index.js

const vowels = /[аеёиоуыэюя]/gi
const deaf = 'длкпстфхцчшщДЛКПСТФХЦЧШЩ'
const signs = ['ь', 'ъ', 'Ь', 'Ъ']
const voiced = ['м', 'р', 'л', 'н', 'М', 'Р', 'Л', 'Н']

const splitLines = source => {
  return source.split('\n')
}

const joinLines = lines => {
  return lines.join('\n')
}

const splitWords = source => {
  return source.split(' ')
}

const joinWords = words => {
  return words.join(' ')
}

const addSyllable = (collection, syllable) => {
  collection.push(syllable)
}

const indexSign = letters => {
  let index = -1
  signs.some(letter => {
    index = letters.indexOf(letter)
    return index !== -1
  })
  return index
}

const indexYoy = (letters, nextLetter) => {
  const index = letters.indexOf('й')

  if (index !== -1) {
    return index
  }
  return -1
}

const indexSonoric = (letters) => {
  let index = -1

  voiced.some(letter => {
    index = letters.indexOf(letter)
    return index !== -1
  })

  if (index !== -1) {
    const nextLetter = letters[index + 1]
    if (deaf.indexOf(nextLetter) === -1) {
      return -1
    }
  }

  return index
}

const _syllabifyWord = (word, { separator = '·' } = {}) => {
  const collection = []

  const vowelsMatches = word.match(vowels)

  if (!vowelsMatches) {
    return word
  }

  const letters = word.match(/[a-яё]/gi)

  let skip = 0
  let currentSyllable = ''
  let currentSyllableCount = 0
  let isWordEnd = false
  let isLastSyllable = false

  const len = letters.length

  letters.forEach((letter, index) => {
    if (skip) {
      return skip--
    }

    currentSyllable += letter

    isWordEnd = (index + 1) === len
    isLastSyllable = (vowelsMatches.length - collection.length) === 1

    if (isLastSyllable) {
      return isWordEnd ? addSyllable(collection, currentSyllable) : null
    }

    if (vowels.test(letter)) {
      vowels.lastIndex = 0
      currentSyllableCount++

      const nextVowel = vowelsMatches[currentSyllableCount]
      const nextWovelIndex = letters.indexOf(nextVowel, index + 1)
      const between = letters.slice(index + 1, nextWovelIndex)

      let indexSlice = -1

      indexSlice = indexSign(between)

      if (indexSlice === -1) {
        indexSlice = indexYoy(between)
      }

      if (indexSlice === -1) {
        indexSlice = indexSonoric(between)
      }

      if (indexSlice !== -1) {
        const sufix = between.slice(0, indexSlice + 1)
        currentSyllable += sufix.join('')
        skip = sufix.length
      }

      addSyllable(collection, currentSyllable)
      currentSyllable = ''
    }
  })

  return word.replace(/[а-яё]+/i, collection.join(separator))
}

const syllabifyWord = (word, { separator = '·' } = {}) => {
  return word
    .split('-')
    .map(word_ => _syllabifyWord(word_, { separator }))
    .join('-')
}

const syllabify = (source, { separator = '·' } = {}) => {
  const lines = splitLines(source)
  const modifyLines = lines.map(line => {
    const words = splitWords(line)
    const syllabifyedWords = words.map(word => syllabifyWord(word, { separator }))
    return joinWords(syllabifyedWords)
  })
  return joinLines(modifyLines)
}

module.exports.syllabify = syllabify
module.exports.syllabifyWord = syllabifyWord