Bolt: Optimize Mel filterbank with sparse loop (~2.3x faster)

[ysdede]

What: Modified MelSpectrogram to precompute start and end indices for non-zero elements in the Mel filterbank matrix. Updated computeRawMel to iterate only over these indices.

Why: The Mel filterbank matrix is ~98.5% sparse (mostly zeros). Iterating over all frequency bins for each Mel filter involves many unnecessary multiplications by zero.

Impact: Reduces computation time significantly. In benchmarks, processing speed improved from ~1167ms to ~509ms per minute of audio (~2.3x speedup).

Measurement: Verified with npm test (all tests passed) and a custom benchmark script measuring total processing time for 50 minutes of audio.

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PR created automatically by Jules for task 16533373355711190474 started by @ysdede

Summary by CodeRabbit

Release Notes

• Documentation

• Added documentation on Mel filterbank optimization strategies.

• Refactor

• Optimized Mel spectrogram computation performance, achieving approximately 2.3x speedup in the audio processing pipeline.

[coderabbitai]

📝 Walkthrough

Walkthrough

The changes optimize Mel spectrogram computation by leveraging the ~98.5% sparsity of the Mel filterbank matrix. Precomputed index ranges (_fbStart, _fbEnd) are stored for each mel bin, enabling the inner computation loop to skip zero-valued filter coefficients and reduce unnecessary iterations.

Changes

Cohort / File(s)	Summary
Sparse Filterbank Optimization src/mel.js	Added precomputed index fields (_fbStart, _fbEnd) to track non-zero support ranges per mel bin; modified inner loop to iterate only over the non-zero filterband region instead of all frequency bins.
Documentation .jules/bolt.md	New entry documenting the Sparse Mel Filterbank Optimization, explaining the ~98.5% sparsity observation and resulting 2.3x pipeline speedup.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

Poem

🐰 Sparse matrices, oh what a sight,
Zeros skipped left and right,
Precomputed bounds so neat,
Make the loops less bittersweet,
Two-point-three times quicker, yay!

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately describes the main change: sparse Mel filterbank optimization with a quantified performance improvement (2.3x faster), directly matching the core change in the changeset.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Merge Conflict Detection	✅ Passed	✅ No merge conflicts detected when merging into main

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✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch bolt/mel-filterbank-optimization-16533373355711190474

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[kiloconnect]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Files Reviewed (2 files)

• .jules/bolt.md - Documentation file (new)
• src/mel.js - Sparse filterbank optimization

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Review Details:

The PR implements a valid optimization for Mel spectrogram computation by precomputing start/end indices for non-zero filterbank elements. The implementation is correct:

1. Filterbank generation uses Math.max(0, ...) ensuring all values are non-negative
2. Edge cases handled - empty mel bins result in zero values as expected
3. Type safety - Int32Array appropriately stores array indices
4. Performance gain - ~2.3x speedup as noted in documentation

No inline comments were necessary as no issues were identified.

[coderabbitai]

Actionable comments posted: 1

🤖 Fix all issues with AI agents

Verify each finding against the current code and only fix it if needed.


In @.jules/bolt.md:
- Around line 1-3: Update the journal heading date in .jules/bolt.md to reflect
the actual PR creation date (replace "2024-05-22" with "2026-02-16" or the
correct date intended) so the entry's timestamp matches the change; ensure only
the heading text is modified and leave the rest of the note about the Sparse Mel
Filterbank Optimization unchanged.

[coderabbitai]

⚠️ Potential issue | 🟡 Minor

Date appears incorrect.

The heading says 2024-05-22, but this PR was created on 2026-02-16. If this is meant to record when the optimization was made, the date should be updated.

🤖 Prompt for AI Agents

Verify each finding against the current code and only fix it if needed.

In @.jules/bolt.md around lines 1 - 3, Update the journal heading date in
.jules/bolt.md to reflect the actual PR creation date (replace "2024-05-22" with
"2026-02-16" or the correct date intended) so the entry's timestamp matches the
change; ensure only the heading text is modified and leave the rest of the note
about the Sparse Mel Filterbank Optimization unchanged.