Let ss->ply denote the number of plies from the root to the current node

[stockfish] / src / position.cpp

diff --git a/src/position.cpp b/src/position.cpp
index b6dc31b4ed0cfa6271d719da333f3c4d94a4ad05..42c1adff71591a270148adb43b37d2588cd2fe29 100644 (file)
--- a/src/position.cpp
+++ b/src/position.cpp
@@ -272,7 +272,7 @@ Position& Position::set(const string& fenStr, bool isChess960, StateInfo* si, Th
   // 5-6. Halfmove clock and fullmove number
   ss >> std::skipws >> st->rule50 >> gamePly;
 
-  // Convert from fullmove starting from 1 to ply starting from 0,
+  // Convert from fullmove starting from 1 to gamePly starting from 0,
   // handle also common incorrect FEN with fullmove = 0.
   gamePly = std::max(2 * (gamePly - 1), 0) + (sideToMove == BLACK);
 
@@ -1071,11 +1071,10 @@ bool Position::is_draw(int ply) const {
   {
       stp = stp->previous->previous;
 
-      // At root position ply is 1, so return a draw score if a position
-      // repeats once earlier but strictly after the root, or repeats twice
-      // before or at the root.
+      // Return a draw score if a position repeats once earlier but strictly
+      // after the root, or repeats twice before or at the root.
       if (   stp->key == st->key
-          && ++cnt + (ply - 1 > i) == 2)
+          && ++cnt + (ply > i) == 2)
           return true;
   }