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Readme

llm-diff-patcher

A TypeScript library for applying "fuzzy" diffs, particularly designed for LLM-generated code modifications where line numbers may not be accurate.

Installation

npm install llm-diff-patcher

Usage

import { applyPatchToFiles } from 'llm-diff-patcher';

// Original source code
const sourceText = `function hello() {
  console.log("Hello, world!");
  return true;
}`;

// A diff to apply (in unified diff format)
const diffText = `@@ -1,4 +1,4 @@
 function hello() {
-  console.log("Hello, world!");
+  console.log("Hello, universe!");
   return true;
 }`;

// Apply the fuzzy diff
const result = applyDiff(sourceText, diffText);

console.log(result);
// Output:
// function hello() {
//   console.log("Hello, universe!");
//   return true;
// }

// For applying to files in a directory:
const patchResult = applyPatchToFiles(diffText, {
  basePath: '/path/to/project',
  dryRun: false
});

console.log(`Files: ${patchResult.successfulFiles}/${patchResult.totalFiles}, Hunks: ${patchResult.appliedHunks}/${patchResult.totalHunks}`);

Configuration Options

You can customize the behavior with options:

// When applying to files
const result = applyPatchToFiles(diffText, {
  // Base directory for resolving file paths
  basePath: '/path/to/project',
  
  // If true, doesn't actually write to files (default: false)
  dryRun: true,
  
  // Minimum number of context lines required for a match
  minContextLines: 2,
  
  // Options passed to the underlying diff library
  jsDiffApplyPatchOptions: {
    // Maximum Levenshtein distance between context in patch and actual file lines
    // Higher values allow more fuzzy matches (default: 0)
    fuzzFactor: 2,
    
    // Automatically handle different line endings between patch and source file
    // (default: true)
    autoConvertLineEndings: true,
    
    // Custom function to compare lines for equality when patching
    // Allows for very flexible fuzzy matching
    compareLine: (lineNumber, line, operation, patchContent) => {

      // Custom comparison logic here
      // Return true if lines should be considered equal, false otherwise
      return line.trim() === patchContent.trim(); // Example: ignore whitespace
    }
  }
});

Why llm-diff-patcher?

Unlike traditional diff tools that require exact line numbers to match, llm-diff-patcher is designed to handle the imprecise nature of diffs generated by Large Language Models (LLMs).

Key benefits:

Fuzzy Matching: Applies changes based on context even when line numbers are incorrect
Robust Error Handling: Provides detailed information about successful and failed patches
Flexible Configuration: Adjust matching strictness to your needs
Dry Run Mode: Test patches without modifying files

Use Cases

Applying code changes suggested by AI assistants like GPT, Claude, or other LLMs
Implementing code modifications across slightly different versions of files
Working with diffs that may not have perfect context or line numbers
Automated code refactoring with LLM-generated patches

API

The library provides several key functions:

applyPatchToFiles: Apply patch to multiple files in a directory
applyDiff: Apply a single diff to a string
parsePatch: Parse a patch string into a structured format
cleanPatch: Clean up and normalize patch text

Error Handling

The library provides detailed error information to help you understand what went wrong:

if (!patchResult.success) {
  console.error(`Failed to apply patch. Errors:`);
  patchResult.errors.forEach(error => {
    console.error(`- ${error.message} (${error.context})`);
  });
}

License

MIT