index.js

/* vim: set shiftwidth=2 tabstop=2 noexpandtab textwidth=80 wrap : */
"use strict";

module.exports = diff;

function diff(a, b, eql) {
	if (!eql)
		eql = function (a, b) { return a === b; };

	var d = new Diff(a, b, eql);
	return d.editscript();
}

diff.Diff = Diff;

function Diff(a, b, eql) {
	this.a = a;
	this.b = b;
	this.eql = eql;

	this.moda = Array.apply(null, new Array(a.length)).map(true.valueOf, false);
	this.modb = Array.apply(null, new Array(b.length)).map(true.valueOf, false);

	// just to save some allocations:
	this.down = {};
	this.up = {};

	this.lcs(0, a.length, 0, b.length);
}

Diff.NOOP    = 'nop';
Diff.DELETE  = 'del';
Diff.INSERT  = 'ins';
Diff.REPLACE = 'rep';

Diff.prototype.editscript = function Diff_editscript() {
	var moda = this.moda, modb = this.modb;
	var astart = 0, aend = moda.length;
	var bstart = 0, bend = modb.length;
	var result = [];
	while (astart < aend || bstart < bend) {
		if (astart < aend && bstart < bend) {
			if (!moda[astart] && !modb[bstart]) {
				result.push(Diff.NOOP);
				astart++; bstart++;
				continue;
			} else if (moda[astart] && modb[bstart]) {
				result.push(Diff.REPLACE);
				astart++; bstart++;
				continue;
			}
		}
		if (astart < aend && (bstart >= bend || moda[astart])) {
			result.push(Diff.DELETE);
			astart++;
		}
		if (bstart < bend && (astart >= aend || modb[bstart])) {
			result.push(Diff.INSERT);
			bstart++;
		}
	}
	return result;
};

Diff.prototype.lcs = function Diff_lcs(astart, aend, bstart, bend) {
	var a = this.a, b = this.b, eql = this.eql;
	// separate common head
	while (astart < aend && bstart < bend && eql(a[astart], b[bstart])) {
		astart++; bstart++;
	}
	// separate common tail
	while (astart < aend && bstart < bend && eql(a[aend - 1], b[bend - 1])) {
		aend--; bend--;
	}

	if (astart === aend) {
		// only insertions
		while (bstart < bend) {
			this.modb[bstart] = true;
			bstart++;
		}
	} else if (bend === bstart) {
		// only deletions
		while (astart < aend) {
			this.moda[astart] = true;
			astart++;
		}
	} else {
		var snake = this.snake(astart, aend, bstart, bend);

		this.lcs(astart, snake.x, bstart, snake.y);
		this.lcs(snake.u, aend, snake.v, bend);
	}
};

Diff.prototype.snake = function Diff_snake(astart, aend, bstart, bend) {
	var a = this.a, b = this.b, eql = this.eql;

	var N = aend - astart,
	    M = bend - bstart;

	var kdown = astart - bstart;
	var kup   = aend   - bend;

	var delta = N - M;
	var deltaOdd = delta & 1;

	var down = this.down;
	down[kdown + 1] = astart;
	var up = this.up;
	up[kup - 1] = aend;

	var Dmax = (N + M + 1) / 2;
	for (var D = 0; D <= Dmax; D++) {
		var k, x, y;
		// forward path
		for (k = kdown - D; k <= kdown + D; k += 2) {
			if (k === kdown - D) {
				x = down[k + 1]; // down
			} else {
				x = down[k - 1] + 1; // right
				if ((k < kdown + D) && (down[k + 1] >= x)) {
					x = down[k + 1]; // down
				}
			}
			y = x - k;

			while (x < aend && y < bend && eql(a[x], b[y])) {
				x++; y++; // diagonal
			}
			down[k] = x;

			if (deltaOdd && (kup - D < k) && (k < kup + D) &&
			    up[k] <= down[k]) {
				return {
					x: down[k],
					y: down[k] - k,
					u: up[k],
					v: up[k] - k,
				};
			}
		}

		// reverse path
		for (k = kup - D; k <= kup + D; k += 2) {
			if (k === kup + D) {
				x = up[k - 1]; // up
			} else {
				x = up[k + 1] - 1; // left
				if ((k > kup - D) && (up[k - 1] < x)) {
					x = up[k - 1]; // up
				}
			}
			y = x - k;

			while (x > astart && y > bstart && eql(a[x - 1], b[y - 1])) {
				x--; y--; // diagonal
			}
			up[k] = x;

			if (!deltaOdd && (kdown - D <= k) && (k <= kdown + D) &&
			    up[k] <= down[k]) {
				return {
					x: down[k],
					y: down[k] - k,
					u: up[k],
					v: up[k] - k,
				};
			}
		}
	}
};