react-musical-keyboard/src/services/getOctaveCompleteness.ts

import mem from 'mem'
import * as caches from './caches'
import getKeyXOffsetUnmemoized from './getKeyXOffset'
import isNaturalKeyUnmemoized from './isNaturalKey'

const getKeyXOffset = mem(getKeyXOffsetUnmemoized, { cache: caches.getKeyXOffset })
const isNaturalKey = mem(isNaturalKeyUnmemoized, { cache: caches.isNaturalKey })

type GetOctaveCompleteness = (firstKey: number, lastKey: number) => number

// expect firstKey and lastKey within the same octave
const getOctaveCompleteness: GetOctaveCompleteness = (firstKey, lastKey) =>
  // see if there are missing higher notes
  getKeyXOffset(lastKey) +
  (isNaturalKey(lastKey) ? 1 / 7 : ((1 / 7) * 18) / 36) -
  // see if there are missing lower notes
  getKeyXOffset(firstKey)

export default getOctaveCompleteness