/// Position::is_draw() tests whether the position is drawn by 50-move rule
/// or by repetition. It does not detect stalemates.
-bool Position::is_draw() const {
+bool Position::is_draw(int ply) const {
if (st->rule50 > 99 && (!checkers() || MoveList<LEGAL>(*this).size()))
return true;
- int e = std::min(st->rule50, st->pliesFromNull);
+ int end = std::min(st->rule50, st->pliesFromNull);
- if (e < 4)
+ if (end < 4)
return false;
StateInfo* stp = st->previous->previous;
+ int cnt = 0;
- do {
+ for (int i = 4; i <= end; i += 2)
+ {
stp = stp->previous->previous;
- if (stp->key == st->key)
- return true; // Draw at first repetition
-
- } while ((e -= 2) >= 4);
+ // At root position ply is 1, so return a draw score if a position
+ // repeats once earlier but after or at the root, or repeats twice
+ // strictly before the root.
+ if ( stp->key == st->key
+ && ++cnt + (ply - i > 0) == 2)
+ return true;
+ }
return false;
}