⚡ perf: optimize pentomino solver inner loop constraints

[lemononmars]

💡 What:

• The constraints calculation was removed from the innermost backtrack step. Instead, all valid piece placements on the board were calculated dynamically upfront into a placements pre-computed lookup.
• Converted the 2D grid array to a 1D array (flatGrid) across the engine for significantly improved data locality and lookup speed.
• Transformed the Set for tracking usedPieces into a bitmask (usedPiecesMask).
• Removed array bounds checks during the recursion since the static constraints filter guarantees the placements are within bounds.
• Flattened recursive flood-fill for checkConnectivity to reuse static memory arrays instead of allocating a fresh array stack each pass.

🎯 Why:

• The recursion depth in backtracking is extremely high in exact-cover algorithms. Specifically, checking isSafe involved repeatedly multiplying offset indices, doing bounds checks, checking logic for forced grid collisions, and Set.has() checks for every permutation step. This resulted in O(n) computations repeating operations known to be static since initialization.
• Precomputing valid placements drops the inner recursion work down to checking bitmasks and array values, which avoids thousands of loops entirely.

📊 Measured Improvement:

• countSolutions(blockedCells, forcedCells) was reduced from 53.1s to ~5.5s (almost a 10x speed boost).
• solvePentomino was reduced from ~120ms to ~50ms (over a 2x speed boost).

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PR created automatically by Jules for task 9477540874381475785 started by @lemononmars

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Project	Deployment	Actions	Updated (UTC)
codebreaker	Ready	Preview, Comment	Jun 28, 2026 4:23pm