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

import getKeyXOffset from './getKeyXOffset'
import getOctaveCount from './getOctaveCount'
import generateKeys from './generateKeys'
import groupKeysIntoOctaves from './groupKeysIntoOctaves'
import getOctaveCompleteness from './getOctaveCompleteness'

interface GetKeyLeft {
  (k: number): number,
}

interface GetKeyLeftDecorator {
  (startKey: number, endKey: number): GetKeyLeft,
}

const getKeyLeftDecorator: GetKeyLeftDecorator = (startKey, endKey): GetKeyLeft => (k) => {
  const dummyKeys = generateKeys(startKey, endKey)
  const keysGroupedIntoOctaves = groupKeysIntoOctaves(dummyKeys)
  const octaveCompleteness = Object
    .entries(keysGroupedIntoOctaves)
    .map<number[]>(([octave, keys]) => [
      octave as unknown as number,
      keys[0],
      keys.slice(-1)[0],
    ])
    .reduce<Record<number, number>>(
      (theOctaveCompleteness, [octave, firstKey, lastKey]) => ({
        ...theOctaveCompleteness,
        [octave]: getOctaveCompleteness(firstKey, lastKey),
      }),
      {}
    )

  const fractionalOctaveCount = Object
    .values(octaveCompleteness)
    .reduce(
      (a, b) => a + b,
      0
    )
  const octaveCount = getOctaveCount(startKey, endKey)

  const startOctave = Math.floor(startKey! / 12)
  const octave = Math.floor(k / 12)
  const octaveOffset = (100 * octaveCount / fractionalOctaveCount) / octaveCount * (octave - startOctave)
  const theKeyOffset = octaveOffset + (100 * octaveCount / fractionalOctaveCount / octaveCount * getKeyXOffset(k))
  const firstKeyOffset = (100 * octaveCount / fractionalOctaveCount / octaveCount * getKeyXOffset(startKey + 12))
  return theKeyOffset - firstKeyOffset
}

export default getKeyLeftDecorator