Add conservative LMR and skip NMP based on static

[bdmendes]

Summary by CodeRabbit

• Refactor
  • Improved search engine internals: tighter pruning conditions and refined null-move handling to reduce wasted search branches.
  • Fewer redundant lookups and improved move-ordering heuristics.
  • New late-move reduction logic and adjusted depth reductions for certain quiet moves, yielding more efficient and consistent search performance.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro

Run ID: a14f3595-2ee0-4586-a98d-00ffc9e23f99

📥 Commits

Reviewing files that changed from the base of the PR and between 29d617b and e03fbb9.

📒 Files selected for processing (1)

• src/search/mod.rs

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Walkthrough

Tightens null-move gating to require a window cutoff against static_evaluation + MAX_POSITIONAL_WEIGHT, caches hash_move for reuse, introduces a may_late_move_reduce condition for quiet/non-frontier moves, increases null-search reduction for certain late quiet moves, and simplifies window-based return logic.

Changes

Cohort / File(s)

Summary

Search core src/search/mod.rs

Require window.cuts_off(static_evaluation + MAX_POSITIONAL_WEIGHT) for null-move eligibility; compute and pass a single hash_move = self.table.hash_move(position) into MovePicker::new(...); add may_late_move_reduce for non-frontier/quiet or absent hash moves; apply an extra depth subtraction for quiet moves with i > 5; remove window.cuts_off(null_score) gating and use window.improves(null_score) only.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Possibly related PRs

• Use mem::transmute in hot code and cache candidate ep squares #342: Introduces/changes ScoreTable::hash_move usage and related search-table API adjustments used here.
• Bootstrap search #333: Adds/uses the hash_move API and alters search flow where MovePicker::new() is invoked.
• Cut depth in half with NMP #412: Modifies null-move pruning and depth/reduction logic in Searcher::pvs, overlapping this change set.

Poem

🐰
I cached a hop, one hash to find,
Trimmed a branch that wasted time.
Quiet steps reduced at late-night tide,
Window checks guide which paths to hide.
Happy hops — the search runs spry! 🥕✨

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 28.57% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately summarizes the two main changes: adding conservative late-move reduction (LMR) and skipping null-move pruning (NMP) based on static evaluation criteria.

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

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[coderabbitai]

Actionable comments posted: 2

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro

Run ID: b377f43c-5e6b-4ae5-9529-dae22c6508eb

📥 Commits

Reviewing files that changed from the base of the PR and between 789450f and 7baf809.

📒 Files selected for processing (4)

• src/position/piece.rs
• src/search/mod.rs
• src/search/picker.rs
• src/search/see.rs

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92%. Comparing base (be20971) to head (e03fbb9).
⚠️ Report is 7 commits behind head on master.

Additional details and impacted files

@@          Coverage Diff          @@
##           master   #437   +/-   ##
=====================================
+ Coverage      92%    92%   +1%     
=====================================
  Files          33     33           
  Lines        3757   3760    +3     
  Branches     3757   3760    +3     
=====================================
+ Hits         3419   3422    +3     
  Misses        319    319           
  Partials       19     19           

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[coderabbitai]

Caution

Some comments are outside the diff and can’t be posted inline due to platform limitations.

⚠️ Outside diff range comments (1)

src/search/mod.rs (1)

234-244: ⚠️ Potential issue | 🟠 Major

Verify reduced fail-highs at full depth.

After Line 234 reduces late quiet moves, a reduced search that returns >= beta skips the full-depth re-search because Line 243 excludes cutoffs. That lets Line 246 feed a shallow score into Line 252 and prune the node based on an unverified reduced-depth result.

🐛 Proposed fix

-                let reduction = if may_late_move_reduce && mov.is_quiet() && i > 5 { 1 } else { 0 };
+                let full_depth = depth.saturating_sub(1);
+                let reduction = if may_late_move_reduce && mov.is_quiet() && i > 5 { 1 } else { 0 };
 
                 let null_score = -self.pvs(
                     &next_position,
-                    depth.saturating_sub(1).saturating_sub(reduction),
+                    full_depth.saturating_sub(reduction),
                     ply.saturating_add(1),
                     window.reverse_null_around_alpha(),
                 );
 
-                if window.improves(null_score) && !window.cuts_off(null_score) {
-                    -self.pvs(&next_position, depth.saturating_sub(1), ply.saturating_add(1), window.reverse())
+                if window.improves(null_score) && (reduction > 0 || !window.cuts_off(null_score)) {
+                    -self.pvs(&next_position, full_depth, ply.saturating_add(1), window.reverse())
                 } else {
                     null_score
                 }

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro

Run ID: e1a30aef-622b-4865-937b-bab57d602ab8

📥 Commits

Reviewing files that changed from the base of the PR and between 7baf809 and 29d617b.

📒 Files selected for processing (1)

• src/search/mod.rs

[github-actions]

Against master [hash=64]: 🆗 Elo: 5.56 +/- 18.74, nElo: 6.40 +/- 21.53

[github-actions]

Against v1.6.0 [hash=64]: ✅ Elo: 75.15 +/- 18.68, nElo: 89.46 +/- 21.53