feat(editor): detect key from a part's notes (Krumhansl–Schmuckler)

[ChrisBeWithYou]

DAW roadmap 4.17 — the passive-key third of the item, sibling to the chord-at-cursor readout (#130). Editor-only.

What

A new Detect button in the key controls guesses the current part's key from its pitch-class content and sets it, turning the in-key highlight on so you see the result immediately.

• @pure:key-detect (_detectKeyPure): the Krumhansl–Schmuckler algorithm — Pearson correlation of a 12-bin pitch-class histogram against the standard major/minor key profiles, across all 24 keys. Correlation (not a raw dot product) is the load-bearing choice: the two profiles have different magnitudes, so a dot product would systematically favour one mode, and correlation makes a profile score a perfect 1.0 against its own key. Returns null for an empty or perfectly flat histogram (no tonal centre) so the caller shows nothing.
• editorDetectKey: builds the histogram from the active arrangement — fretted parts resolved to sounding pitch (capo/tuning-aware via _rollMidiForNote + _rollPitchCtx), keys parts to their packed pitch — duration-weighted (a held note counts more than a passing one, with a small floor so staccato still registers). Sets S.editorKey, enables the highlight, and reports the guess. A suggestion — the tonic/scale pickers stay editable.

Tests — tests/key_detect.test.js (8, all fail on main)

Exact profiles detect their own key (C major, G major, A minor via rotation); a textbook C-major distribution; uses-all-weights (shifting the histogram shifts the detected tonic by exactly the same amount); degenerate inputs → null (empty / all-zero / short / non-array / flat); a single pitch class; and the NaN/negative-bin guard. Full JS suite green.

Scope / fresh region

New @pure:key-detect block by @pure:scale, an editorDetectKey handler by the other key handlers, and one HTML button. No core, no spec; independent of the chord-readout PR (#130) — different block, different control.

Summary by CodeRabbit

• New Features
  • Added a Detect button in the key controls to suggest the current part’s key from the notes.
  • Key suggestions can now enable in-key highlighting and update the on-screen key display.
• Bug Fixes
  • Improved handling for passages without a clear tonal center, returning no suggestion instead of a misleading result.
  • Key detection now accounts for note duration, capo, and tuning so results better match the played material.

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Review details

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: f05b37ca-d5e5-4cd0-9c2f-2de8d2573059

📥 Commits

Reviewing files that changed from the base of the PR and between 09ae61e and 47b3066.

📒 Files selected for processing (3)

• CHANGELOG.md
• screen.html
• screen.js

📝 Walkthrough

Walkthrough

This PR adds a key-detection feature: a pure Krumhansl–Kessler/Pearson-correlation algorithm (_detectKeyPure) in screen.js, a global window.editorDetectKey function that builds a duration-weighted pitch-class histogram from arrangement notes and updates editor state/UI, a new "Detect" button in screen.html, a corresponding test suite, and a changelog entry.

Changes

Key Detection

Layer / File(s)	Summary
Key-detection algorithm and editor integration screen.js	Adds Krumhansl-Kessler profile constants and _detectKeyPure (Pearson correlation over rotated profiles, returns tonic/scale/score or null), and window.editorDetectKey, which builds a duration-weighted 12-bin pitch-class histogram from arrangement notes, runs detection, updates S.editorKey, enables key highlighting, persists state, refreshes controls, redraws, and sets a status message.
Detect button UI wiring screen.html	Adds a "Detect" button (editor-key-detect-btn) in the key/scale controls that calls editorDetectKey() on click.
Key detection test suite tests/key_detect.test.js	Extracts the @pure:key-detect block from screen.js, evaluates it in a sandbox, and tests exact/rotated profile matching plus degenerate/invalid input handling, reporting pass/fail counts.
Changelog entry CHANGELOG.md	Documents the new "Detect" key-control feature under Unreleased → Added, referencing the algorithm and test file.

Estimated code review effort: 3 (Moderate) | ~25 minutes

Sequence Diagram(s)

sequenceDiagram
  participant User
  participant DetectButton as Detect Button
  participant editorDetectKey
  participant _detectKeyPure
  participant EditorState as Editor State/UI

  User->>DetectButton: click
  DetectButton->>editorDetectKey: editorDetectKey()
  editorDetectKey->>editorDetectKey: build weighted pitch-class histogram from notes
  editorDetectKey->>_detectKeyPure: pcWeights
  _detectKeyPure-->>editorDetectKey: {tonic, scale, score} or null
  editorDetectKey->>EditorState: update S.editorKey, enable highlighting, persist key
  editorDetectKey->>EditorState: refresh key controls, redraw, set status message

Loading

Possibly related PRs

• got-feedBack/feedBack-plugin-editor#108: Modifies the same key/scale and in-key highlighting infrastructure in screen.js that editorDetectKey updates on successful detection.

🚥 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 clearly summarizes the main editor change: detecting a part's key from notes using the Krumhansl–Schmuckler method.
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.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/editor-detect-key

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