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function notesBetween(a, b) {
const [tonicA, tonicB] = [a[0], b[0]]
const abs = tonicB.charCodeAt() - tonicA.charCodeAt();
if (abs < 0) {
return abs + 7;
} else {
return abs;
}
}
const ringHandler = {
get(target, prop) {
return target[prop % target.length] || Reflect.get(...arguments);
}
};
export const chromaticScale = new Proxy(
['C', 'C♯', 'D', 'D♯', 'E', 'F', 'F♯', 'G', 'G♯', 'A', 'A♯', 'B'],
ringHandler);
// todo: handle tonics with a flat ‘♭’, so we can deal with [BE]♭.
function scaleFromIntervals(tonic, intervals) {
const scaleIndex = chromaticScale.indexOf(tonic);
if (scaleIndex < 0) {
throw new Error(`tonic ${tonic} not found in scale ${chromaticScale.toString()}`);
}
let scale = [tonic];
let steps = 0;
let lastBase = tonic[0];
for (let i = 0; i < intervals.length; i++) {
steps += intervals[i];
const note = chromaticScale[scaleIndex + steps];
// don't display two base notes in a row by changing
// accidentals.
const delta = notesBetween(lastBase, note);
if (delta === 0) {
const nextBase = chromaticScale[scaleIndex + steps + 1][0];
scale.push(`${nextBase}♭`)
} else if (delta === 1) {
scale.push(`${note}`)
} else {
// we can only be two away, so take the sharp of the
// previous note.
const nextBase = chromaticScale[scaleIndex + steps - 1][0];
scale.push(`${nextBase}♯`)
}
lastBase = scale[scale.length-1];
}
return new Proxy(scale, ringHandler);
}
class Scale {
constructor(tonic, intervals) {
this.scale = scaleFromIntervals(tonic, intervals);
return new Proxy(this, {
get(target, prop) {
if (prop in target) {
return target[prop];
} else {
return target.scale[prop] || Reflect.get(...arguments);
}
}
});
}
get tonic() {
return this.scale[0];
}
get third() {
return this.scale[2];
}
get fifth() {
return this.scale[4];
}
get length() {
return this.scale.length;
}
toString() {
return `[${this.scale.join(", ")}]`;
}
}
export function MajorScale(tonic) {
console.debug(`MajorScale(${tonic})`);
const intervals = [2, 2, 1, 2, 2, 2];
return new Scale(tonic, intervals);
}
export function MinorScale(tonic) {
console.debug(`MinorScale(${tonic})`);
const intervals = [2, 1, 2, 2, 1, 2];
return new Scale(tonic, intervals);
}
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