fix longstanding negamax bug

we needed to be saving the alpha before the search loop in which we modified it. At this point, though i can't remember, i expect that this is the cause of whatever "instability" the comment was talking about that would have prevented us from using >= in the TT lookup.

1 files changed, 4 insertions, 3 deletions

diff --git a/include/negamax.c b/include/negamax.c
index dd769ad..995caa6 100644
--- a/include/negamax.c
+++ b/include/negamax.c
@@ -89,8 +89,7 @@ static float negamax(tak_state_p state, const uint8_t cur_depth,
 
   tt_entry_t *entry = tt_seek(hash);
 
-  // CAUTION: ≥ breaks search stability (vs =) on shallow depths
-  if (entry != NULL && entry->depth == cur_depth) {
+  if (entry != NULL && entry->depth >= cur_depth) {
     if (entry->flag == TT_EXACT) {
       return entry->value;
     } else if (entry->flag == TT_LOWERBOUND && entry->value > alpha) {
@@ -116,6 +115,8 @@ static float negamax(tak_state_p state, const uint8_t cur_depth,
     action_move_to_front(negamax_best_action, actions, num_actions);
   }
 
+  const float orig_alpha = alpha;
+
   action_t best_action = actions[0];
   float best_value = -infty;
 
@@ -162,7 +163,7 @@ static float negamax(tak_state_p state, const uint8_t cur_depth,
   flag = TT_EXACT;
   if (best_value >= beta)
     flag = TT_LOWERBOUND;
-  else if (best_value <= alpha)
+  else if (best_value <= orig_alpha)
     flag = TT_UPPERBOUND;
 
   if (entry == NULL) {