cur_depth == negamax_search_depth)
action_to_ptn(node, negamax_ptn);
}
alpha = fmax(value, alpha);
if (alpha >= beta) break;
}
action_list_free(list);
flag = TT_EXACT;
if (value >= beta) flag = TT_LOWERBOUND;
else if (value <= alpha) flag = TT_UPPERBOUND;
if (entry == NULL) {
tt_insert(hash, flag, cur_depth, value);
} else {
entry->flag = flag;
entry->value = value;
entry->depth = cur_depth;
// entry->action = best;
}
return value;
}
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