Did I abuse generators 🫠

reducer-with-generator.js

function byCallbacks(s0, inp) {
  if (inp == null) {
    let s = s0
    if (s0.integerPart === '') {
      s = { ...s0, integerPart: '0' }
    }
    return s.integerPart + (s.decimalPart ? `.${s.decimalPart}` : '')
  } if (s0.readingIntegerPart) {
    if (inp === '.') {
      return { ...s0, readingIntegerPart: false }
    } else if (inp === '0' && s0.integerPart === '') {
      return s0
    } else {
      const integerPart = s0.integerPart + inp
      return { ...s0, integerPart }
    }
  } else {
    if (inp === '0') {
      const zeroCount = s0.zeroCount + 1
      return { ...s0, zeroCount }
    } else {
      const decimalPart = s0.decimalPart + '0'.repeat(s0.zeroCount) + inp
      return { ...s0, zeroCount: 0, decimalPart }
    }
  }
}

function reducer(str) {
  let s = {
    readingIntegerPart: true,
    integerPart: '',
    decimalPart: '',
    zeroCount: 0,
  }
  for (const c of str.split('')) {
    s = byCallbacks(s, c)
  }
  return byCallbacks(s, null)
}

// ----------

function* byGenerator() {
  let inp
  let s = {integerPart: '', decimalPart: ''}
  while (inp = yield s) {
    if (inp === '.') break
    if (!(inp === '0' && s.integerPart === '')) {
      const integerPart = s.integerPart + inp
      s = { ...s, integerPart }
    }
  }
  let zeroCount = 0
  while (inp !== null && (inp = yield s)) {
    if (inp === '0') {
      zeroCount += 1
    } else {
      const decimalPart = s.decimalPart + '0'.repeat(zeroCount) + inp
      zeroCount = 0
      s = { ...s, decimalPart }
    }
  }
  if (s.integerPart === '') {
    const integerPart = s.integerPart = '0'
    s = { ...s, integerPart }
  }
  return s.integerPart + (s.decimalPart ? `.${s.decimalPart}` : '')
}

function reducerWithGenerator(str) {
  const gen = byGenerator()
  gen.next()
  for (const c of str.split('')) {
    gen.next(c)
  }
  const result = gen.next(null)
  if (!result.done) throw new Error('generator not ended')
  return result.value
}

const testString = '00420.123040050000000000'

const r1 = reducer(testString)
const r2 = reducerWithGenerator(testString)

console.log('result', r1, r2)