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, 22 insertions, 10 deletions

diff --git a/src/move.c b/src/move.c
index 44e7073..27d86df 100755
--- a/src/move.c
+++ b/src/move.c
@@ -64,6 +64,8 @@ Return value:
     pos->cNonPawns[c][uIndex] = cTo;
     pos->bbPieces[c][PIECE_TYPE(p)] &= ~COOR_TO_BB(cFrom);
     pos->bbPieces[c][PIECE_TYPE(p)] |= COOR_TO_BB(cTo);
+    pos->bbOccupied &= ~COOR_TO_BB(cFrom);
+    pos->bbOccupied |= COOR_TO_BB(cTo);
     pos->u64NonPawnSig ^= g_u64SigSeeds[cFrom][PIECE_TYPE(p)][c];
     pos->u64NonPawnSig ^= g_u64SigSeeds[cTo][PIECE_TYPE(p)][c];
 #ifdef DEBUG
@@ -123,6 +125,8 @@ Return value:
     pos->cPawns[c][uIndex] = cTo;
     pos->bbPawns[c] &= ~COOR_TO_BB(cFrom);
     pos->bbPawns[c] |= COOR_TO_BB(cTo);
+    pos->bbOccupied &= ~COOR_TO_BB(cFrom);
+    pos->bbOccupied |= COOR_TO_BB(cTo);
     pos->u64PawnSig ^= g_u64PawnSigSeeds[cFrom][c];
     pos->u64PawnSig ^= g_u64PawnSigSeeds[cTo][c];
     pos->rgSquare[cTo].pPiece = p;
@@ -177,6 +181,8 @@ Return value:
     pos->cNonPawns[c][uIndex] = cTo;
     pos->bbPieces[c][PIECE_TYPE(p)] &= ~COOR_TO_BB(cFrom);
     pos->bbPieces[c][PIECE_TYPE(p)] |= COOR_TO_BB(cTo);
+    pos->bbOccupied &= ~COOR_TO_BB(cFrom);
+    pos->bbOccupied |= COOR_TO_BB(cTo);
     pos->rgSquare[cTo].pPiece = p;
     pos->rgSquare[cTo].uIndex = uIndex;
 #ifdef DEBUG
@@ -185,7 +191,7 @@ Return value:
 }
 
 
-void 
+void
 SlidePawnWithoutSigs(POSITION *pos, COOR cFrom, COOR cTo)
 /**
 
@@ -228,6 +234,8 @@ Return value:
     pos->cPawns[c][uIndex] = cTo;
     pos->bbPawns[c] &= ~COOR_TO_BB(cFrom);
     pos->bbPawns[c] |= COOR_TO_BB(cTo);
+    pos->bbOccupied &= ~COOR_TO_BB(cFrom);
+    pos->bbOccupied |= COOR_TO_BB(cTo);
     pos->rgSquare[cTo].pPiece = p;
     pos->rgSquare[cTo].uIndex = uIndex;
 #ifdef DEBUG
@@ -275,6 +283,7 @@ Return value:
     pos->rgSquare[cSquare].pPiece = EMPTY;
     ASSERT(IS_VALID_PIECE(pLifted));
     ASSERT(!IS_KING(pLifted));
+    pos->bbOccupied &= ~COOR_TO_BB(cSquare);
     uIndex = pos->rgSquare[cSquare].uIndex;
 #ifdef DEBUG
     ASSERT(IS_VALID_PIECE_INDEX(uIndex));
@@ -287,12 +296,12 @@ Return value:
     pos->iMaterialBalance[color] -= pv;
     pos->iMaterialBalance[FLIP(color)] += pv;
     ASSERT(pos->iMaterialBalance[WHITE] * -1 == pos->iMaterialBalance[BLACK]);
-    
+
     if (IS_PAWN(pLifted))
     {
         ASSERT(pos->cPawns[color][uIndex] == cSquare);
         ASSERT(pv == VALUE_PAWN);
-        
+
         pos->uPawnMaterial[color] -= pv;
         ASSERT(pos->uPawnMaterial[color] <= (7 * VALUE_PAWN));
 
@@ -410,7 +419,8 @@ Return value:
     pos->rgSquare[cSquare].pPiece = EMPTY;
     ASSERT(IS_VALID_PIECE(pLifted));
     ASSERT(!IS_KING(pLifted));
-    
+    pos->bbOccupied &= ~COOR_TO_BB(cSquare);
+
     uIndex = pos->rgSquare[cSquare].uIndex;
 #ifdef DEBUG
     ASSERT(IS_VALID_PIECE_INDEX(uIndex));
@@ -423,12 +433,12 @@ Return value:
     pos->iMaterialBalance[color] -= pv;
     pos->iMaterialBalance[FLIP(color)] += pv;
     ASSERT(pos->iMaterialBalance[WHITE] * -1 == pos->iMaterialBalance[BLACK]);
-    
+
     if (IS_PAWN(pLifted))
     {
         ASSERT(pos->cPawns[color][uIndex] == cSquare);
         ASSERT(pv == VALUE_PAWN);
-        
+
         pos->uPawnMaterial[color] -= pv;
         ASSERT(pos->uPawnMaterial[color] <= (7 * VALUE_PAWN));
         pos->bbPawns[color] &= ~COOR_TO_BB(cSquare);
@@ -533,7 +543,8 @@ Return value:
     pos->iMaterialBalance[color] += pv;
     pos->iMaterialBalance[FLIP(color)] -= pv;
     ASSERT(pos->iMaterialBalance[WHITE] * -1 == pos->iMaterialBalance[BLACK]);
-    
+    pos->bbOccupied |= COOR_TO_BB(cSquare);
+
     if (IS_PAWN(pPiece))
     {
         //
@@ -542,7 +553,7 @@ Return value:
         ASSERT(pv == VALUE_PAWN);
         pos->uPawnMaterial[color] += pv;
         ASSERT(pos->uPawnMaterial[color] <= (8 * VALUE_PAWN));
-        
+
         uIndex = pos->uPawnCount[color];
         ASSERT((uIndex >= 0) && (uIndex <= 7));
         pos->uPawnCount[color]++;
@@ -626,7 +637,8 @@ Return value:
     pos->iMaterialBalance[color] += pv;
     pos->iMaterialBalance[FLIP(color)] -= pv;
     ASSERT(pos->iMaterialBalance[WHITE] * -1 == pos->iMaterialBalance[BLACK]);
-    
+    pos->bbOccupied |= COOR_TO_BB(cSquare);
+
     if (IS_PAWN(pPiece))
     {
         //
@@ -635,7 +647,7 @@ Return value:
         ASSERT(pv == VALUE_PAWN);
         pos->uPawnMaterial[color] += pv;
         ASSERT(pos->uPawnMaterial[color] <= (8 * VALUE_PAWN));
-        
+
         uIndex = pos->uPawnCount[color];
         ASSERT((uIndex >= 0) && (uIndex <= 7));
         pos->uPawnCount[color]++;