tonleiter experiment

packages/tonal/test/tonleiter.test.mjs

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+import { describe, it, expect } from 'vitest';
+import { Note } from '../tonleiter.mjs';
+
+describe('tonleiter', () => {
+  it('Should tokenize notes', () => {
+    expect(Note.tokenize('c')).toEqual(['c', '']);
+    expect(Note.tokenize('C')).toEqual(['C', '']);
+    expect(Note.tokenize('c#')).toEqual(['c', '#']);
+    expect(Note.tokenize('Bb')).toEqual(['B', 'b']);
+  });
+});

packages/tonal/tonleiter.mjs

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+const steps = [1, 0, 2, 0, 3, 4, 0, 5, 0, 6, 0, 7];
+const notes = ['C', '', 'D', '', 'E', 'F', '', 'G', '', 'A', '', 'B'];
+const noteLetters = ['C', 'D', 'E', 'F', 'G', 'A', 'B'];
+
+const accidentalOffset = (accidentals) => {
+  return accidentals.split('#').length - accidentals.split('b').length;
+};
+
+const accidentalString = (offset) => {
+  if (offset < 0) {
+    return 'b'.repeat(-offset);
+  }
+  if (offset > 0) {
+    return '#'.repeat(offset);
+  }
+  return '';
+};
+
+export const Step = {
+  tokenize(step) {
+    const [accidentals, stepNumber] = step.match(/^([#b]*)([1-9]+)$/).slice(1);
+    return [accidentals, parseInt(stepNumber)];
+  },
+  accidentals(step) {
+    return accidentalOffset(Step.tokenize(step)[0]);
+  },
+};
+
+Step.tokenize('#11');
+Step.tokenize('b13');
+Step.tokenize('bb6');
+Step.accidentals('3');
+Step.accidentals('b3');
+
+export const Note = {
+  // TODO: support octave numbers
+  tokenize(note) {
+    return [note[0], note.slice(1)];
+  },
+  accidentals(note) {
+    return accidentalOffset(this.tokenize(note)[1]);
+  },
+};
+
+Note.tokenize('C##');
+Note.accidentals('C#');
+Note.accidentals('C##');
+Note.accidentals('Eb');
+Note.accidentals('Bbb');
+
+// TODO: support octave numbers
+function transpose(note, step) {
+  // example: E, 3
+  const stepNumber = Step.tokenize(step)[1]; // 3
+  const noteLetter = Note.tokenize(note)[0]; // E
+  const noteIndex = noteLetters.indexOf(noteLetter); // 2 "E is C+2"
+  const targetNote = noteLetters[(noteIndex + stepNumber - 1) % 8]; // G "G is a third above E"
+  const rootIndex = notes.indexOf(noteLetter); // 4 "E is 4 semitones above C"
+  const targetIndex = notes.indexOf(targetNote); // 7 "G is 7 semitones above C"
+  const indexOffset = targetIndex - rootIndex; // 3 (E to G is normally a 3 semitones)
+  const stepIndex = steps.indexOf(stepNumber); // 4 ("3" is normally 4 semitones)
+  const offsetAccidentals = accidentalString(Step.accidentals(step) + Note.accidentals(note) + stepIndex - indexOffset); // "we need to add a # to to the G to make it a major third from E"
+  return [targetNote, offsetAccidentals].join('');
+}
+
+transpose('F#', '3');
+transpose('C', '3');
+transpose('D', '3');
+transpose('E', '3');
+transpose('Eb', '3');
+transpose('Ebb', '3');