Add a known draw case in kpk bitbase generation

Early classify as known draws the positions
where white king is trapped on the rook file.

Suggested by Dan Honeycutt.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

diff --git a/src/bitbase.cpp b/src/bitbase.cpp
index b01a03d4bc8815ea3c20794057dfc1b3cde159c5..ceb94a0f9ef002f7c5bcc6218c8bf5a09df47b7a 100644 (file)
--- a/src/bitbase.cpp
+++ b/src/bitbase.cpp
@@ -156,7 +156,7 @@ namespace {
         && rank_of(psq) < RANK_7)
         return DRAW;
 
-    //  Case 4: White king in front of pawn and black has opposition
+    // Case 4: White king in front of pawn and black has opposition
     if (   stm == WHITE
         && wksq == psq + DELTA_N
         && bksq == wksq + DELTA_N + DELTA_N
@@ -168,6 +168,13 @@ namespace {
         && file_of(psq) == FILE_A)
         return DRAW;
 
+    // Case 6: White king trapped on the rook file
+    if (   file_of(wksq) == FILE_A
+        && file_of(psq) == FILE_A
+        && rank_of(wksq) > rank_of(psq)
+        && bksq == wksq + 2)
+        return DRAW;
+
     return UNKNOWN;
   }