Maintain pos->bbOccupied incrementally, dedupe its two from-scratch builders

chess.h: add POSITION::bbOccupied (full-board occupancy, both colors,
every piece including kings), maintained incrementally alongside
bbPieces/bbPawns rather than rebuilt on demand -- resolves the open
question in EVAL.md section 0 about whether this is worth doing given
both generate.c and the planned eval.c mobility rewrite need it.

move.c: SlidePiece/SlidePawn/LiftPiece/PlacePiece and their
WithoutSigs variants now maintain bbOccupied at the same choke points
that already maintain bbPieces/bbPawns -- unconditionally, since
occupancy doesn't care about piece type or color. Kings only ever
move through SlidePiece/SlidePieceWithoutSigs (never Lift/Place), so
no separate king-specific update site was needed.

fen.c: populate bbOccupied when parsing a FEN.

board.c: VerifyPositionConsistency cross-checks pos->bbOccupied
against a from-scratch rebuild, same pattern already used for
bbPieces/bbPawns.

generate.c/movesup.c/see.c: replace call sites that rebuilt full
occupancy via _BuildFullOccupiedBB/_BuildOccupiedBB with direct reads
of pos->bbOccupied; delete see.c's _BuildOccupiedBB, which was a
byte-for-byte duplicate of generate.c's _BuildFullOccupiedBB (kept
only as the from-scratch ground truth for the new consistency check
and testgenerate.c's benchmark harness).

testsup.c: GenerateRandomLegalPosition builds POSITIONs by poking
rgSquare/bbPieces/bbPawns directly, bypassing both move.c and fen.c --
a third construction path the above missed. It never set bbOccupied,
so the new VerifyPositionConsistency check failed on every generated
position, and since generation retries until a position verifies, the
self-test suite spun forever (100% CPU, no progress) instead of
crashing outright. Fixed by setting bbOccupied at all four placement
sites (both kings, pawn, non-pawn piece). Full self-test suite and
precommit_check.sh verified clean afterward.

board_representation/EVAL.md: record the bbOccupied decision and
rationale, resolving section 0's open question.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01P6g6iF6mD1Hau6nCZCzwYj

1 files changed, 19 insertions, 18 deletions

diff --git a/src/generate.c b/src/generate.c
index 69e431f..faecc2a 100755
--- a/src/generate.c
+++ b/src/generate.c
@@ -739,14 +739,14 @@ GenerateWhiteKnight(IN MOVE_STACK *pStack,
 // lookup needs no such color-specific shortcut, it just ANDs off
 // whichever side's occupancy pos->uToMove identifies.
 //
-// Full-board occupancy, both sides -- same formula as see.c's static
-// _BuildOccupiedBB (a separate copy, not shared, since that one is
-// file-local to see.c and this module's own convention keeps its
-// bitboard helpers together). Needed by the slider magic-bitboard
-// generators (_GenerateRookBB/_GenerateBishopBB) to index into
-// g_RookAttackTable/g_BishopAttackTable -- see MOVE_STACK's
-// bbOccupied field comment in chess.h. Non-static so testgenerate.c's
-// harness can call it directly.
+// Full-board occupancy, both sides. Callers should prefer reading
+// pos->bbOccupied directly (incrementally maintained, see chess.h and
+// board_representation/EVAL.md section 0) -- this from-scratch
+// rebuild now exists mainly as the ground truth used to verify that
+// field stays in sync (VerifyPositionConsistency, board.c; the
+// ASSERT(pStack->bbOccupied == _BuildFullOccupiedBB(pos)) calls
+// below) and for testgenerate.c's benchmark harness. Non-static so
+// both of those can call it directly.
 BITBOARD
 _BuildFullOccupiedBB(IN POSITION *pos)
 /**
@@ -850,10 +850,11 @@ _BuildFriendlySideBB(IN POSITION *pos, IN ULONG uSide)
 Routine description:
 
     Full occupancy bitboard for one side only (all piece types
-    including pawns and king) -- see.c's _BuildOccupiedBB ORs both
-    sides together for a different purpose (SEE's "is this square
-    occupied at all" query); move generation needs just one side's
-    squares, to AND off as illegal (self-occupied) destinations.
+    including pawns and king) -- pos->bbOccupied ORs both sides
+    together for a different purpose (SEE's/generation's "is this
+    square occupied at all" query); move generation also needs just
+    one side's squares here, to AND off as illegal (self-occupied)
+    destinations.
 
 Parameters:
 
@@ -2733,7 +2734,7 @@ Return value:
 **/
 {
     BITBOARD bbPawns = pos->bbPawns[uSide];
-    BITBOARD bbOccupied = _BuildFullOccupiedBB(pos);
+    BITBOARD bbOccupied = pos->bbOccupied;
     BITBOARD bbEmpty = ~bbOccupied;
     BITBOARD bbEnemy = bbOccupied & ~_BuildFriendlySideBB(pos, uSide);
     BITBOARD bbSinglePush, bbDoublePush, bbCapLeft, bbCapRight, bb;
@@ -3072,7 +3073,7 @@ Return value:
 **/
 {
     BITBOARD bbPawns = pos->bbPawns[uSide];
-    BITBOARD bbOccupied = _BuildFullOccupiedBB(pos);
+    BITBOARD bbOccupied = pos->bbOccupied;
     BITBOARD bbEmpty = ~bbOccupied;
     BITBOARD bbEnemy = bbOccupied & ~_BuildFriendlySideBB(pos, uSide);
     BITBOARD bbSinglePush, bbDoublePush, bbCapLeft, bbCapRight, bb;
@@ -3382,7 +3383,7 @@ Return value:
 #endif
 #if defined(GENERATE_ROOK_BITBOARD) || defined(GENERATE_BISHOP_BITBOARD) || \
     defined(GENERATE_QUEEN_BITBOARD)
-    pStack->bbOccupied = _BuildFullOccupiedBB(pos);
+    pStack->bbOccupied = pos->bbOccupied;
 #endif
 
     for(u = pos->uNonPawnCount[pos->uToMove][0] - 1;
@@ -3511,7 +3512,7 @@ Return value:
 #endif
 
     pStack->bbFriendlyOccupied = _BuildFriendlySideBB(pos, uSide);
-    pStack->bbOccupied = _BuildFullOccupiedBB(pos);
+    pStack->bbOccupied = pos->bbOccupied;
 
     bb = pos->bbPieces[uSide][KNIGHT];
     while (bb)
@@ -3728,7 +3729,7 @@ Return value:
         (void)pKing;
         BITBOARD bbFriendly = _BuildFriendlySideBB(pos, pos->uToMove);
         BITBOARD bbOccupiedWithoutKing =
-            _BuildFullOccupiedBB(pos) & ~COOR_TO_BB(cKing);
+            pos->bbOccupied & ~COOR_TO_BB(cKing);
         BITBOARD bbDest = g_KingAttacksBB[cKing] & ~bbFriendly;
         ULONG uBitIndex;
 
@@ -3875,7 +3876,7 @@ Return value:
         // _SaveMe*BB call below to share (same amortization reasoning
         // as _GenerateAllMoves's own precompute block).
         pStack->bbFriendlyOccupied = _BuildFriendlySideBB(pos, pos->uToMove);
-        pStack->bbOccupied = _BuildFullOccupiedBB(pos);
+        pStack->bbOccupied = pos->bbOccupied;
         bbTargetMask =
             _ComputeCheckTargetMaskBB(cKing, c, pStack->bbOccupied);