substance/util/diff.js

Substance is a JavaScript library for web-based content editing. It provides building blocks for realizing custom text editors and web-based publishing system. It is developed to power our online editing platform [Substance](http://substance.io).

import isString from './isString'
import levenshtein from './levenshtein'

/*
  Determines a list of changes to transform String a into String b.

  @param {String} a
  @param {String} b
*/
function diff (a, b, offset) {
  if (!isString(a) || !isString(b)) {
    throw new Error('Illegal arguments.')
  }
  offset = offset || 0
  let changes = []
  if (a || b) {
    if (!a && b) {
      changes.push({ type: 'insert', start: offset, text: b })
    } else if (a && !b) {
      changes.push({ type: 'delete', start: offset, end: offset + a.length })
    } else {
      const m = levenshtein(a, b)
      changes = _diff(a, b, m, offset)
    }
  }
  return changes
}

function _diff (a, b, m, offset) {
  let i = b.length
  let j = a.length
  const changes = []
  let current
  while (i > 0 && j > 0) {
    _next()
  }
  _commit()
  return changes

  function _next () {
    const d = m[i][j]
    const ib = i - 1
    const jb = j - 1
    // substitute
    if (m[ib][jb] < d) {
      if (current && current.type === 'replace') {
        current.start--
        current.text.unshift(b.charAt(ib))
      } else {
        _commit()
        current = { type: 'replace', start: jb, end: j, text: [b.charAt(ib)] }
      }
      i--
      j--
    // insert
    } else if (m[ib][j] < d) {
      if (current && current.type === 'insert') {
        current.start--
        current.text.unshift(b.charAt(ib))
      } else {
        _commit()
        current = { type: 'insert', start: jb, text: [b.charAt(ib)] }
      }
      i--
    // delete char
    } else if (m[i][jb] < d) {
      if (current && current.type === 'delete') {
        current.start--
      } else {
        _commit()
        current = { type: 'delete', start: jb, end: j }
      }
      j--
    // preserve
    } else {
      _commit()
      i--
      j--
    }
  }

  function _commit () {
    if (current) {
      switch (current.type) {
        case 'insert':
          current.start += offset
          current.text = current.text.join('')
          break
        case 'delete':
          current.start += offset
          current.end += offset
          break
        case 'replace':
          current.start += offset
          current.end += offset
          current.text = current.text.join('')
          break
        default:
          throw new Error('Invalid state')
      }
      changes.push(current)
      current = null
    }
  }
}

export default diff