diff.ts

/*---------------------------------------------------------------------------------------------
 *  Copyright (c) Microsoft Corporation. All rights reserved.
 *  Licensed under the MIT License. See License.txt in the project root for license information.
 *--------------------------------------------------------------------------------------------*/

import { DiffChange } from './diffChange.js';
import { stringHash } from '../hash.js';
import { Constants } from '../uint.js';

export class StringDiffSequence implements ISequence {

	constructor(private source: string) { }

	getElements(): Int32Array | number[] | string[] {
		const source = this.source;
		const characters = new Int32Array(source.length);
		for (let i = 0, len = source.length; i < len; i++) {
			characters[i] = source.charCodeAt(i);
		}
		return characters;
	}
}

export function stringDiff(original: string, modified: string, pretty: boolean): IDiffChange[] {
	return new LcsDiff(new StringDiffSequence(original), new StringDiffSequence(modified)).ComputeDiff(pretty).changes;
}

export interface ISequence {
	getElements(): Int32Array | number[] | string[];
	getStrictElement?(index: number): string;
}

export interface IDiffChange {
	/**
	 * The position of the first element in the original sequence which
	 * this change affects.
	 */
	originalStart: number;

	/**
	 * The number of elements from the original sequence which were
	 * affected.
	 */
	originalLength: number;

	/**
	 * The position of the first element in the modified sequence which
	 * this change affects.
	 */
	modifiedStart: number;

	/**
	 * The number of elements from the modified sequence which were
	 * affected (added).
	 */
	modifiedLength: number;
}

export interface IContinueProcessingPredicate {
	(furthestOriginalIndex: number, matchLengthOfLongest: number): boolean;
}

export interface IDiffResult {
	quitEarly: boolean;
	changes: IDiffChange[];
}

//
// The code below has been ported from a C# implementation in VS
//

class Debug {

	public static Assert(condition: boolean, message: string): void {
		if (!condition) {
			throw new Error(message);
		}
	}
}

class MyArray {
	/**
	 * Copies a range of elements from an Array starting at the specified source index and pastes
	 * them to another Array starting at the specified destination index. The length and the indexes
	 * are specified as 64-bit integers.
	 * sourceArray:
	 *		The Array that contains the data to copy.
	 * sourceIndex:
	 *		A 64-bit integer that represents the index in the sourceArray at which copying begins.
	 * destinationArray:
	 *		The Array that receives the data.
	 * destinationIndex:
	 *		A 64-bit integer that represents the index in the destinationArray at which storing begins.
	 * length:
	 *		A 64-bit integer that represents the number of elements to copy.
	 */
	public static Copy(sourceArray: unknown[], sourceIndex: number, destinationArray: unknown[], destinationIndex: number, length: number) {
		for (let i = 0; i < length; i++) {
			destinationArray[destinationIndex + i] = sourceArray[sourceIndex + i];
		}
	}
	public static Copy2(sourceArray: Int32Array, sourceIndex: number, destinationArray: Int32Array, destinationIndex: number, length: number) {
		for (let i = 0; i < length; i++) {
			destinationArray[destinationIndex + i] = sourceArray[sourceIndex + i];
		}
	}
}

//*****************************************************************************
// LcsDiff.cs
//
// An implementation of the difference algorithm described in
// "An O(ND) Difference Algorithm and its variations" by Eugene W. Myers
//
// Copyright (C) 2008 Microsoft Corporation @minifier_do_not_preserve
//*****************************************************************************

// Our total memory usage for storing history is (worst-case):
// 2 * [(MaxDifferencesHistory + 1) * (MaxDifferencesHistory + 1) - 1] * sizeof(int)
// 2 * [1448*1448 - 1] * 4 = 16773624 = 16MB
const enum LocalConstants {
	MaxDifferencesHistory = 1447
}

/**
 * A utility class which helps to create the set of DiffChanges from
 * a difference operation. This class accepts original DiffElements and
 * modified DiffElements that are involved in a particular change. The
 * MarkNextChange() method can be called to mark the separation between
 * distinct changes. At the end, the Changes property can be called to retrieve
 * the constructed changes.
 */
class DiffChangeHelper {

	private m_changes: DiffChange[];
	private m_originalStart: number;
	private m_modifiedStart: number;
	private m_originalCount: number;
	private m_modifiedCount: number;

	/**
	 * Constructs a new DiffChangeHelper for the given DiffSequences.
	 */
	constructor() {
		this.m_changes = [];
		this.m_originalStart = Constants.MAX_SAFE_SMALL_INTEGER;
		this.m_modifiedStart = Constants.MAX_SAFE_SMALL_INTEGER;
		this.m_originalCount = 0;
		this.m_modifiedCount = 0;
	}

	/**
	 * Marks the beginning of the next change in the set of differences.
	 */
	public MarkNextChange(): void {
		// Only add to the list if there is something to add
		if (this.m_originalCount > 0 || this.m_modifiedCount > 0) {
			// Add the new change to our list
			this.m_changes.push(new DiffChange(this.m_originalStart, this.m_originalCount,
				this.m_modifiedStart, this.m_modifiedCount));
		}

		// Reset for the next change
		this.m_originalCount = 0;
		this.m_modifiedCount = 0;
		this.m_originalStart = Constants.MAX_SAFE_SMALL_INTEGER;
		this.m_modifiedStart = Constants.MAX_SAFE_SMALL_INTEGER;
	}

	/**
	 * Adds the original element at the given position to the elements
	 * affected by the current change. The modified index gives context
	 * to the change position with respect to the original sequence.
	 * @param originalIndex The index of the original element to add.
	 * @param modifiedIndex The index of the modified element that provides corresponding position in the modified sequence.
	 */
	public AddOriginalElement(originalIndex: number, modifiedIndex: number) {
		// The 'true' start index is the smallest of the ones we've seen
		this.m_originalStart = Math.min(this.m_originalStart, originalIndex);
		this.m_modifiedStart = Math.min(this.m_modifiedStart, modifiedIndex);

		this.m_originalCount++;
	}

	/**
	 * Adds the modified element at the given position to the elements
	 * affected by the current change. The original index gives context
	 * to the change position with respect to the modified sequence.
	 * @param originalIndex The index of the original element that provides corresponding position in the original sequence.
	 * @param modifiedIndex The index of the modified element to add.
	 */
	public AddModifiedElement(originalIndex: number, modifiedIndex: number): void {
		// The 'true' start index is the smallest of the ones we've seen
		this.m_originalStart = Math.min(this.m_originalStart, originalIndex);
		this.m_modifiedStart = Math.min(this.m_modifiedStart, modifiedIndex);

		this.m_modifiedCount++;
	}

	/**
	 * Retrieves all of the changes marked by the class.
	 */
	public getChanges(): DiffChange[] {
		if (this.m_originalCount > 0 || this.m_modifiedCount > 0) {
			// Finish up on whatever is left
			this.MarkNextChange();
		}

		return this.m_changes;
	}

	/**
	 * Retrieves all of the changes marked by the class in the reverse order
	 */
	public getReverseChanges(): DiffChange[] {
		if (this.m_originalCount > 0 || this.m_modifiedCount > 0) {
			// Finish up on whatever is left
			this.MarkNextChange();
		}

		this.m_changes.reverse();
		return this.m_changes;
	}

}

/**
 * An implementation of the difference algorithm described in
 * "An O(ND) Difference Algorithm and its variations" by Eugene W. Myers
 */
export class LcsDiff {

	private readonly ContinueProcessingPredicate: IContinueProcessingPredicate | null;

	private readonly _originalSequence: ISequence;
	private readonly _modifiedSequence: ISequence;
	private readonly _hasStrings: boolean;
	private readonly _originalStringElements: string[];
	private readonly _originalElementsOrHash: Int32Array;
	private readonly _modifiedStringElements: string[];
	private readonly _modifiedElementsOrHash: Int32Array;

	private m_forwardHistory: Int32Array[];
	private m_reverseHistory: Int32Array[];

	/**
	 * Constructs the DiffFinder
	 */
	constructor(originalSequence: ISequence, modifiedSequence: ISequence, continueProcessingPredicate: IContinueProcessingPredicate | null = null) {
		this.ContinueProcessingPredicate = continueProcessingPredicate;

		this._originalSequence = originalSequence;
		this._modifiedSequence = modifiedSequence;

		const [originalStringElements, originalElementsOrHash, originalHasStrings] = LcsDiff._getElements(originalSequence);
		const [modifiedStringElements, modifiedElementsOrHash, modifiedHasStrings] = LcsDiff._getElements(modifiedSequence);

		this._hasStrings = (originalHasStrings && modifiedHasStrings);
		this._originalStringElements = originalStringElements;
		this._originalElementsOrHash = originalElementsOrHash;
		this._modifiedStringElements = modifiedStringElements;
		this._modifiedElementsOrHash = modifiedElementsOrHash;

		this.m_forwardHistory = [];
		this.m_reverseHistory = [];
	}

	private static _isStringArray(arr: Int32Array | number[] | string[]): arr is string[] {
		return (arr.length > 0 && typeof arr[0] === 'string');
	}

	private static _getElements(sequence: ISequence): [string[], Int32Array, boolean] {
		const elements = sequence.getElements();

		if (LcsDiff._isStringArray(elements)) {
			const hashes = new Int32Array(elements.length);
			for (let i = 0, len = elements.length; i < len; i++) {
				hashes[i] = stringHash(elements[i], 0);
			}
			return [elements, hashes, true];
		}

		if (elements instanceof Int32Array) {
			return [[], elements, false];
		}

		return [[], new Int32Array(elements), false];
	}

	private ElementsAreEqual(originalIndex: number, newIndex: number): boolean {
		if (this._originalElementsOrHash[originalIndex] !== this._modifiedElementsOrHash[newIndex]) {
			return false;
		}
		return (this._hasStrings ? this._originalStringElements[originalIndex] === this._modifiedStringElements[newIndex] : true);
	}

	private ElementsAreStrictEqual(originalIndex: number, newIndex: number): boolean {
		if (!this.ElementsAreEqual(originalIndex, newIndex)) {
			return false;
		}
		const originalElement = LcsDiff._getStrictElement(this._originalSequence, originalIndex);
		const modifiedElement = LcsDiff._getStrictElement(this._modifiedSequence, newIndex);
		return (originalElement === modifiedElement);
	}

	private static _getStrictElement(sequence: ISequence, index: number): string | null {
		if (typeof sequence.getStrictElement === 'function') {
			return sequence.getStrictElement(index);
		}
		return null;
	}

	private OriginalElementsAreEqual(index1: number, index2: number): boolean {
		if (this._originalElementsOrHash[index1] !== this._originalElementsOrHash[index2]) {
			return false;
		}
		return (this._hasStrings ? this._originalStringElements[index1] === this._originalStringElements[index2] : true);
	}

	private ModifiedElementsAreEqual(index1: number, index2: number): boolean {
		if (this._modifiedElementsOrHash[index1] !== this._modifiedElementsOrHash[index2]) {
			return false;
		}
		return (this._hasStrings ? this._modifiedStringElements[index1] === this._modifiedStringElements[index2] : true);
	}

	public ComputeDiff(pretty: boolean): IDiffResult {
		return this._ComputeDiff(0, this._originalElementsOrHash.length - 1, 0, this._modifiedElementsOrHash.length - 1, pretty);
	}

	/**
	 * Computes the differences between the original and modified input
	 * sequences on the bounded range.
	 * @returns An array of the differences between the two input sequences.
	 */
	private _ComputeDiff(originalStart: number, originalEnd: number, modifiedStart: number, modifiedEnd: number, pretty: boolean): IDiffResult {
		const quitEarlyArr = [false];
		let changes = this.ComputeDiffRecursive(originalStart, originalEnd, modifiedStart, modifiedEnd, quitEarlyArr);

		if (pretty) {
			// We have to clean up the computed diff to be more intuitive
			// but it turns out this cannot be done correctly until the entire set
			// of diffs have been computed
			changes = this.PrettifyChanges(changes);
		}

		return {
			quitEarly: quitEarlyArr[0],
			changes: changes
		};
	}

	/**
	 * Private helper method which computes the differences on the bounded range
	 * recursively.
	 * @returns An array of the differences between the two input sequences.
	 */
	private ComputeDiffRecursive(originalStart: number, originalEnd: number, modifiedStart: number, modifiedEnd: number, quitEarlyArr: boolean[]): DiffChange[] {
		quitEarlyArr[0] = false;

		// Find the start of the differences
		while (originalStart <= originalEnd && modifiedStart <= modifiedEnd && this.ElementsAreEqual(originalStart, modifiedStart)) {
			originalStart++;
			modifiedStart++;
		}

		// Find the end of the differences
		while (originalEnd >= originalStart && modifiedEnd >= modifiedStart && this.ElementsAreEqual(originalEnd, modifiedEnd)) {
			originalEnd--;
			modifiedEnd--;
		}

		// In the special case where we either have all insertions or all deletions or the sequences are identical
		if (originalStart > originalEnd || modifiedStart > modifiedEnd) {
			let changes: DiffChange[];

			if (modifiedStart <= modifiedEnd) {
				Debug.Assert(originalStart === originalEnd + 1, 'originalStart should only be one more than originalEnd');

				// All insertions
				changes = [
					new DiffChange(originalStart, 0, modifiedStart, modifiedEnd - modifiedStart + 1)
				];
			} else if (originalStart <= originalEnd) {
				Debug.Assert(modifiedStart === modifiedEnd + 1, 'modifiedStart should only be one more than modifiedEnd');

				// All deletions
				changes = [
					new DiffChange(originalStart, originalEnd - originalStart + 1, modifiedStart, 0)
				];
			} else {
				Debug.Assert(originalStart === originalEnd + 1, 'originalStart should only be one more than originalEnd');
				Debug.Assert(modifiedStart === modifiedEnd + 1, 'modifiedStart should only be one more than modifiedEnd');

				// Identical sequences - No differences
				changes = [];
			}

			return changes;
		}

		// This problem can be solved using the Divide-And-Conquer technique.
		const midOriginalArr = [0];
		const midModifiedArr = [0];
		const result = this.ComputeRecursionPoint(originalStart, originalEnd, modifiedStart, modifiedEnd, midOriginalArr, midModifiedArr, quitEarlyArr);

		const midOriginal = midOriginalArr[0];
		const midModified = midModifiedArr[0];

		if (result !== null) {
			// Result is not-null when there was enough memory to compute the changes while
			// searching for the recursion point
			return result;
		} else if (!quitEarlyArr[0]) {
			// We can break the problem down recursively by finding the changes in the
			// First Half:   (originalStart, modifiedStart) to (midOriginal, midModified)
			// Second Half:  (midOriginal + 1, minModified + 1) to (originalEnd, modifiedEnd)
			// NOTE: ComputeDiff() is inclusive, therefore the second range starts on the next point

			const leftChanges = this.ComputeDiffRecursive(originalStart, midOriginal, modifiedStart, midModified, quitEarlyArr);
			let rightChanges: DiffChange[] = [];

			if (!quitEarlyArr[0]) {
				rightChanges = this.ComputeDiffRecursive(midOriginal + 1, originalEnd, midModified + 1, modifiedEnd, quitEarlyArr);
			} else {
				// We didn't have time to finish the first half, so we don't have time to compute this half.
				// Consider the entire rest of the sequence different.
				rightChanges = [
					new DiffChange(midOriginal + 1, originalEnd - (midOriginal + 1) + 1, midModified + 1, modifiedEnd - (midModified + 1) + 1)
				];
			}

			return this.ConcatenateChanges(leftChanges, rightChanges);
		}

		// If we hit here, we quit early, and so can't return anything meaningful
		return [
			new DiffChange(originalStart, originalEnd - originalStart + 1, modifiedStart, modifiedEnd - modifiedStart + 1)
		];
	}

	private WALKTRACE(diagonalForwardBase: number, diagonalForwardStart: number, diagonalForwardEnd: number, diagonalForwardOffset: number,
		diagonalReverseBase: number, diagonalReverseStart: number, diagonalReverseEnd: number, diagonalReverseOffset: number,
		forwardPoints: Int32Array, reversePoints: Int32Array,
		originalIndex: number, originalEnd: number, midOriginalArr: number[],
		modifiedIndex: number, modifiedEnd: number, midModifiedArr: number[],
		deltaIsEven: boolean, quitEarlyArr: boolean[]
	): DiffChange[] {
		let forwardChanges: DiffChange[] | null = null;
		let reverseChanges: DiffChange[] | null = null;

		// First, walk backward through the forward diagonals history
		let changeHelper = new DiffChangeHelper();
		let diagonalMin = diagonalForwardStart;
		let diagonalMax = diagonalForwardEnd;
		let diagonalRelative = (midOriginalArr[0] - midModifiedArr[0]) - diagonalForwardOffset;
		let lastOriginalIndex = Constants.MIN_SAFE_SMALL_INTEGER;
		let historyIndex = this.m_forwardHistory.length - 1;

		do {
			// Get the diagonal index from the relative diagonal number
			const diagonal = diagonalRelative + diagonalForwardBase;

			// Figure out where we came from
			if (diagonal === diagonalMin || (diagonal < diagonalMax && forwardPoints[diagonal - 1] < forwardPoints[diagonal + 1])) {
				// Vertical line (the element is an insert)
				originalIndex = forwardPoints[diagonal + 1];
				modifiedIndex = originalIndex - diagonalRelative - diagonalForwardOffset;
				if (originalIndex < lastOriginalIndex) {
					changeHelper.MarkNextChange();
				}
				lastOriginalIndex = originalIndex;
				changeHelper.AddModifiedElement(originalIndex + 1, modifiedIndex);
				diagonalRelative = (diagonal + 1) - diagonalForwardBase; //Setup for the next iteration
			} else {
				// Horizontal line (the element is a deletion)
				originalIndex = forwardPoints[diagonal - 1] + 1;
				modifiedIndex = originalIndex - diagonalRelative - diagonalForwardOffset;
				if (originalIndex < lastOriginalIndex) {
					changeHelper.MarkNextChange();
				}
				lastOriginalIndex = originalIndex - 1;
				changeHelper.AddOriginalElement(originalIndex, modifiedIndex + 1);
				diagonalRelative = (diagonal - 1) - diagonalForwardBase; //Setup for the next iteration
			}

			if (historyIndex >= 0) {
				forwardPoints = this.m_forwardHistory[historyIndex];
				diagonalForwardBase = forwardPoints[0]; //We stored this in the first spot
				diagonalMin = 1;
				diagonalMax = forwardPoints.length - 1;
			}
		} while (--historyIndex >= -1);

		// Ironically, we get the forward changes as the reverse of the
		// order we added them since we technically added them backwards
		forwardChanges = changeHelper.getReverseChanges();

		if (quitEarlyArr[0]) {
			// TODO: Calculate a partial from the reverse diagonals.
			//       For now, just assume everything after the midOriginal/midModified point is a diff

			let originalStartPoint = midOriginalArr[0] + 1;
			let modifiedStartPoint = midModifiedArr[0] + 1;

			if (forwardChanges !== null && forwardChanges.length > 0) {
				const lastForwardChange = forwardChanges[forwardChanges.length - 1];
				originalStartPoint = Math.max(originalStartPoint, lastForwardChange.getOriginalEnd());
				modifiedStartPoint = Math.max(modifiedStartPoint, lastForwardChange.getModifiedEnd());
			}

			reverseChanges = [
				new DiffChange(originalStartPoint, originalEnd - originalStartPoint + 1,
					modifiedStartPoint, modifiedEnd - modifiedStartPoint + 1)
			];
		} else {
			// Now walk backward through the reverse diagonals history
			changeHelper = new DiffChangeHelper();
			diagonalMin = diagonalReverseStart;
			diagonalMax = diagonalReverseEnd;
			diagonalRelative = (midOriginalArr[0] - midModifiedArr[0]) - diagonalReverseOffset;
			lastOriginalIndex = Constants.MAX_SAFE_SMALL_INTEGER;
			historyIndex = (deltaIsEven) ? this.m_reverseHistory.length - 1 : this.m_reverseHistory.length - 2;

			do {
				// Get the diagonal index from the relative diagonal number
				const diagonal = diagonalRelative + diagonalReverseBase;

				// Figure out where we came from
				if (diagonal === diagonalMin || (diagonal < diagonalMax && reversePoints[diagonal - 1] >= reversePoints[diagonal + 1])) {
					// Horizontal line (the element is a deletion))
					originalIndex = reversePoints[diagonal + 1] - 1;
					modifiedIndex = originalIndex - diagonalRelative - diagonalReverseOffset;
					if (originalIndex > lastOriginalIndex) {
						changeHelper.MarkNextChange();
					}
					lastOriginalIndex = originalIndex + 1;
					changeHelper.AddOriginalElement(originalIndex + 1, modifiedIndex + 1);
					diagonalRelative = (diagonal + 1) - diagonalReverseBase; //Setup for the next iteration
				} else {
					// Vertical line (the element is an insertion)
					originalIndex = reversePoints[diagonal - 1];
					modifiedIndex = originalIndex - diagonalRelative - diagonalReverseOffset;
					if (originalIndex > lastOriginalIndex) {
						changeHelper.MarkNextChange();
					}
					lastOriginalIndex = originalIndex;
					changeHelper.AddModifiedElement(originalIndex + 1, modifiedIndex + 1);
					diagonalRelative = (diagonal - 1) - diagonalReverseBase; //Setup for the next iteration
				}

				if (historyIndex >= 0) {
					reversePoints = this.m_reverseHistory[historyIndex];
					diagonalReverseBase = reversePoints[0]; //We stored this in the first spot
					diagonalMin = 1;
					diagonalMax = reversePoints.length - 1;
				}
			} while (--historyIndex >= -1);

			// There are cases where the reverse history will find diffs that
			// are correct, but not intuitive, so we need shift them.
			reverseChanges = changeHelper.getChanges();
		}

		return this.ConcatenateChanges(forwardChanges, reverseChanges);
	}

	/**
	 * Given the range to compute the diff on, this method finds the point:
	 * (midOriginal, midModified)
	 * that exists in the middle of the LCS of the two sequences and
	 * is the point at which the LCS problem may be broken down recursively.
	 * This method will try to keep the LCS trace in memory. If the LCS recursion
	 * point is calculated and the full trace is available in memory, then this method
	 * will return the change list.
	 * @param originalStart The start bound of the original sequence range
	 * @param originalEnd The end bound of the original sequence range
	 * @param modifiedStart The start bound of the modified sequence range
	 * @param modifiedEnd The end bound of the modified sequence range
	 * @param midOriginal The middle point of the original sequence range
	 * @param midModified The middle point of the modified sequence range
	 * @returns The diff changes, if available, otherwise null
	 */
	private ComputeRecursionPoint(originalStart: number, originalEnd: number, modifiedStart: number, modifiedEnd: number, midOriginalArr: number[], midModifiedArr: number[], quitEarlyArr: boolean[]) {
		let originalIndex = 0, modifiedIndex = 0;
		let diagonalForwardStart = 0, diagonalForwardEnd = 0;
		let diagonalReverseStart = 0, diagonalReverseEnd = 0;

		// To traverse the edit graph and produce the proper LCS, our actual
		// start position is just outside the given boundary
		originalStart--;
		modifiedStart--;

		// We set these up to make the compiler happy, but they will
		// be replaced before we return with the actual recursion point
		midOriginalArr[0] = 0;
		midModifiedArr[0] = 0;

		// Clear out the history
		this.m_forwardHistory = [];
		this.m_reverseHistory = [];

		// Each cell in the two arrays corresponds to a diagonal in the edit graph.
		// The integer value in the cell represents the originalIndex of the furthest
		// reaching point found so far that ends in that diagonal.
		// The modifiedIndex can be computed mathematically from the originalIndex and the diagonal number.
		const maxDifferences = (originalEnd - originalStart) + (modifiedEnd - modifiedStart);
		const numDiagonals = maxDifferences + 1;
		const forwardPoints = new Int32Array(numDiagonals);
		const reversePoints = new Int32Array(numDiagonals);
		// diagonalForwardBase: Index into forwardPoints of the diagonal which passes through (originalStart, modifiedStart)
		// diagonalReverseBase: Index into reversePoints of the diagonal which passes through (originalEnd, modifiedEnd)
		const diagonalForwardBase = (modifiedEnd - modifiedStart);
		const diagonalReverseBase = (originalEnd - originalStart);
		// diagonalForwardOffset: Geometric offset which allows modifiedIndex to be computed from originalIndex and the
		//    diagonal number (relative to diagonalForwardBase)
		// diagonalReverseOffset: Geometric offset which allows modifiedIndex to be computed from originalIndex and the
		//    diagonal number (relative to diagonalReverseBase)
		const diagonalForwardOffset = (originalStart - modifiedStart);
		const diagonalReverseOffset = (originalEnd - modifiedEnd);

		// delta: The difference between the end diagonal and the start diagonal. This is used to relate diagonal numbers
		//   relative to the start diagonal with diagonal numbers relative to the end diagonal.
		// The Even/Oddn-ness of this delta is important for determining when we should check for overlap
		const delta = diagonalReverseBase - diagonalForwardBase;
		const deltaIsEven = (delta % 2 === 0);

		// Here we set up the start and end points as the furthest points found so far
		// in both the forward and reverse directions, respectively
		forwardPoints[diagonalForwardBase] = originalStart;
		reversePoints[diagonalReverseBase] = originalEnd;

		// Remember if we quit early, and thus need to do a best-effort result instead of a real result.
		quitEarlyArr[0] = false;



		// A couple of points:
		// --With this method, we iterate on the number of differences between the two sequences.
		//   The more differences there actually are, the longer this will take.
		// --Also, as the number of differences increases, we have to search on diagonals further
		//   away from the reference diagonal (which is diagonalForwardBase for forward, diagonalReverseBase for reverse).
		// --We extend on even diagonals (relative to the reference diagonal) only when numDifferences
		//   is even and odd diagonals only when numDifferences is odd.
		for (let numDifferences = 1; numDifferences <= (maxDifferences / 2) + 1; numDifferences++) {
			let furthestOriginalIndex = 0;
			let furthestModifiedIndex = 0;

			// Run the algorithm in the forward direction
			diagonalForwardStart = this.ClipDiagonalBound(diagonalForwardBase - numDifferences, numDifferences, diagonalForwardBase, numDiagonals);
			diagonalForwardEnd = this.ClipDiagonalBound(diagonalForwardBase + numDifferences, numDifferences, diagonalForwardBase, numDiagonals);
			for (let diagonal = diagonalForwardStart; diagonal <= diagonalForwardEnd; diagonal += 2) {
				// STEP 1: We extend the furthest reaching point in the present diagonal
				// by looking at the diagonals above and below and picking the one whose point
				// is further away from the start point (originalStart, modifiedStart)
				if (diagonal === diagonalForwardStart || (diagonal < diagonalForwardEnd && forwardPoints[diagonal - 1] < forwardPoints[diagonal + 1])) {
					originalIndex = forwardPoints[diagonal + 1];
				} else {
					originalIndex = forwardPoints[diagonal - 1] + 1;
				}
				modifiedIndex = originalIndex - (diagonal - diagonalForwardBase) - diagonalForwardOffset;

				// Save the current originalIndex so we can test for false overlap in step 3
				const tempOriginalIndex = originalIndex;

				// STEP 2: We can continue to extend the furthest reaching point in the present diagonal
				// so long as the elements are equal.
				while (originalIndex < originalEnd && modifiedIndex < modifiedEnd && this.ElementsAreEqual(originalIndex + 1, modifiedIndex + 1)) {
					originalIndex++;
					modifiedIndex++;
				}
				forwardPoints[diagonal] = originalIndex;

				if (originalIndex + modifiedIndex > furthestOriginalIndex + furthestModifiedIndex) {
					furthestOriginalIndex = originalIndex;
					furthestModifiedIndex = modifiedIndex;
				}

				// STEP 3: If delta is odd (overlap first happens on forward when delta is odd)
				// and diagonal is in the range of reverse diagonals computed for numDifferences-1
				// (the previous iteration; we haven't computed reverse diagonals for numDifferences yet)
				// then check for overlap.
				if (!deltaIsEven && Math.abs(diagonal - diagonalReverseBase) <= (numDifferences - 1)) {
					if (originalIndex >= reversePoints[diagonal]) {
						midOriginalArr[0] = originalIndex;
						midModifiedArr[0] = modifiedIndex;

						if (tempOriginalIndex <= reversePoints[diagonal] && LocalConstants.MaxDifferencesHistory > 0 && numDifferences <= (LocalConstants.MaxDifferencesHistory + 1)) {
							// BINGO! We overlapped, and we have the full trace in memory!
							return this.WALKTRACE(diagonalForwardBase, diagonalForwardStart, diagonalForwardEnd, diagonalForwardOffset,
								diagonalReverseBase, diagonalReverseStart, diagonalReverseEnd, diagonalReverseOffset,
								forwardPoints, reversePoints,
								originalIndex, originalEnd, midOriginalArr,
								modifiedIndex, modifiedEnd, midModifiedArr,
								deltaIsEven, quitEarlyArr
							);
						} else {
							// Either false overlap, or we didn't have enough memory for the full trace
							// Just return the recursion point
							return null;
						}
					}
				}
			}

			// Check to see if we should be quitting early, before moving on to the next iteration.
			const matchLengthOfLongest = ((furthestOriginalIndex - originalStart) + (furthestModifiedIndex - modifiedStart) - numDifferences) / 2;

			if (this.ContinueProcessingPredicate !== null && !this.ContinueProcessingPredicate(furthestOriginalIndex, matchLengthOfLongest)) {
				// We can't finish, so skip ahead to generating a result from what we have.
				quitEarlyArr[0] = true;

				// Use the furthest distance we got in the forward direction.
				midOriginalArr[0] = furthestOriginalIndex;
				midModifiedArr[0] = furthestModifiedIndex;

				if (matchLengthOfLongest > 0 && LocalConstants.MaxDifferencesHistory > 0 && numDifferences <= (LocalConstants.MaxDifferencesHistory + 1)) {
					// Enough of the history is in memory to walk it backwards
					return this.WALKTRACE(diagonalForwardBase, diagonalForwardStart, diagonalForwardEnd, diagonalForwardOffset,
						diagonalReverseBase, diagonalReverseStart, diagonalReverseEnd, diagonalReverseOffset,
						forwardPoints, reversePoints,
						originalIndex, originalEnd, midOriginalArr,
						modifiedIndex, modifiedEnd, midModifiedArr,
						deltaIsEven, quitEarlyArr
					);
				} else {
					// We didn't actually remember enough of the history.

					//Since we are quitting the diff early, we need to shift back the originalStart and modified start
					//back into the boundary limits since we decremented their value above beyond the boundary limit.
					originalStart++;
					modifiedStart++;

					return [
						new DiffChange(originalStart, originalEnd - originalStart + 1,
							modifiedStart, modifiedEnd - modifiedStart + 1)
					];
				}
			}

			// Run the algorithm in the reverse direction
			diagonalReverseStart = this.ClipDiagonalBound(diagonalReverseBase - numDifferences, numDifferences, diagonalReverseBase, numDiagonals);
			diagonalReverseEnd = this.ClipDiagonalBound(diagonalReverseBase + numDifferences, numDifferences, diagonalReverseBase, numDiagonals);
			for (let diagonal = diagonalReverseStart; diagonal <= diagonalReverseEnd; diagonal += 2) {
				// STEP 1: We extend the furthest reaching point in the present diagonal
				// by looking at the diagonals above and below and picking the one whose point
				// is further away from the start point (originalEnd, modifiedEnd)
				if (diagonal === diagonalReverseStart || (diagonal < diagonalReverseEnd && reversePoints[diagonal - 1] >= reversePoints[diagonal + 1])) {
					originalIndex = reversePoints[diagonal + 1] - 1;
				} else {
					originalIndex = reversePoints[diagonal - 1];
				}
				modifiedIndex = originalIndex - (diagonal - diagonalReverseBase) - diagonalReverseOffset;

				// Save the current originalIndex so we can test for false overlap
				const tempOriginalIndex = originalIndex;

				// STEP 2: We can continue to extend the furthest reaching point in the present diagonal
				// as long as the elements are equal.
				while (originalIndex > originalStart && modifiedIndex > modifiedStart && this.ElementsAreEqual(originalIndex, modifiedIndex)) {
					originalIndex--;
					modifiedIndex--;
				}
				reversePoints[diagonal] = originalIndex;

				// STEP 4: If delta is even (overlap first happens on reverse when delta is even)
				// and diagonal is in the range of forward diagonals computed for numDifferences
				// then check for overlap.
				if (deltaIsEven && Math.abs(diagonal - diagonalForwardBase) <= numDifferences) {
					if (originalIndex <= forwardPoints[diagonal]) {
						midOriginalArr[0] = originalIndex;
						midModifiedArr[0] = modifiedIndex;

						if (tempOriginalIndex >= forwardPoints[diagonal] && LocalConstants.MaxDifferencesHistory > 0 && numDifferences <= (LocalConstants.MaxDifferencesHistory + 1)) {
							// BINGO! We overlapped, and we have the full trace in memory!
							return this.WALKTRACE(diagonalForwardBase, diagonalForwardStart, diagonalForwardEnd, diagonalForwardOffset,
								diagonalReverseBase, diagonalReverseStart, diagonalReverseEnd, diagonalReverseOffset,
								forwardPoints, reversePoints,
								originalIndex, originalEnd, midOriginalArr,
								modifiedIndex, modifiedEnd, midModifiedArr,
								deltaIsEven, quitEarlyArr
							);
						} else {
							// Either false overlap, or we didn't have enough memory for the full trace
							// Just return the recursion point
							return null;
						}
					}
				}
			}

			// Save current vectors to history before the next iteration
			if (numDifferences <= LocalConstants.MaxDifferencesHistory) {
				// We are allocating space for one extra int, which we fill with
				// the index of the diagonal base index
				let temp = new Int32Array(diagonalForwardEnd - diagonalForwardStart + 2);
				temp[0] = diagonalForwardBase - diagonalForwardStart + 1;
				MyArray.Copy2(forwardPoints, diagonalForwardStart, temp, 1, diagonalForwardEnd - diagonalForwardStart + 1);
				this.m_forwardHistory.push(temp);

				temp = new Int32Array(diagonalReverseEnd - diagonalReverseStart + 2);
				temp[0] = diagonalReverseBase - diagonalReverseStart + 1;
				MyArray.Copy2(reversePoints, diagonalReverseStart, temp, 1, diagonalReverseEnd - diagonalReverseStart + 1);
				this.m_reverseHistory.push(temp);
			}

		}

		// If we got here, then we have the full trace in history. We just have to convert it to a change list
		// NOTE: This part is a bit messy
		return this.WALKTRACE(diagonalForwardBase, diagonalForwardStart, diagonalForwardEnd, diagonalForwardOffset,
			diagonalReverseBase, diagonalReverseStart, diagonalReverseEnd, diagonalReverseOffset,
			forwardPoints, reversePoints,
			originalIndex, originalEnd, midOriginalArr,
			modifiedIndex, modifiedEnd, midModifiedArr,
			deltaIsEven, quitEarlyArr
		);
	}

	/**
	 * Shifts the given changes to provide a more intuitive diff.
	 * While the first element in a diff matches the first element after the diff,
	 * we shift the diff down.
	 *
	 * @param changes The list of changes to shift
	 * @returns The shifted changes
	 */
	private PrettifyChanges(changes: DiffChange[]): DiffChange[] {

		// Shift all the changes down first
		for (let i = 0; i < changes.length; i++) {
			const change = changes[i];
			const originalStop = (i < changes.length - 1) ? changes[i + 1].originalStart : this._originalElementsOrHash.length;
			const modifiedStop = (i < changes.length - 1) ? changes[i + 1].modifiedStart : this._modifiedElementsOrHash.length;
			const checkOriginal = change.originalLength > 0;
			const checkModified = change.modifiedLength > 0;

			while (
				change.originalStart + change.originalLength < originalStop
				&& change.modifiedStart + change.modifiedLength < modifiedStop
				&& (!checkOriginal || this.OriginalElementsAreEqual(change.originalStart, change.originalStart + change.originalLength))
				&& (!checkModified || this.ModifiedElementsAreEqual(change.modifiedStart, change.modifiedStart + change.modifiedLength))
			) {
				const startStrictEqual = this.ElementsAreStrictEqual(change.originalStart, change.modifiedStart);
				const endStrictEqual = this.ElementsAreStrictEqual(change.originalStart + change.originalLength, change.modifiedStart + change.modifiedLength);
				if (endStrictEqual && !startStrictEqual) {
					// moving the change down would create an equal change, but the elements are not strict equal
					break;
				}
				change.originalStart++;
				change.modifiedStart++;
			}

			const mergedChangeArr: Array<DiffChange | null> = [null];
			if (i < changes.length - 1 && this.ChangesOverlap(changes[i], changes[i + 1], mergedChangeArr)) {
				changes[i] = mergedChangeArr[0]!;
				changes.splice(i + 1, 1);
				i--;
				continue;
			}
		}

		// Shift changes back up until we hit empty or whitespace-only lines
		for (let i = changes.length - 1; i >= 0; i--) {
			const change = changes[i];

			let originalStop = 0;
			let modifiedStop = 0;
			if (i > 0) {
				const prevChange = changes[i - 1];
				originalStop = prevChange.originalStart + prevChange.originalLength;
				modifiedStop = prevChange.modifiedStart + prevChange.modifiedLength;
			}

			const checkOriginal = change.originalLength > 0;
			const checkModified = change.modifiedLength > 0;

			let bestDelta = 0;
			let bestScore = this._boundaryScore(change.originalStart, change.originalLength, change.modifiedStart, change.modifiedLength);

			for (let delta = 1; ; delta++) {
				const originalStart = change.originalStart - delta;
				const modifiedStart = change.modifiedStart - delta;

				if (originalStart < originalStop || modifiedStart < modifiedStop) {
					break;
				}

				if (checkOriginal && !this.OriginalElementsAreEqual(originalStart, originalStart + change.originalLength)) {
					break;
				}

				if (checkModified && !this.ModifiedElementsAreEqual(modifiedStart, modifiedStart + change.modifiedLength)) {
					break;
				}

				const touchingPreviousChange = (originalStart === originalStop && modifiedStart === modifiedStop);
				const score = (
					(touchingPreviousChange ? 5 : 0)
					+ this._boundaryScore(originalStart, change.originalLength, modifiedStart, change.modifiedLength)
				);

				if (score > bestScore) {
					bestScore = score;
					bestDelta = delta;
				}
			}

			change.originalStart -= bestDelta;
			change.modifiedStart -= bestDelta;

			const mergedChangeArr: Array<DiffChange | null> = [null];
			if (i > 0 && this.ChangesOverlap(changes[i - 1], changes[i], mergedChangeArr)) {
				changes[i - 1] = mergedChangeArr[0]!;
				changes.splice(i, 1);
				i++;
				continue;
			}
		}

		// There could be multiple longest common substrings.
		// Give preference to the ones containing longer lines
		if (this._hasStrings) {
			for (let i = 1, len = changes.length; i < len; i++) {
				const aChange = changes[i - 1];
				const bChange = changes[i];
				const matchedLength = bChange.originalStart - aChange.originalStart - aChange.originalLength;
				const aOriginalStart = aChange.originalStart;
				const bOriginalEnd = bChange.originalStart + bChange.originalLength;
				const abOriginalLength = bOriginalEnd - aOriginalStart;
				const aModifiedStart = aChange.modifiedStart;
				const bModifiedEnd = bChange.modifiedStart + bChange.modifiedLength;
				const abModifiedLength = bModifiedEnd - aModifiedStart;
				// Avoid wasting a lot of time with these searches
				if (matchedLength < 5 && abOriginalLength < 20 && abModifiedLength < 20) {
					const t = this._findBetterContiguousSequence(
						aOriginalStart, abOriginalLength,
						aModifiedStart, abModifiedLength,
						matchedLength
					);
					if (t) {
						const [originalMatchStart, modifiedMatchStart] = t;
						if (originalMatchStart !== aChange.originalStart + aChange.originalLength || modifiedMatchStart !== aChange.modifiedStart + aChange.modifiedLength) {
							// switch to another sequence that has a better score
							aChange.originalLength = originalMatchStart - aChange.originalStart;
							aChange.modifiedLength = modifiedMatchStart - aChange.modifiedStart;
							bChange.originalStart = originalMatchStart + matchedLength;
							bChange.modifiedStart = modifiedMatchStart + matchedLength;
							bChange.originalLength = bOriginalEnd - bChange.originalStart;
							bChange.modifiedLength = bModifiedEnd - bChange.modifiedStart;
						}
					}
				}
			}
		}

		return changes;
	}

	private _findBetterContiguousSequence(originalStart: number, originalLength: number, modifiedStart: number, modifiedLength: number, desiredLength: number): [number, number] | null {
		if (originalLength < desiredLength || modifiedLength < desiredLength) {
			return null;
		}
		const originalMax = originalStart + originalLength - desiredLength + 1;
		const modifiedMax = modifiedStart + modifiedLength - desiredLength + 1;
		let bestScore = 0;
		let bestOriginalStart = 0;
		let bestModifiedStart = 0;
		for (let i = originalStart; i < originalMax; i++) {
			for (let j = modifiedStart; j < modifiedMax; j++) {
				const score = this._contiguousSequenceScore(i, j, desiredLength);
				if (score > 0 && score > bestScore) {
					bestScore = score;
					bestOriginalStart = i;
					bestModifiedStart = j;
				}
			}
		}
		if (bestScore > 0) {
			return [bestOriginalStart, bestModifiedStart];
		}
		return null;
	}

	private _contiguousSequenceScore(originalStart: number, modifiedStart: number, length: number): number {
		let score = 0;
		for (let l = 0; l < length; l++) {
			if (!this.ElementsAreEqual(originalStart + l, modifiedStart + l)) {
				return 0;
			}
			score += this._originalStringElements[originalStart + l].length;
		}
		return score;
	}

	private _OriginalIsBoundary(index: number): boolean {
		if (index <= 0 || index >= this._originalElementsOrHash.length - 1) {
			return true;
		}
		return (this._hasStrings && /^\s*$/.test(this._originalStringElements[index]));
	}

	private _OriginalRegionIsBoundary(originalStart: number, originalLength: number): boolean {
		if (this._OriginalIsBoundary(originalStart) || this._OriginalIsBoundary(originalStart - 1)) {
			return true;
		}
		if (originalLength > 0) {
			const originalEnd = originalStart + originalLength;
			if (this._OriginalIsBoundary(originalEnd - 1) || this._OriginalIsBoundary(originalEnd)) {
				return true;
			}
		}
		return false;
	}

	private _ModifiedIsBoundary(index: number): boolean {
		if (index <= 0 || index >= this._modifiedElementsOrHash.length - 1) {
			return true;
		}
		return (this._hasStrings && /^\s*$/.test(this._modifiedStringElements[index]));
	}

	private _ModifiedRegionIsBoundary(modifiedStart: number, modifiedLength: number): boolean {
		if (this._ModifiedIsBoundary(modifiedStart) || this._ModifiedIsBoundary(modifiedStart - 1)) {
			return true;
		}
		if (modifiedLength > 0) {
			const modifiedEnd = modifiedStart + modifiedLength;
			if (this._ModifiedIsBoundary(modifiedEnd - 1) || this._ModifiedIsBoundary(modifiedEnd)) {
				return true;
			}
		}
		return false;
	}

	private _boundaryScore(originalStart: number, originalLength: number, modifiedStart: number, modifiedLength: number): number {
		const originalScore = (this._OriginalRegionIsBoundary(originalStart, originalLength) ? 1 : 0);
		const modifiedScore = (this._ModifiedRegionIsBoundary(modifiedStart, modifiedLength) ? 1 : 0);
		return (originalScore + modifiedScore);
	}

	/**
	 * Concatenates the two input DiffChange lists and returns the resulting
	 * list.
	 * @param The left changes
	 * @param The right changes
	 * @returns The concatenated list
	 */
	private ConcatenateChanges(left: DiffChange[], right: DiffChange[]): DiffChange[] {
		const mergedChangeArr: DiffChange[] = [];

		if (left.length === 0 || right.length === 0) {
			return (right.length > 0) ? right : left;
		} else if (this.ChangesOverlap(left[left.length - 1], right[0], mergedChangeArr)) {
			// Since we break the problem down recursively, it is possible that we
			// might recurse in the middle of a change thereby splitting it into
			// two changes. Here in the combining stage, we detect and fuse those
			// changes back together
			const result = new Array<DiffChange>(left.length + right.length - 1);
			MyArray.Copy(left, 0, result, 0, left.length - 1);
			result[left.length - 1] = mergedChangeArr[0];
			MyArray.Copy(right, 1, result, left.length, right.length - 1);

			return result;
		} else {
			const result = new Array<DiffChange>(left.length + right.length);
			MyArray.Copy(left, 0, result, 0, left.length);
			MyArray.Copy(right, 0, result, left.length, right.length);

			return result;
		}
	}

	/**
	 * Returns true if the two changes overlap and can be merged into a single
	 * change
	 * @param left The left change
	 * @param right The right change
	 * @param mergedChange The merged change if the two overlap, null otherwise
	 * @returns True if the two changes overlap
	 */
	private ChangesOverlap(left: DiffChange, right: DiffChange, mergedChangeArr: Array<DiffChange | null>): boolean {
		Debug.Assert(left.originalStart <= right.originalStart, 'Left change is not less than or equal to right change');
		Debug.Assert(left.modifiedStart <= right.modifiedStart, 'Left change is not less than or equal to right change');

		if (left.originalStart + left.originalLength >= right.originalStart || left.modifiedStart + left.modifiedLength >= right.modifiedStart) {
			const originalStart = left.originalStart;
			let originalLength = left.originalLength;
			const modifiedStart = left.modifiedStart;
			let modifiedLength = left.modifiedLength;

			if (left.originalStart + left.originalLength >= right.originalStart) {
				originalLength = right.originalStart + right.originalLength - left.originalStart;
			}
			if (left.modifiedStart + left.modifiedLength >= right.modifiedStart) {
				modifiedLength = right.modifiedStart + right.modifiedLength - left.modifiedStart;
			}

			mergedChangeArr[0] = new DiffChange(originalStart, originalLength, modifiedStart, modifiedLength);
			return true;
		} else {
			mergedChangeArr[0] = null;
			return false;
		}
	}

	/**
	 * Helper method used to clip a diagonal index to the range of valid
	 * diagonals. This also decides whether or not the diagonal index,
	 * if it exceeds the boundary, should be clipped to the boundary or clipped
	 * one inside the boundary depending on the Even/Odd status of the boundary
	 * and numDifferences.
	 * @param diagonal The index of the diagonal to clip.
	 * @param numDifferences The current number of differences being iterated upon.
	 * @param diagonalBaseIndex The base reference diagonal.
	 * @param numDiagonals The total number of diagonals.
	 * @returns The clipped diagonal index.
	 */
	private ClipDiagonalBound(diagonal: number, numDifferences: number, diagonalBaseIndex: number, numDiagonals: number): number {
		if (diagonal >= 0 && diagonal < numDiagonals) {
			// Nothing to clip, its in range
			return diagonal;
		}

		// diagonalsBelow: The number of diagonals below the reference diagonal
		// diagonalsAbove: The number of diagonals above the reference diagonal
		const diagonalsBelow = diagonalBaseIndex;
		const diagonalsAbove = numDiagonals - diagonalBaseIndex - 1;
		const diffEven = (numDifferences % 2 === 0);

		if (diagonal < 0) {
			const lowerBoundEven = (diagonalsBelow % 2 === 0);
			return (diffEven === lowerBoundEven) ? 0 : 1;
		} else {
			const upperBoundEven = (diagonalsAbove % 2 === 0);
			return (diffEven === upperBoundEven) ? numDiagonals - 1 : numDiagonals - 2;
		}
	}
}


/**
 * Precomputed equality array for character codes.
 */
const precomputedEqualityArray = new Uint32Array(0x10000);

/**
 * Computes the Levenshtein distance for strings of length <= 32.
 * @param firstString - The first string.
 * @param secondString - The second string.
 * @returns The Levenshtein distance.
 */
const computeLevenshteinDistanceForShortStrings = (firstString: string, secondString: string): number => {
	const firstStringLength = firstString.length;
	const secondStringLength = secondString.length;
	const lastBitMask = 1 << (firstStringLength - 1);
	let positiveVector = -1;
	let negativeVector = 0;
	let distance = firstStringLength;
	let index = firstStringLength;

	// Initialize precomputedEqualityArray for firstString
	while (index--) {
		precomputedEqualityArray[firstString.charCodeAt(index)] |= 1 << index;
	}

	// Process each character of secondString
	for (index = 0; index < secondStringLength; index++) {
		let equalityMask = precomputedEqualityArray[secondString.charCodeAt(index)];
		const combinedVector = equalityMask | negativeVector;
		equalityMask |= ((equalityMask & positiveVector) + positiveVector) ^ positiveVector;
		negativeVector |= ~(equalityMask | positiveVector);
		positiveVector &= equalityMask;
		if (negativeVector & lastBitMask) {
			distance++;
		}
		if (positiveVector & lastBitMask) {
			distance--;
		}
		negativeVector = (negativeVector << 1) | 1;
		positiveVector = (positiveVector << 1) | ~(combinedVector | negativeVector);
		negativeVector &= combinedVector;
	}

	// Reset precomputedEqualityArray
	index = firstStringLength;
	while (index--) {
		precomputedEqualityArray[firstString.charCodeAt(index)] = 0;
	}

	return distance;
};

/**
 * Computes the Levenshtein distance for strings of length > 32.
 * @param firstString - The first string.
 * @param secondString - The second string.
 * @returns The Levenshtein distance.
 */
function computeLevenshteinDistanceForLongStrings(firstString: string, secondString: string): number {
	const firstStringLength = firstString.length;
	const secondStringLength = secondString.length;
	const horizontalBitArray = [];
	const verticalBitArray = [];
	const horizontalSize = Math.ceil(firstStringLength / 32);
	const verticalSize = Math.ceil(secondStringLength / 32);

	// Initialize horizontal and vertical bit arrays
	for (let i = 0; i < horizontalSize; i++) {
		horizontalBitArray[i] = -1;
		verticalBitArray[i] = 0;
	}

	let verticalIndex = 0;
	for (; verticalIndex < verticalSize - 1; verticalIndex++) {
		let negativeVector = 0;
		let positiveVector = -1;
		const start = verticalIndex * 32;
		const verticalLength = Math.min(32, secondStringLength) + start;

		// Initialize precomputedEqualityArray for secondString
		for (let k = start; k < verticalLength; k++) {
			precomputedEqualityArray[secondString.charCodeAt(k)] |= 1 << k;
		}

		// Process each character of firstString
		for (let i = 0; i < firstStringLength; i++) {
			const equalityMask = precomputedEqualityArray[firstString.charCodeAt(i)];
			const previousBit = (horizontalBitArray[(i / 32) | 0] >>> i) & 1;
			const matchBit = (verticalBitArray[(i / 32) | 0] >>> i) & 1;
			const combinedVector = equalityMask | negativeVector;
			const combinedHorizontalVector = ((((equalityMask | matchBit) & positiveVector) + positiveVector) ^ positiveVector) | equalityMask | matchBit;
			let positiveHorizontalVector = negativeVector | ~(combinedHorizontalVector | positiveVector);
			let negativeHorizontalVector = positiveVector & combinedHorizontalVector;
			if ((positiveHorizontalVector >>> 31) ^ previousBit) {
				horizontalBitArray[(i / 32) | 0] ^= 1 << i;
			}
			if ((negativeHorizontalVector >>> 31) ^ matchBit) {
				verticalBitArray[(i / 32) | 0] ^= 1 << i;
			}
			positiveHorizontalVector = (positiveHorizontalVector << 1) | previousBit;
			negativeHorizontalVector = (negativeHorizontalVector << 1) | matchBit;
			positiveVector = negativeHorizontalVector | ~(combinedVector | positiveHorizontalVector);
			negativeVector = positiveHorizontalVector & combinedVector;
		}

		// Reset precomputedEqualityArray
		for (let k = start; k < verticalLength; k++) {
			precomputedEqualityArray[secondString.charCodeAt(k)] = 0;
		}
	}

	let negativeVector = 0;
	let positiveVector = -1;
	const start = verticalIndex * 32;
	const verticalLength = Math.min(32, secondStringLength - start) + start;

	// Initialize precomputedEqualityArray for secondString
	for (let k = start; k < verticalLength; k++) {
		precomputedEqualityArray[secondString.charCodeAt(k)] |= 1 << k;
	}

	let distance = secondStringLength;

	// Process each character of firstString
	for (let i = 0; i < firstStringLength; i++) {
		const equalityMask = precomputedEqualityArray[firstString.charCodeAt(i)];
		const previousBit = (horizontalBitArray[(i / 32) | 0] >>> i) & 1;
		const matchBit = (verticalBitArray[(i / 32) | 0] >>> i) & 1;
		const combinedVector = equalityMask | negativeVector;
		const combinedHorizontalVector = ((((equalityMask | matchBit) & positiveVector) + positiveVector) ^ positiveVector) | equalityMask | matchBit;
		let positiveHorizontalVector = negativeVector | ~(combinedHorizontalVector | positiveVector);
		let negativeHorizontalVector = positiveVector & combinedHorizontalVector;
		distance += (positiveHorizontalVector >>> (secondStringLength - 1)) & 1;
		distance -= (negativeHorizontalVector >>> (secondStringLength - 1)) & 1;
		if ((positiveHorizontalVector >>> 31) ^ previousBit) {
			horizontalBitArray[(i / 32) | 0] ^= 1 << i;
		}
		if ((negativeHorizontalVector >>> 31) ^ matchBit) {
			verticalBitArray[(i / 32) | 0] ^= 1 << i;
		}
		positiveHorizontalVector = (positiveHorizontalVector << 1) | previousBit;
		negativeHorizontalVector = (negativeHorizontalVector << 1) | matchBit;
		positiveVector = negativeHorizontalVector | ~(combinedVector | positiveHorizontalVector);
		negativeVector = positiveHorizontalVector & combinedVector;
	}

	// Reset precomputedEqualityArray
	for (let k = start; k < verticalLength; k++) {
		precomputedEqualityArray[secondString.charCodeAt(k)] = 0;
	}

	return distance;
}

/**
 * Computes the Levenshtein distance between two strings.
 * @param firstString - The first string.
 * @param secondString - The second string.
 * @returns The Levenshtein distance.
 */
export function computeLevenshteinDistance(firstString: string, secondString: string): number {
	if (firstString.length < secondString.length) {
		const temp = secondString;
		secondString = firstString;
		firstString = temp;
	}
	if (secondString.length === 0) {
		return firstString.length;
	}
	if (firstString.length <= 32) {
		return computeLevenshteinDistanceForShortStrings(firstString, secondString);
	}
	return computeLevenshteinDistanceForLongStrings(firstString, secondString);
}