PhragmentPatchUtil.php
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	<?php

	final class PhragmentPatchUtil extends Phobject {

	const EMPTY_HASH = '0000000000000000000000000000000000000000';

	/**
	* Calculate the DiffMatchPatch patch between two Phabricator files.
	*
	* @phutil-external-symbol class diff_match_patch
	*/
	public static function calculatePatch(
	PhabricatorFile $old = null,
	PhabricatorFile $new = null) {

	$root = dirname(phutil_get_library_root('phabricator'));
	require_once $root.'/externals/diff_match_patch/diff_match_patch.php';

	$old_hash = self::EMPTY_HASH;
	$new_hash = self::EMPTY_HASH;

	if ($old !== null) {
	$old_hash = $old->getContentHash();
	}
	if ($new !== null) {
	$new_hash = $new->getContentHash();
	}

	$old_content = '';
	$new_content = '';

	if ($old_hash === $new_hash) {
	return null;
	}

	if ($old_hash !== self::EMPTY_HASH) {
	$old_content = $old->loadFileData();
	} else {
	$old_content = '';
	}

	if ($new_hash !== self::EMPTY_HASH) {
	$new_content = $new->loadFileData();
	} else {
	$new_content = '';
	}

	$dmp = new diff_match_patch();
	$dmp_patches = $dmp->patch_make($old_content, $new_content);
	return $dmp->patch_toText($dmp_patches);
	}

	}