perf: gallop the backward trace upper bound from lower

[nh13]

PR #9 of the v0.2 stack (carries 8cbee1f). Base is feat/v0.2-prefetch (#21).

Gallops the backward trace upper bound up from the lower bound instead of a full-range search. Pure perf — byte-identical (the backward-spectrum oracle proptests pass unchanged). Clean cherry-pick onto #8.

Green-pass: build / clippy -D warnings / nightly fmt / cargo test --workspace all ✅ (162 lib + all FFI; only Plan-3 deferrals ignored).

Summary by CodeRabbit

• Refactor
  • Internal reorganization to centralize and simplify a core search loop, improving maintainability and reducing duplicated logic.
  • No public API or behavior changes; end-user experience remains unchanged.

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⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

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Run ID: ef7689e4-aa86-4f32-a4c1-a15cc0e61577

📥 Commits

Reviewing files that changed from the base of the PR and between 89fa2e5 and 7c8ae2a.

📒 Files selected for processing (1)

• prmi/src/index/spectrum.rs

🚧 Files skipped from review as they are similar to previous changes (1)

• prmi/src/index/spectrum.rs

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📝 Walkthrough

Walkthrough

This PR extracts the per-step SA-interval location from backward_spectrum_inner_into into backward_locate_step, which recomputes per-step 32-mer keys, derives a model-seeded hint window (or uses seed_override), performs two seeded boundary searches, and returns the located [lower, upper) interval or None. The main loop now calls this helper and exits on None.

Changes

SA Interval Location Refactoring

Layer / File(s)	Summary
SA interval location helper prmi/src/index/spectrum.rs	New backward_locate_step function encapsulates 32-mer key recomputation, hint window derivation from model prediction or seed override, and two exponential-from-hint boundary searches to locate the SA interval [lower, upper), returning None if empty.
Left-extension loop integration prmi/src/index/spectrum.rs	backward_spectrum_inner_into refactored to form per-step p_slice and delegate interval computation to backward_locate_step, breaking the loop on None. Prior inline key derivation, window launch, and boundary search logic removed from loop body.

Estimated Code Review Effort

🎯 4 (Complex) | ⏱️ ~45 minutes

Possibly related PRs

• fg-labs/prmi#21: Modifies find_boundary/probe loops to add SA-entry prefetching; touches the same backward boundary-search control path used by the new helper.

Poem

I nibble keys by lantern light,
Recompute 32-mers through the night,
Seeded hints and boundaries hop,
The loop steps left until it stops,
A rabbit hums — the index sings bright. 🐰✨

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'perf: gallop the backward trace upper bound from lower' directly describes the core performance optimization in the changeset—switching to a gallop search strategy for the upper bound starting from the lower bound instead of full-range search.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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🧹 Nitpick comments (1)

prmi/src/index/spectrum.rs (1)

2112-2127: ⚡ Quick win

Update these doc comments to match the current implementation.

These comments now describe mem_search_backward_from_hint, not mem_search_backward: Lines 1577-1580 still route mem_search_backward through backward_spectrum_inner_into, and the lower search is still launched from [pred - err, pred + err + 1) at Lines 2151-2153 rather than a unit seed at pred.

Also applies to: 2166-2171

🤖 Prompt for AI Agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@prmi/src/index/spectrum.rs` around lines 2112 - 2127, Update the doc comment
to accurately describe the current behavior: state that these comments document
mem_search_backward_from_hint (not mem_search_backward), and clarify that
mem_search_backward now routes through backward_spectrum_inner_into; change the
description of the lower-bound search to say it is launched from a unit seed at
`pred` (not from the full model window `[pred - err, pred + err + 1)`), and note
that the model window is only a hint for the gallop but the actual boundary
search uses unit seeds at `pred` (lower) and at the chosen lower probe (upper).
Also update the nearby similar comment block referenced around
`find_boundary`/`mem_search_backward` so both blocks consistently describe the
seeding and probe-count behavior of `mem_search_backward_from_hint`,
`mem_search_backward`, and `find_boundary`.

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Nitpick comments:
In `@prmi/src/index/spectrum.rs`:
- Around line 2112-2127: Update the doc comment to accurately describe the
current behavior: state that these comments document
mem_search_backward_from_hint (not mem_search_backward), and clarify that
mem_search_backward now routes through backward_spectrum_inner_into; change the
description of the lower-bound search to say it is launched from a unit seed at
`pred` (not from the full model window `[pred - err, pred + err + 1)`), and note
that the model window is only a hint for the gallop but the actual boundary
search uses unit seeds at `pred` (lower) and at the chosen lower probe (upper).
Also update the nearby similar comment block referenced around
`find_boundary`/`mem_search_backward` so both blocks consistently describe the
seeding and probe-count behavior of `mem_search_backward_from_hint`,
`mem_search_backward`, and `find_boundary`.

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 0f6db5cc-1ebf-4c75-ada4-ad16d6d333e3

📥 Commits

Reviewing files that changed from the base of the PR and between 942787e and 89fa2e5.

📒 Files selected for processing (1)

• prmi/src/index/spectrum.rs

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