1 files changed, 19 insertions, 1 deletions
diff --git a/src/testsup.c b/src/testsup.c
index 3ed50e3..d1b102d 100644
--- a/src/testsup.c
+++ b/src/testsup.c
@@ -87,7 +87,25 @@ GenerateRandomLegalPosition(POSITION *pos)
         {
             pos->cNonPawns[WHITE][u] = pos->cNonPawns[BLACK][u] = ILLEGAL_COOR;
         }
-        
+
+        // memset above zeroes cEpSquare to 0x00 (A8), a real on-board
+        // square, not the "no en passant" sentinel -- every random
+        // position generated by this function had a bogus "en passant
+        // available on a8" flag set regardless of whether that made
+        // any sense. Both the mailbox and bitboard pawn generators
+        // trust this field and will act on it (mailbox: any pawn whose
+        // normal diagonal-capture target happens to coincide with a8;
+        // bitboard: proactively, once per side per node, since
+        // _GenerateAllPawnMovesBB/_SaveMeAllPawnMovesBB gate their
+        // en-passant block on IS_ON_BOARD(cEpSquare) directly) --
+        // constructing a garbage "en passant capture" move whenever a
+        // pawn of the right color happens to sit on b7 by pure chance,
+        // which corrupts position state and was traced back to an
+        // intermittent TestSearch segfault (board_representation/
+        // MOVEGEN_MIGRATION.md section 6b).
+        pos->cEpSquare = ILLEGAL_COOR;
+
+
         //
         // Place both kings legally
         //