1 files changed, 20 insertions(+), 0 deletions(-)

M src/attack.zig

@@ 282,3 282,23 @@ fn numBishopRelativeOccupancies() [64]u64 {
         return occupancies;
     }
 }
+
+// Given a blockMask, a mask of rays containing potentially blocking pieces for
+// a sliding piece, return a bitboard where 1s are a subset of the 1s in the
+// mask. There are many such subsets, they are indexed by 'i'.
+fn subOccupancy(blockMask: u64, i: u64) u64 {
+    var blockBitCount = @popCount(u64, blockMask);
+    var occupancy: u64 = 0;
+    var mask = blockMask;
+    var count: u6 = 0;
+
+    while (count < blockBitCount) : (count += 1) {
+        var lsbIndex = @intCast(u6, @ctz(u64, mask));
+        mask = resetBit(mask, @intToEnum(Square, lsbIndex));
+
+        if (i & (@as(u64, 1) << count) != 0) {
+            occupancy |= @as(u64, 1) << lsbIndex;
+        }
+    }
+    return occupancy;
+}