Bolt: Optimize Mel filterbank application using sparsity

[ysdede]

What: Precompute start and end indices for each Mel filter to iterate only over non-zero elements.
Why: The Mel filterbank matrix is ~98% sparse. Iterating over all frequency bins for each Mel filter involves many unnecessary multiplications by zero.
Impact: Reduces computation time for Mel spectrogram generation by ~2.8x (measured on 1-minute audio sample).
Measurement: Verified with benchmark script measuring execution time for 10 iterations of 1-minute audio processing.

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PR created automatically by Jules for task 1941292631891831797 started by @ysdede

Summary by CodeRabbit

• Performance Improvements
• Optimized Mel spectrogram computation for improved processing speed.

[coderabbitai]

📝 Walkthrough

Walkthrough

A new documentation note explains an optimization strategy for sparse Mel filterbanks, achieving ~2.8x speedup by precomputing non-zero index ranges. The parallel implementation adds internal bounds arrays to MelSpectrogram and restricts computation to those ranges during feature extraction.

Changes

Cohort / File(s)	Summary
Documentation .jules/bolt.md	New note documenting sparse Mel filterbank optimization strategy and performance gains.
Implementation src/mel.js	Added internal _fbStart and _fbEnd arrays to precompute non-zero frequency bin ranges per mel bin. Modified computation loop to restrict dot-product calculations to precomputed bounds instead of iterating over all frequency bins. Constructor updated to populate bounds during initialization.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~12 minutes

Poem

🐰 Sparse matrices make us hop with glee,
Non-zeros precomputed, swift and free!
From full iteration to bounds so tight,
Mel filterbanks leap 2.8 times in flight! ✨

🚥 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 clearly identifies the main change: optimizing Mel filterbank application by leveraging sparsity, which is the primary focus of 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 perf/sparse-mel-filterbank-1941292631891831797

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[coderabbitai]

Actionable comments posted: 1

🤖 Fix all issues with AI agents

In @.jules/bolt.md:
- Around line 1-3: The changelog heading "2024-05-22 - Sparse Mel Filterbank
Optimization" is stale; update the date in that heading to the actual PR/change
date (e.g., "2026-02-15") so the entry reflects when the optimization was made;
edit the heading line in .jules/bolt.md (the line starting with "2024-05-22 -
Sparse Mel Filterbank Optimization") and replace the old date with the correct
one.

[coderabbitai]

⚠️ Potential issue | 🟡 Minor

Stale date in the heading.

The date 2024-05-22 doesn't match the PR creation date (2026-02-15). Consider updating it to reflect the actual date of this change.

🤖 Prompt for AI Agents

In @.jules/bolt.md around lines 1 - 3, The changelog heading "2024-05-22 -
Sparse Mel Filterbank Optimization" is stale; update the date in that heading to
the actual PR/change date (e.g., "2026-02-15") so the entry reflects when the
optimization was made; edit the heading line in .jules/bolt.md (the line
starting with "2024-05-22 - Sparse Mel Filterbank Optimization") and replace the
old date with the correct one.

[kiloconnect]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Files Reviewed (2 files)

• .jules/bolt.md - Documentation (1 minor comment from CodeRabbit about stale date)
• src/mel.js - Implementation changes

Code Analysis

Reviewed the sparse Mel filterbank optimization changes:

1. Constructor changes (lines 269-295): Precomputes _fbStart and _fbEnd arrays to track non-zero index ranges for each mel filter. The logic correctly finds first/last non-zero elements and handles edge cases.

2. computeRawMel changes (lines 356-358): Uses the precomputed indices to iterate only over non-zero portions of each filter, reducing unnecessary multiplications.

The optimization is well-implemented and should provide significant performance improvements as documented (~2.8x speedup).

Note

CodeRabbit already noted a minor documentation issue in .jules/bolt.md regarding a stale date (2024-05-22 vs PR date 2026-02-15), which is a style preference rather than a code issue.