diffStrings.ts

/**
 * Copyright (c) Meta Platforms, Inc. and affiliates.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 */

import * as diff from 'diff-sequences'
import { DIFF_DELETE, DIFF_EQUAL, DIFF_INSERT, Diff } from './cleanupSemantic'
import type { DiffOptions } from './types'

// platforms compatible
function getNewLineSymbol(string: string) {
  return string.includes('\r\n') ? '\r\n' : '\n'
}

function diffStrings(a: string, b: string, options?: DiffOptions): [Array<Diff>, boolean] {
  const truncate = options?.truncateThreshold ?? false
  const truncateThreshold = Math.max(Math.floor(options?.truncateThreshold ?? 0), 0)
  let aLength = a.length
  let bLength = b.length
  if (truncate) {
    const aMultipleLines = a.includes('\n')
    const bMultipleLines = b.includes('\n')
    const aNewLineSymbol = getNewLineSymbol(a)
    const bNewLineSymbol = getNewLineSymbol(b)
    // multiple-lines string expects a newline to be appended at the end
    const _a = aMultipleLines ? `${a.split(aNewLineSymbol, truncateThreshold).join(aNewLineSymbol)}\n` : a
    const _b = bMultipleLines ? `${b.split(bNewLineSymbol, truncateThreshold).join(bNewLineSymbol)}\n` : b
    aLength = _a.length
    bLength = _b.length
  }
  const truncated = aLength !== a.length || bLength !== b.length

  const isCommon = (aIndex: number, bIndex: number) => a[aIndex] === b[bIndex]

  let aIndex = 0
  let bIndex = 0
  const diffs: Array<Diff> = []

  const foundSubsequence = (
    nCommon: number,
    aCommon: number,
    bCommon: number,
  ) => {
    if (aIndex !== aCommon)
      diffs.push(new Diff(DIFF_DELETE, a.slice(aIndex, aCommon)))

    if (bIndex !== bCommon)
      diffs.push(new Diff(DIFF_INSERT, b.slice(bIndex, bCommon)))

    aIndex = aCommon + nCommon // number of characters compared in a
    bIndex = bCommon + nCommon // number of characters compared in b
    diffs.push(new Diff(DIFF_EQUAL, b.slice(bCommon, bIndex)))
  }

  // @ts-expect-error wrong bundling
  const diffSequences = diff.default.default || diff.default

  diffSequences(aLength, bLength, isCommon, foundSubsequence)

  // After the last common subsequence, push remaining change items.
  if (aIndex !== aLength)
    diffs.push(new Diff(DIFF_DELETE, a.slice(aIndex)))

  if (bIndex !== bLength)
    diffs.push(new Diff(DIFF_INSERT, b.slice(bIndex)))

  return [diffs, truncated]
}

export default diffStrings