Fast Diff

const diff = require('fast-diff');

import * as diff from 'fast-diff';
// or
import diff = require('fast-diff');

const diff = require('fast-diff');

// Basic diff comparison
const result = diff('Good dog', 'Bad dog');
// Returns: [[-1, "Goo"], [1, "Ba"], [0, "d dog"]]

// Using operation constants
const operations = result.map(([op, text]) => {
  switch (op) {
    case diff.DELETE: return `Delete: "${text}"`;
    case diff.INSERT: return `Insert: "${text}"`;
    case diff.EQUAL: return `Keep: "${text}"`;
  }
});

/**
 * Find the differences between two texts using Myers' O(ND) Difference Algorithm
 * @param {string} text1 - Old string to be diffed
 * @param {string} text2 - New string to be diffed
 * @param {number|CursorInfo} [cursor_pos] - Edit position in text1 or cursor info object
 * @param {boolean} [cleanup] - Apply semantic cleanup before returning
 * @returns {Array<[number, string]>} Array of diff tuples [operation, text]
 */
function diff(text1, text2, cursor_pos, cleanup);

// Basic text comparison
const result = diff('abc', 'aec');
// Returns: [[0, "a"], [-1, "b"], [1, "e"], [0, "c"]]

// With cursor position (respects edit location for better results)
const result = diff('aaa', 'aaaa', 1);
// Returns: [[0, "a"], [1, "a"], [0, "aa"]]
// Insertion is positioned at cursor location

// With semantic cleanup (improves readability)
const result = diff('The quick brown fox', 'A quick brown fox', null, true);
// Applies semantic cleanup to make changes more meaningful

// Using cursor info object for complex edits
const result = diff('hello world', 'hello there world', {
  oldRange: { index: 6, length: 0 },
  newRange: { index: 6, length: 6 }
});

/** Constant representing a delete operation (-1) */
const DELETE = -1;

/** Constant representing an insert operation (1) */  
const INSERT = 1;

/** Constant representing an equality/no change (0) */
const EQUAL = 0;

const result = diff('old text', 'new text');

result.forEach(([operation, text]) => {
  switch (operation) {
    case diff.DELETE:
      console.log(`Remove: "${text}"`);
      break;
    case diff.INSERT:
      console.log(`Add: "${text}"`);
      break;
    case diff.EQUAL:
      console.log(`Keep: "${text}"`);
      break;
  }
});

// Check for specific operations
const hasInsertions = result.some(([op]) => op === diff.INSERT);
const hasDelations = result.some(([op]) => op === diff.DELETE);

/**
 * Main diff function signature for TypeScript
 */
declare function diff(
  text1: string,
  text2: string,
  cursorPos?: number | diff.CursorInfo,
  cleanup?: boolean
): diff.Diff[];

/**
 * Diff tuple representing an operation and its associated text
 */
type Diff = [-1 | 0 | 1, string];

/**
 * Interface for providing detailed cursor position information
 */
interface CursorInfo {
  /** Range in the old text */
  oldRange: { index: number; length: number };
  /** Range in the new text */  
  newRange: { index: number; length: number };
}

/**
 * Operation constants
 */
const DELETE: -1;
const INSERT: 1;
const EQUAL: 0;

// Example return value for diff('abc', 'ac'):
[
  [-1, "b"],    // Delete "b"
  [0, "ac"]     // Keep "ac"
]

// Example return value for diff('ac', 'abc'):
[
  [0, "a"],     // Keep "a"
  [1, "b"],     // Insert "b"  
  [0, "c"]      // Keep "c"
]