feat: `applyPatch` reversely

[benjycui]

It seems that we cannot apply patch on newStr to calculate oldStr now. I read source code https://github.com/kpdecker/jsdiff/blob/master/src/patch/apply.js#L85 , and thought that we can add an option to do so.

If you accept this feature, I will create a pull request :-)

[kpdecker]

I'm not sure I fully understand the feature, but if you want to write some tests to demonstrate it, go for it.

Generally I'm fine with adding contributed features as long as they don't add a lot of overhead relative to their general usefulness and have proper tests.

[Luiz-N]

@kpdecker I think @benjycui might be talking about the ability to "reverse" or "revert" a patch. I have a similar use case where if I apply a series of patches to a document how can I then "revert" them in the order they were applied. Is this possible or do I need to manually create a "reversed patch" for every one applied?

[ericbiewener]

Here's my method for reversing a patch. Happy to open a PR if this is the right approach, but I'm unsure if it would cover all use cases.

const reversePatch = (patch: string) => {
  const parsedDiff = diff.parsePatch(patch)[0]

  const { oldFileName, newFileName, oldHeader, newHeader, hunks } = parsedDiff

  parsedDiff.oldFileName = newFileName
  parsedDiff.oldHeader = newHeader
  parsedDiff.newFileName = oldFileName
  parsedDiff.newHeader = oldHeader

  for (const hunk of hunks) {
    const { oldLines, oldStart, newLines, newStart, lines } = hunk
    hunk.oldLines = newLines
    hunk.oldStart = newStart
    hunk.newLines = oldLines
    hunk.newStart = oldStart

    hunk.lines = lines.map(l => {
      if (l.startsWith('-')) return `+${l.slice(1)}`
      if (l.startsWith('+')) return `-${l.slice(1)}`
      return l
    })
  }

  return parsedDiff
}

[gh-andre]

I was looking for the same feature and found this issue. It would be very useful to be able to apply a patch in reverse, like patch --reverse does. This allows one to avoid having to store the very first original text and instead makes it possible to apply patches in reverse against the latest version.

For example, for a Wiki post, blog post, etc, I would store just the latest text version and all patches from edits and I could show limited history of patches going back from the latest version and when people want to see some older version, I would apply patches in reverse to get the full text for that version.