Alto Code Diff

A modern PHP library to generate, render and apply diffs, featuring advanced algorithms, versatile rendering, and full patching support.

---

         

Features

Advanced Diff Algorithms

• Myers Diff Algorithm (default): Fast and accurate line-by-line and word-by-word diffing (O(ND)).
• LCS Diff Algorithm: Opt-in Longest Common Subsequence engine (O(MN) time and memory) for deterministic academic use cases.
• Binary Detection: Automatic detection and rejection of binary content.

Versatile Rendering

Visualize and format diffs for any output medium:

HTML Output

ANSI Side-by-Side

Full Patching Support

• Unified Diff Parsing: Parse standard unified diff patches into objects.
• Patch Application: Apply patches to files with "fuzz" factor support.
• Multi-file Bundles: Handle complex patches affecting multiple files.

Requirements

• PHP 8.3 or higher

Installation

composer require alto/code-diff

Quick Start

Basic Diff

use Alto\Code\Diff\Diff;
use Alto\Code\Diff\Renderer\UnifiedRenderer;

$old = "line1\nline2\nline3\n";
$new = "line1\nline2 modified\nline3\n";

$result = Diff::build()->compare($old, $new);

$renderer = new UnifiedRenderer('old.txt', 'new.txt');
echo $renderer->render($result);

Output:

--- old.txt
+++ new.txt
@@ -1,3 +1,3 @@
 line1
-line2
+line2 modified
 line3

Word-Level Diff

$result = Diff::build()
    ->withWordDiff()
    ->compare($old, $new);

HTML Output

use Alto\Code\Diff\Renderer\HtmlRenderer;

$renderer = new HtmlRenderer(
    showLineNumbers: true,
    wrapLines: false,
    classPrefix: 'diff-'
);

echo $renderer->render($result);

JSON Output

use Alto\Code\Diff\Renderer\JsonRenderer;

$renderer = new JsonRenderer(prettyPrint: true);
echo $renderer->render($result);

ANSI Side-by-Side Output

use Alto\Code\Diff\Renderer\AnsiSideBySideRenderer;

$renderer = new AnsiSideBySideRenderer(
    showLineNumbers: true,
    width: 120
);

echo $renderer->render($result);

Configuration Options

Context Lines

Control how many unchanged lines to show around changes:

$result = Diff::build()
    ->contextLines(5)  // Default is 3
    ->compare($old, $new);

Ignore Whitespace

Ignore whitespace differences:

$result = Diff::build()
    ->ignoreWhitespace()
    ->compare($old, $new);

Size Limits

Set maximum input size (default 5MB):

$result = Diff::build()
    ->maxBytes(10_000_000)  // 10MB
    ->compare($old, $new);

Parsing and Applying Patches

Parse a Unified Diff

use Alto\Code\Diff\Patch\UnifiedParser;

$patch = <<<'PATCH'
--- old.txt
+++ new.txt
@@ -1,3 +1,3 @@
 line1
-line2
+line2 modified
 line3
PATCH;

$parser = new UnifiedParser();
$bundle = $parser->parse($patch);

foreach ($bundle->files() as $file) {
    echo "File: {$file->oldPath} -> {$file->newPath}\n";
    echo "Hunks: " . count($file->result->hunks()) . "\n";
}

Apply a Patch

use Alto\Code\Diff\Patch\PatchApplier;

$original = "line1\nline2\nline3\n";

$applier = new PatchApplier();
$patched = $applier->apply($original, $patch);

echo $patched;
// Output: line1\nline2 modified\nline3\n

Note: PatchApplier::apply() accepts a single-file patch. For multi-file diffs, parse the patch and call applyBundle() instead.

Apply Patch with Fuzz Factor

$applier = new PatchApplier(fuzz: 2);
$patched = $applier->apply($original, $patch);

Apply Patch to Multiple Files

use Alto\Code\Diff\Model\DiffBundle;

$files = [
    'file1.txt' => "content1\n",
    'file2.txt' => "content2\n",
];

$applier = new PatchApplier();
$patchedFiles = $applier->applyBundle($files, $bundle);

Emitting Unified Diffs

From DiffResult

use Alto\Code\Diff\Patch\UnifiedEmitter;

$result = Diff::build()->compare($old, $new);

$emitter = new UnifiedEmitter();
$patch = $emitter->emit($result);

From DiffBundle

use Alto\Code\Diff\Model\DiffBundle;
use Alto\Code\Diff\Model\DiffFile;

$files = [
    new DiffFile('file1.txt', 'file1.txt', $result1),
    new DiffFile('file2.txt', 'file2.txt', $result2),
];

$bundle = new DiffBundle($files);
$emitter = new UnifiedEmitter();
$patch = $emitter->emit($bundle);

Renderer Options

UnifiedRenderer

new UnifiedRenderer(
    oldLabel: 'a/file.txt',  // Label for old version
    newLabel: 'b/file.txt'   // Label for new version
);

HtmlRenderer

new HtmlRenderer(
    showLineNumbers: true,      // Show line numbers
    wrapLines: false,           // Wrap long lines
    classPrefix: 'diff-'        // CSS class prefix
);

JsonRenderer

new JsonRenderer(
    prettyPrint: true  // Format with indentation
);

AnsiSideBySideRenderer

new AnsiSideBySideRenderer(
    showLineNumbers: true,  // Show line numbers
    width: 120              // Terminal width
);

Advanced Usage

Custom Diff Engine

You can choose between the built-in engines or implement your own.

MyersDiffEngine (Default): Uses the O(ND) Myers algorithm. Best for most use cases, especially when differences are small.

LcsDiffEngine: Uses the standard O(MN) LCS algorithm. Enable it explicitly with ->withEngine(new LcsDiffEngine()) only for small inputs, because its quadratic memory footprint is intended for controlled, academic scenarios.

use Alto\Code\Diff\Engine\LcsDiffEngine;

$result = Diff::build()
    ->withEngine(new LcsDiffEngine())
    ->compare($old, $new);

Implementing a Custom Engine

use Alto\Code\Diff\Engine\DiffEngineInterface;

class MyCustomEngine implements DiffEngineInterface
{
    public function diff(string $old, string $new, Options $opts): DiffResult
    {
        // Custom implementation
    }
}

$result = Diff::build()
    ->withEngine(new MyCustomEngine())
    ->compare($old, $new);

Working with Git Patches

The library supports parsing git-style unified diffs with headers:

$patch = <<<'PATCH'
diff --git a/file.txt b/file.txt
index abcdef..123456 100644
--- a/file.txt
+++ b/file.txt
@@ -1,3 +1,3 @@
 line1
-line2
+line2 modified
 line3
PATCH;

$parser = new UnifiedParser();
$bundle = $parser->parse($patch);

// Access headers
$file = $bundle->files()[0];
$file->headers['diff'];   // 'diff --git a/file.txt b/file.txt'
$file->headers['index'];  // 'index abcdef..123456 100644'

Documentation

• Installation: install the package and verify its requirements.
• Getting started: compare two strings and render a unified diff.
• Diffing: configure comparisons, inspect results, and select an engine.
• Rendering: produce unified text, HTML, JSON, or ANSI output.
• Formats: emit, parse, and apply unified patches.
• Errors: recover from rejected inputs and patches.
• Complete documentation: review the package scope and every guide.

Contributing

Contributions of all kinds are welcome. Visit the project on GitHub to report a bug, suggest a feature, or open a pull request.

Before submitting code, run:

# Runs PHP CS Fixer, PHPStan, and PHPUnit
composer qa

Changes to public behavior should include tests and documentation.

Support

ALTO Code Diff is open source and independently maintained by Simon André. If it is useful to your work, you can support its continued development through GitHub Sponsors.

Sharing the package or starring it on GitHub also helps.

License

ALTO Code Diff is released by ALTO PHP under the MIT License.