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v1.3.0 2021-09-27 22:08 UTC

This package is auto-updated.

Last update: 2023-02-03 03:11:35 UTC


Continuous Integration

DiffLib is a Hack library for creating and parsing diffs. Diffs can be created between any two sequences of items. Additional helpers (such as support for unified diffs, and colored diffs) are provided for string diffs.

Diffs are represented as a sequence of DiffOp operations; these can 'keep' an element (i.e. it's unchanged in both sequences), insert an element, or delete an element. For example, in a unified diff, these would be:


String diffs are typically represented as a sequence of lines, but can also be represented as a sequence of characters, allowing intra-line diffs.

Diff output formats

difflib can create standard unified diffs:

@@ -1 +1@@ -Foo Bat Baz +Foo Bar Baz

difflib can also create colored diffs, with intra-line edits highlighted:

same diff as above, but with 'Bat' highlighted red, 'Bar' highlighted green

A concrete implementation is provided for standard CLI terminals, and an abstract base class for other implementations, such as HTML or XHP.


use namespace Facebook\DiffLib;

function create_unified_diff(string $from, string $to): string {
  return DiffLib\StringDiff::lines($from, $to)->getUnifiedDiff();

function create_colored_diff(string $from, string $to): string {
  return DiffLib\CLIColoredUnifiedDiff::create($from, $to);

final class IntDiff extends DiffLib\Diff {
  const type TContent = int;

function dump_int_diff(vec<int> $from, vec<int> $to): void {
  $diff = (new IntDiff($from, $to))->getDiff();
  foreach ($diff as $op) {
    if ($op is DiffLib\DiffKeepOp<_>) {
      \printf("  %d\n", $op->getContent());
    } else if ($op is DiffLib\DiffDeleteOp<_>) {
      \printf("- %d\n", $op->getContent());
    } else {
      $op = $op as DiffLib\DiffInsertOp<_>;
      printf("+ %d\n", $op->getContent());

dump_int_diff(vec[1, 3, 9], vec[1, 4, 9]) will produce this output:

- 3
+ 4


  • The current release version of HHVM

Installing DiffLib

composer require facebook/difflib

How DiffLib works

Diffs are created using Myers' diff algorithm: Myers, E.W. Algorithmica (1986) 1: 251. https://doi.org/10.1007/BF01840446

For more details, see the commented implementation.

Full documentation

  • DiffLib\Diff: abstract base class. Needs to be subclassed to operate on any particular type.
  • DiffLib\StringDiff: final class for diffing strings, and adds support for creating unified diffs
  • DiffLib\ColoredUnifiedDiff: abstract class for rendering unified diffs. Output may be any type - for example, strings or XHP.
  • DiffLib\CLIColoredUnifiedDiff: abstract final class for rendering unified diffs to a terminal that supports color escape sequences. Provides intra-line highlighting.
  • DiffLib\DiffOp: abstract class for a diff operation. Sealed to DiffInsertOp, DiffKeepOp, and DiffDeleteOp.
  • DiffLib\cluster(): utility function to group together sequential operations of the same kind. Converts vec<DiffOp<T>> to a generally shorter number of vec<DiffOp<vec<T>>>


DiffLib is MIT licensed, as found in the LICENSE file.