/**
 * 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'

function diffStrings(a: string, b: string): Array<Diff> {
  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(a.length, b.length, isCommon, foundSubsequence)

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

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

  return diffs
}

export default diffStrings