index.js

'use strict'

const PARTS = {
  Intro: { weight: 1, even: false, spillRank: 4, balance: false },
  Chorus: { weight: 2, even: true, spillRank: 6, balance: true },
  Verse: { weight: 4, even: true, spillRank: 5, balance: true },
  Bridge: { weight: 2.5, even: true, spillRank: 1, balance: false },
  Outro: { weight: 1, even: false, spillRank: 3, balance: false },
  'Short Break': { weight: 1, even: true, spillRank: 1, balance: false },
  'Long Break': { weight: 3, even: true, spillRank: 2, balance: false }
}

const rankedEntries = Object.entries(PARTS).sort((a, b) => {
  return a[1].spillRank - b[1].spillRank
})

const suggestedStructure = (structure = [], beats, bpMeasure) => {
  if (!structure.length) return []
  if (!beats || !bpMeasure) return structure

  const weightSum = structure.reduce((accum, p) => accum + PARTS[p.part].weight, 0)
  const { out, spill } = structure.reduce((obj, { part }) => {
    const weight = PARTS[part].weight
    const rawVal = (beats * (weight / weightSum)) / bpMeasure
    const remainder = PARTS[part].even ? rawVal % 2 : 0
    const val = Math.floor(rawVal - remainder)

    obj.spill += rawVal - val
    obj.out.push({ part, measures: val, beats: val * bpMeasure })

    return obj
  }, { out: [], spill: 0 })

  let overflow = Math.floor(spill)
  const fade = Math.floor((spill - overflow) * bpMeasure)

  while (overflow > 0) {
    const byType = out.reduce((obj, partObj) => {
      obj[partObj.part] = obj[partObj.part] || []
      obj[partObj.part].push(partObj)
      return obj
    }, {})

    const assignMeasures = (part, addMeasures) => {
      part.measures += addMeasures
      part.beats += (addMeasures * bpMeasure)
      overflow -= addMeasures
    }

    rankedEntries.slice().forEach(([key, { even, balance }]) => {
      const addMeasures = even ? 2 : 1
      const typeParts = byType[key] || []

      if (overflow < addMeasures) return

      if (balance) {
        if ((typeParts.length * addMeasures) > overflow) return
        while (typeParts.length) assignMeasures(typeParts.shift(), addMeasures)
        return
      }

      const part = typeParts.shift()
      if (part) assignMeasures(part, addMeasures)
    })
  }

  if (fade) out.push({ part: 'Fade', beats: fade })

  return out
}

module.exports = { suggestedStructure, partsDefinition: PARTS }