1 files changed, 12 insertions, 1 deletions
diff --git a/src/bitboard.c b/src/bitboard.c
index 0478fc1..9e9df48 100755
--- a/src/bitboard.c
+++ b/src/bitboard.c
@@ -389,10 +389,21 @@ Return value:
     {
         ASSERT(*pbb);
         uLastBit--;
-        *pbb &= (*pbb - 1);
         c = BIT_NUMBER_TO_COOR(uLastBit);
         ASSERT(c == SLOW_BIT_NUMBER_TO_COOR(uLastBit));
         ASSERT(IS_ON_BOARD(c));
+        // BUG (fixed): this used to be `*pbb &= (*pbb - 1)`, which
+        // clears the LOWEST set bit -- correct for
+        // CoorFromBitBoardRank8ToRank1's first-bit semantics, wrong
+        // here, where uLastBit is the HIGHEST set bit. With more than
+        // one bit set (e.g. doubled pawns on a file), that cleared the
+        // wrong bit: the reported (highest) bit was never actually
+        // removed, so a caller looping on this function would see it
+        // again next call (a duplicate) while the true lowest bit was
+        // silently skipped forever. Single-bit inputs never exposed
+        // this (lowest-bit-clear and highest-bit-clear coincide when
+        // there's only one bit).
+        *pbb &= ~COOR_TO_BB(c);
     }
     return(c);
 }