Extract repeated chord-note retrieval into getChordPitchClasses utility

[Copilot]

Five files duplicated the same conditional to resolve pitch classes from a Chord — a maintenance hazard whenever a new chord variant is added. This replaces all occurrences with a single canonical utility.

New utility

client/src/features/chord/utils/getChordPitchClasses.ts

export function getChordPitchClasses(chord: Chord): number[] {
  if (Array.isArray(chord.customNotes)) return chord.customNotes;
  return getChordNoteIndices(chord.root, chord.quality);
}

Re-exported from a new client/src/features/chord/utils/index.ts. Uses Array.isArray directly (rather than importing isCustomChord from current-chord/utils) to avoid a cross-feature circular dependency.

Call-site updates

• CurrentChordPanel.tsx, ChordTile.tsx, midiBuilder.ts — replaced inline ternary with getChordPitchClasses(chord); dropped now-unused getChordNoteIndices imports.
• pairMetrics.ts — removed the local getChordPitchClasses (which returned Set<number> and duplicated the same logic); now imports from @/features/chord/utils and wraps with new Set(...) where set semantics are needed.
• pairMetrics.test.ts — import updated to the canonical location; assertions updated from Set to number[] to match the new return type.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>ISSUE-E7-02 — Extract Repeated Chord-Note Retrieval into a Shared Utility</issue_title>
<issue_description># ISSUE-E7-02 — Extract Repeated Chord-Note Retrieval into a Shared Utility

Objective

Replace a copy-pasted conditional that retrieves pitch classes from a Chord value with a single reusable utility function, removing the duplication from at least five files.

Background

The following pattern appears verbatim (or near-verbatim) in multiple files across the codebase:

isCustomChord(chord) ? chord.customNotes : getChordNoteIndices(chord.root, chord.type)

Known locations (non-exhaustive):

• client/src/features/current-chord/components/CurrentChordPanel.tsx
• client/src/features/progression-sidebar/components/ChordTile.tsx
• client/src/features/progression-sidebar/components/ProgressionSidebar.tsx
• client/src/features/midi-export/utils/midiBuilder.ts
• client/src/features/progression-sidebar/utils/pairMetrics.ts

Each duplicate is a maintenance hazard: introducing a new chord variant (e.g., quartal chords) requires hunting down and updating every occurrence. A single function eliminates that risk.

Proposed API

// client/src/features/chord/utils/getChordPitchClasses.ts
export function getChordPitchClasses(chord: Chord): number[]

The function should:

1. Check isCustomChord(chord) and return chord.customNotes if true.
2. Otherwise call getChordNoteIndices(chord.root, chord.type) and return the result.

Files To Add

• client/src/features/chord/utils/getChordPitchClasses.ts — new utility function.

Files To Edit

• client/src/features/chord/utils/index.ts — re-export getChordPitchClasses.
• All files listed in Known locations above — replace inline conditionals with a call to getChordPitchClasses(chord).

Acceptance Criteria

• getChordPitchClasses(chord: Chord): number[] exists in client/src/features/chord/utils/.
• No file outside getChordPitchClasses.ts contains the pattern isCustomChord(chord) ? chord.customNotes.
• All existing tests pass.
• npm run lint passes with --max-warnings=0.
• npm run build succeeds with no TypeScript errors.

Verification Commands

cd client
npm run lint
npm run build
npm test
```</issue_description>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes ISSUE-E7-02 — Extract Repeated Chord-Note Retrieval into a Shared Utility #255

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