[6/6] Add futility pruning

[luccabb]

Summary

• Skip quiet moves at low depths when static evaluation is far below alpha
• Margin-based pruning: depth 1 = 100cp, depth 2 = 200cp

Details

Futility Pruning

If the static evaluation plus a safety margin is still below alpha, quiet moves
are unlikely to improve the position enough to beat alpha. We skip them.

Implementation:

if depth <= 2 and not in_check:
    static_eval = self.eval_board(board)
    futility_margin = 100 * depth  # 100cp at depth 1, 200cp at depth 2
    can_futility_prune = static_eval + futility_margin < alpha

# In move loop:
if can_futility_prune and move_index > 0 and is_quiet_move:
    continue  # Skip this move

Safety Conditions

Pruning is conservative to avoid missing important moves:

• Only at low depths (1-2 ply)
• Never when in check
• Never prune captures, checks, or promotions
• Never prune the first move
• Margin allows for reasonable positional improvement

Why it Works

At low depths near leaf nodes:

• Quiet moves have limited potential to swing evaluation
• If we're already losing by > margin, a quiet move won't save us
• Captures/checks are still searched (tactical opportunities)

Test plan

• All 64 unit tests pass
• Verified pruning only happens for quiet moves at low depth

---

🤖 Generated with Claude Code

[github-actions]

🔬 Stockfish Benchmark Results

vs Stockfish Skill Level 3

Metric	Wins	Losses	Draws	Total	Win %
Overall	0	100	0	100	0%
As White	0	50	0	50	0%
As Black	0	50	0	50	0%

vs Stockfish Skill Level 4

Metric	Wins	Losses	Draws	Total	Win %
Overall	1	99	0	100	1.0%
As White	0	50	0	50	0%
As Black	1	49	0	50	2.0%

vs Stockfish Skill Level 5

Metric	Wins	Losses	Draws	Total	Win %
Overall	0	100	0	100	0%
As White	0	50	0	50	0%
As Black	0	50	0	50	0%

Configuration

• 5 chunks × 20 rounds × 3 skill levels = 300 total games
• Each opening played with colors reversed (-repeat) for fairness
• Moonfish: 60s per move
• Stockfish: 60+5 time control

[luccabb]

/run-nps-benchmark

[github-actions]

⚡ NPS Benchmark Results

Metric	Value
Depth	5
Positions	48
Total nodes	6462485
Total time	819.07s
Nodes/second	7889

Node count is the primary signal — it's deterministic and catches search behavior changes. If the node count changes, the PR changed search behavior. NPS is informational only (CI runner performance varies).

Per-position breakdown

Position  1/48: nodes=21252      time=2.75s  nps=7724
Position  2/48: nodes=265680     time=39.08s  nps=6799
Position  3/48: nodes=14666      time=1.16s  nps=12605
Position  4/48: nodes=384303     time=48.37s  nps=7945
Position  5/48: nodes=68572      time=9.78s  nps=7009
Position  6/48: nodes=137992     time=18.97s  nps=7274
Position  7/48: nodes=176141     time=24.54s  nps=7178
Position  8/48: nodes=100297     time=12.82s  nps=7824
Position  9/48: nodes=179833     time=23.02s  nps=7811
Position 10/48: nodes=93999      time=12.79s  nps=7352
Position 11/48: nodes=86459      time=10.80s  nps=8009
Position 12/48: nodes=318364     time=43.67s  nps=7290
Position 13/48: nodes=221369     time=30.23s  nps=7322
Position 14/48: nodes=43382      time=6.17s  nps=7036
Position 15/48: nodes=80374      time=9.60s  nps=8374
Position 16/48: nodes=22219      time=2.74s  nps=8106
Position 17/48: nodes=3802       time=0.30s  nps=12697
Position 18/48: nodes=2911       time=0.22s  nps=13119
Position 19/48: nodes=5644       time=0.49s  nps=11552
Position 20/48: nodes=30857      time=2.52s  nps=12227
Position 21/48: nodes=12801      time=1.20s  nps=10635
Position 22/48: nodes=636        time=0.05s  nps=13413
Position 23/48: nodes=1115       time=0.08s  nps=14605
Position 24/48: nodes=10577      time=0.84s  nps=12552
Position 25/48: nodes=6579       time=0.54s  nps=12178
Position 26/48: nodes=12469      time=1.03s  nps=12156
Position 27/48: nodes=25801      time=2.64s  nps=9772
Position 28/48: nodes=115697     time=12.68s  nps=9127
Position 29/48: nodes=28258      time=2.81s  nps=10057
Position 30/48: nodes=2774       time=0.22s  nps=12538
Position 31/48: nodes=202496     time=22.76s  nps=8898
Position 32/48: nodes=436165     time=56.73s  nps=7688
Position 33/48: nodes=150218     time=22.77s  nps=6598
Position 34/48: nodes=123790     time=15.23s  nps=8127
Position 35/48: nodes=195976     time=22.34s  nps=8773
Position 36/48: nodes=1742889    time=206.18s  nps=8453
Position 37/48: nodes=790867     time=114.52s  nps=6905
Position 38/48: nodes=1562       time=0.10s  nps=16115
Position 39/48: nodes=3156       time=0.23s  nps=13652
Position 40/48: nodes=3435       time=0.12s  nps=28423
Position 41/48: nodes=14228      time=1.18s  nps=12025
Position 42/48: nodes=13842      time=1.28s  nps=10823
Position 43/48: nodes=2321       time=0.16s  nps=14151
Position 44/48: nodes=25708      time=2.34s  nps=10972
Position 45/48: nodes=14544      time=1.58s  nps=9211
Position 46/48: nodes=266465     time=29.47s  nps=9040
Position 47/48: nodes=0          time=0.00s  nps=0  (terminal)
Position 48/48: nodes=0          time=0.00s  nps=0  (terminal)