Expand lookup tables to KWG and cross set computation

[olaugh]

Extract BIT_MASK from movegen.c to shared inc/bit_tables.h and add BITS_BELOW_MASK table for (1U << n) - 1 patterns. This consolidates the 68000 shift optimization across the codebase.

Updated locations:

• kwg.h: kwg_get_letter_sets() uses BIT_MASK for letter set bitmaps
• kwg.c: compute_cross_set() uses BIT_MASK for cross set computation
• klv.c: leave_map_init() uses BIT_MASK and BITS_BELOW_MASK
• graphics.c/main.c: blanks bitmask operations (consistency)
• movegen.c: now includes shared header instead of local table

On 68000, variable shifts cost 2 cycles/bit (up to 52 cycles for n=26), while table lookups cost ~8 cycles regardless of shift amount.

https://claude.ai/code/session_01Wfwz5QT34PfaYXp8kK6Q8V

[Copilot]

Pull request overview

This PR consolidates bit manipulation operations across the codebase by extracting the BIT_MASK lookup table from movegen.c into a shared header file (inc/bit_tables.h) and adding a new BITS_BELOW_MASK table. The optimization targets the Motorola 68000 architecture where variable shifts are significantly slower (2 cycles/bit, up to 52 cycles) than table lookups (~8 cycles).

Changes:

• Extracted BIT_MASK table to shared inc/bit_tables.h header and added BITS_BELOW_MASK table for (1U << n) - 1 patterns
• Updated cross-set computation, leave map indexing, and blank tracking to use lookup tables instead of shift operations
• Consolidated the 68000 optimization pattern across 6 source files

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
inc/bit_tables.h	New shared header defining BIT_MASK[32] and BITS_BELOW_MASK[33] lookup tables with documentation
src/movegen.c	Removed local BIT_MASK definition, now includes shared header
inc/kwg.h	Uses BIT_MASK in kwg_get_letter_sets() for letter set bitmap operations
src/kwg.c	Uses BIT_MASK in compute_cross_set() for cross-set computation
src/klv.c	Uses BIT_MASK and BITS_BELOW_MASK in leave_map_init() for rack indexing
src/graphics.c	Uses BIT_MASK for blank bitmask operations in draw_history()
src/main.c	Uses BIT_MASK for blank bitmask operations in add_to_history()

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[Copilot]

The use of static const arrays in this header creates a separate copy in each translation unit that includes it. The existing codebase uses extern const for data tables (see TILE_SCORES, IS_VOWEL, TILE_COUNTS in scrabble.h). While static const in headers is a valid pattern for small lookup tables used in performance-critical code, it deviates from the established convention in this codebase and will increase binary size.

Consider using extern const declarations here with definitions in a corresponding bit_tables.c file to match the existing pattern. This would reduce code duplication across object files while still allowing the compiler to optimize access patterns.

[github-actions]

Speed Test Benchmark Results

Performance Comparison

Metric	Test (refs/pull/33/merge)	Baseline (main)	Change
NWL23 Hybrid	285.8 frames	285.8 frames	0%
CSW24 Hybrid	315.3 frames	315.3 frames	0%

Detailed Results

Test Results (refs/pull/33/merge)

{
  "num_games": 50,
  "lexicons": [
    {
      "name": "NWL23",
      "shadow_mean": 308.2,
      "noshadow_mean": 332.6,
      "hybrid_mean": 285.8,
      "move_count": 1148
    },
    {
      "name": "CSW24",
      "shadow_mean": 335.4,
      "noshadow_mean": 386.4,
      "hybrid_mean": 315.3,
      "move_count": 1091
    }
  ]
}

Baseline Results (main)

{
  "num_games": 50,
  "lexicons": [
    {
      "name": "NWL23",
      "shadow_mean": 308.2,
      "noshadow_mean": 332.6,
      "hybrid_mean": 285.8,
      "move_count": 1148
    },
    {
      "name": "CSW24",
      "shadow_mean": 335.4,
      "noshadow_mean": 386.4,
      "hybrid_mean": 315.3,
      "move_count": 1091
    }
  ]
}

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View full workflow run

[olaugh]

no speedup, closing