dev/third_party/pi/packages/chord/src/delta/diff.ts

import type { JsonValue } from "../json.ts";
import { type NonEmptyPath, type Op, overlap, type Path, RESERVED_SEGMENTS, type Seg } from "./index.ts";

const DEFAULT_OVERLAP_SCAN = 65_536;
const MAX_DELTA_OPERATIONS = 4_096;
const overflowedBatches = new WeakSet<Op[]>();

const emitOperation = (operations: Op[], operation: Op): void => {
	if (overflowedBatches.has(operations)) return;
	if (operations.length >= MAX_DELTA_OPERATIONS) {
		overflowedBatches.add(operations);
		return;
	}
	operations.push(operation);
};

const isContainer = (value: JsonValue): value is JsonValue[] | Record<string, JsonValue> =>
	value !== null && typeof value === "object";

const emitSet = (path: Path, value: JsonValue, operations: Op[]): void => {
	if (path.length === 0) emitOperation(operations, ["r", value]);
	else emitOperation(operations, ["s", path as NonEmptyPath, value]);
};

const equalJson = (left: JsonValue, right: JsonValue): boolean => {
	if (left === right) return true;
	if (!isContainer(left) || !isContainer(right) || Array.isArray(left) !== Array.isArray(right)) return false;
	if (Array.isArray(left)) {
		const other = right as JsonValue[];
		if (left.length !== other.length) return false;
		for (let index = 0; index < left.length; index++) {
			if (!equalJson(left[index]!, other[index]!)) return false;
		}
		return true;
	}
	const other = right as Record<string, JsonValue>;
	const keys = Object.keys(left);
	if (keys.length !== Object.keys(other).length) return false;
	for (const key of keys) {
		if (!Object.hasOwn(other, key) || !equalJson(left[key]!, other[key]!)) return false;
	}
	return true;
};

const permutation = (before: readonly JsonValue[], after: readonly JsonValue[]): number[] | undefined => {
	if (before.length !== after.length) return undefined;
	const positions = new Map<JsonValue, { indices: number[]; used: number }>();
	for (let index = 0; index < before.length; index++) {
		const value = before[index]!;
		const entry = positions.get(value);
		if (entry === undefined) positions.set(value, { indices: [index], used: 0 });
		else entry.indices.push(index);
	}
	const result = new Array<number>(after.length);
	for (let index = 0; index < after.length; index++) {
		const entry = positions.get(after[index]!);
		if (entry === undefined || entry.used === entry.indices.length) return undefined;
		result[index] = entry.indices[entry.used++]!;
	}
	return result;
};

const emitString = (before: string, after: string, path: NonEmptyPath, operations: Op[]): void => {
	if (before === after) return;
	if (after.length > before.length && after.slice(0, before.length) === before) {
		emitOperation(operations, ["a", path, after.slice(before.length)]);
		return;
	}
	const shared = overlap(before, after, DEFAULT_OVERLAP_SCAN);
	if (shared === 0) {
		emitOperation(operations, ["s", path, after]);
		return;
	}
	emitOperation(operations, ["t", path, before.length - shared]);
	if (after.length > shared) emitOperation(operations, ["a", path, after.slice(shared)]);
};

type ArrayMatch = readonly [before: number, after: number];

type MatchCandidate = {
	before: number;
	after: number;
	previous: number;
};

const MAX_IDENTITY_CANDIDATES = 200_000;
const MAX_SEMANTIC_CELLS = 65_536;

const sameValue = (left: JsonValue, right: JsonValue): boolean => left === right || equalJson(left, right);

const lcsMatches = (
	before: readonly JsonValue[],
	after: readonly JsonValue[],
	equal: (left: JsonValue, right: JsonValue) => boolean,
	maxCells: number,
): ArrayMatch[] | undefined => {
	if (before.length === 0 || after.length === 0) return [];
	if (before.length * after.length > maxCells) return undefined;
	const width = after.length + 1;
	const lengths = new Uint32Array((before.length + 1) * width);
	for (let left = before.length - 1; left >= 0; left--) {
		for (let right = after.length - 1; right >= 0; right--) {
			const at = left * width + right;
			lengths[at] = equal(before[left]!, after[right]!)
				? lengths[(left + 1) * width + right + 1]! + 1
				: Math.max(lengths[(left + 1) * width + right]!, lengths[left * width + right + 1]!);
		}
	}
	const matches: ArrayMatch[] = [];
	let left = 0;
	let right = 0;
	while (left < before.length && right < after.length) {
		if (
			equal(before[left]!, after[right]!) &&
			lengths[left * width + right] === lengths[(left + 1) * width + right + 1]! + 1
		) {
			matches.push([left++, right++]);
		} else if (lengths[(left + 1) * width + right]! >= lengths[left * width + right + 1]!) left += 1;
		else right += 1;
	}
	return matches;
};

const semanticallyAligned = (left: JsonValue, right: JsonValue): boolean => {
	if (sameValue(left, right)) return true;
	if (!isContainer(left) || !isContainer(right) || Array.isArray(left) !== Array.isArray(right)) return false;
	if (Array.isArray(left) && Array.isArray(right)) {
		if (left.length !== right.length) return false;
		return left.some((value, index) => isContainer(value) && value === right[index]);
	}
	const leftObject = left as Record<string, JsonValue>;
	const rightObject = right as Record<string, JsonValue>;
	for (const key of Object.keys(leftObject)) {
		const value = leftObject[key];
		if (isContainer(value) && Object.hasOwn(rightObject, key) && value === rightObject[key]) return true;
	}
	return false;
};

const lowerBound = (values: readonly number[], value: number): number => {
	let low = 0;
	let high = values.length;
	while (low < high) {
		const middle = (low + high) >>> 1;
		if (values[middle]! < value) low = middle + 1;
		else high = middle;
	}
	return low;
};

const identitySubsequence = (
	before: readonly JsonValue[],
	beforeStart: number,
	beforeEnd: number,
	after: readonly JsonValue[],
	afterStart: number,
	afterEnd: number,
): ArrayMatch[] | undefined => {
	const beforeCount = beforeEnd - beforeStart;
	const afterCount = afterEnd - afterStart;
	const matches: ArrayMatch[] = [];
	if (afterCount < beforeCount) {
		let beforeIndex = beforeStart;
		for (let afterIndex = afterStart; afterIndex < afterEnd; afterIndex++) {
			while (beforeIndex < beforeEnd && before[beforeIndex] !== after[afterIndex]) beforeIndex += 1;
			if (beforeIndex === beforeEnd) return undefined;
			matches.push([beforeIndex++, afterIndex]);
		}
		return matches;
	}
	if (beforeCount < afterCount) {
		let afterIndex = afterStart;
		for (let beforeIndex = beforeStart; beforeIndex < beforeEnd; beforeIndex++) {
			while (afterIndex < afterEnd && before[beforeIndex] !== after[afterIndex]) afterIndex += 1;
			if (afterIndex === afterEnd) return undefined;
			matches.push([beforeIndex, afterIndex++]);
		}
		return matches;
	}
	return undefined;
};

const greedyIdentityAnchors = (
	positions: ReadonlyMap<JsonValue, readonly number[]>,
	after: readonly JsonValue[],
	afterStart: number,
	afterEnd: number,
): ArrayMatch[] => {
	const matches: ArrayMatch[] = [];
	let previous = -1;
	for (let afterIndex = afterStart; afterIndex < afterEnd; afterIndex++) {
		const candidates = positions.get(after[afterIndex]!);
		if (candidates === undefined) continue;
		const at = lowerBound(candidates, previous + 1);
		const beforeIndex = candidates[at];
		if (beforeIndex === undefined) continue;
		matches.push([beforeIndex, afterIndex]);
		previous = beforeIndex;
	}
	return matches;
};

const identityAnchors = (
	before: readonly JsonValue[],
	beforeStart: number,
	beforeEnd: number,
	after: readonly JsonValue[],
	afterStart: number,
	afterEnd: number,
): ArrayMatch[] => {
	const positions = new Map<JsonValue, number[]>();
	for (let index = beforeStart; index < beforeEnd; index++) {
		const value = before[index]!;
		const existing = positions.get(value);
		if (existing === undefined) positions.set(value, [index]);
		else existing.push(index);
	}
	let candidateCount = 0;
	for (let index = afterStart; index < afterEnd; index++) {
		candidateCount += positions.get(after[index]!)?.length ?? 0;
		if (candidateCount > MAX_IDENTITY_CANDIDATES) {
			return greedyIdentityAnchors(positions, after, afterStart, afterEnd);
		}
	}
	if (candidateCount === 0) return [];

	const candidates: MatchCandidate[] = [];
	const tails: number[] = [];
	const tailValues: number[] = [];
	for (let afterIndex = afterStart; afterIndex < afterEnd; afterIndex++) {
		const beforePositions = positions.get(after[afterIndex]!);
		if (beforePositions === undefined) continue;
		for (let index = beforePositions.length - 1; index >= 0; index--) {
			const beforeIndex = beforePositions[index]!;
			const at = lowerBound(tailValues, beforeIndex);
			const candidateIndex = candidates.length;
			candidates.push({ before: beforeIndex, after: afterIndex, previous: at === 0 ? -1 : tails[at - 1]! });
			tails[at] = candidateIndex;
			tailValues[at] = beforeIndex;
		}
	}
	const matches: ArrayMatch[] = [];
	let candidateIndex = tails[tails.length - 1] ?? -1;
	while (candidateIndex >= 0) {
		const candidate = candidates[candidateIndex]!;
		matches.push([candidate.before, candidate.after]);
		candidateIndex = candidate.previous;
	}
	matches.reverse();
	return matches;
};

const processArrayMatches = (
	before: JsonValue[],
	after: JsonValue[],
	path: Path,
	operations: Op[],
	beforeStart: number,
	beforeEnd: number,
	afterStart: number,
	afterEnd: number,
	outputStart: number,
	matches: readonly ArrayMatch[],
): void => {
	let beforeAt = beforeStart;
	let afterAt = afterStart;
	let outputAt = outputStart;
	for (const [beforeMatch, afterMatch] of matches) {
		if (overflowedBatches.has(operations)) return;
		diffArrayRegion(before, after, path, operations, beforeAt, beforeMatch, afterAt, afterMatch, outputAt);
		outputAt += afterMatch - afterAt;
		if (!sameValue(before[beforeMatch]!, after[afterMatch]!)) {
			diffValue(before[beforeMatch]!, after[afterMatch]!, [...path, outputAt], operations);
		}
		outputAt += 1;
		beforeAt = beforeMatch + 1;
		afterAt = afterMatch + 1;
	}
	if (!overflowedBatches.has(operations)) {
		diffArrayRegion(before, after, path, operations, beforeAt, beforeEnd, afterAt, afterEnd, outputAt);
	}
};

function diffArrayRegion(
	before: JsonValue[],
	after: JsonValue[],
	path: Path,
	operations: Op[],
	beforeStart: number,
	beforeEnd: number,
	afterStart: number,
	afterEnd: number,
	outputStart: number,
): void {
	if (overflowedBatches.has(operations)) return;
	while (beforeStart < beforeEnd && afterStart < afterEnd && sameValue(before[beforeStart]!, after[afterStart]!)) {
		beforeStart += 1;
		afterStart += 1;
		outputStart += 1;
	}
	while (beforeStart < beforeEnd && afterStart < afterEnd && sameValue(before[beforeEnd - 1]!, after[afterEnd - 1]!)) {
		beforeEnd -= 1;
		afterEnd -= 1;
	}
	const beforeCount = beforeEnd - beforeStart;
	const afterCount = afterEnd - afterStart;
	if (beforeCount === 0 && afterCount === 0) return;
	if (beforeCount === 0 || afterCount === 0) {
		emitOperation(operations, ["p", path, outputStart, beforeCount, after.slice(afterStart, afterEnd)]);
		return;
	}

	if (beforeCount === afterCount) {
		const positional: ArrayMatch[] = [];
		for (let offset = 0; offset < beforeCount; offset++) {
			if (sameValue(before[beforeStart + offset]!, after[afterStart + offset]!)) {
				positional.push([beforeStart + offset, afterStart + offset]);
			}
		}
		if (positional.length > 0) {
			processArrayMatches(
				before,
				after,
				path,
				operations,
				beforeStart,
				beforeEnd,
				afterStart,
				afterEnd,
				outputStart,
				positional,
			);
			return;
		}
	}

	const subsequence = identitySubsequence(before, beforeStart, beforeEnd, after, afterStart, afterEnd);
	if (subsequence !== undefined && subsequence.length > 0) {
		processArrayMatches(
			before,
			after,
			path,
			operations,
			beforeStart,
			beforeEnd,
			afterStart,
			afterEnd,
			outputStart,
			subsequence,
		);
		return;
	}

	const identity = identityAnchors(before, beforeStart, beforeEnd, after, afterStart, afterEnd);
	if (identity.length > 0) {
		processArrayMatches(
			before,
			after,
			path,
			operations,
			beforeStart,
			beforeEnd,
			afterStart,
			afterEnd,
			outputStart,
			identity,
		);
		return;
	}

	const semantic = lcsMatches(
		before.slice(beforeStart, beforeEnd),
		after.slice(afterStart, afterEnd),
		semanticallyAligned,
		MAX_SEMANTIC_CELLS,
	);
	if (semantic !== undefined && semantic.length > 0) {
		const absolute = semantic.map(
			([beforeIndex, afterIndex]) => [beforeStart + beforeIndex, afterStart + afterIndex] as const,
		);
		processArrayMatches(
			before,
			after,
			path,
			operations,
			beforeStart,
			beforeEnd,
			afterStart,
			afterEnd,
			outputStart,
			absolute,
		);
		return;
	}

	if (beforeCount === 1 && afterCount === 1) {
		diffValue(before[beforeStart]!, after[afterStart]!, [...path, outputStart], operations);
		return;
	}
	emitOperation(operations, ["p", path, outputStart, beforeCount, after.slice(afterStart, afterEnd)]);
}

const diffArray = (before: JsonValue[], after: JsonValue[], path: Path, operations: Op[]): void => {
	if (before === after || equalJson(before, after)) return;
	if (
		before.length === after.length &&
		before.length > 1 &&
		!sameValue(before[0]!, after[0]!) &&
		!sameValue(before[before.length - 1]!, after[after.length - 1]!)
	) {
		const order = permutation(before, after);
		if (order !== undefined) {
			emitOperation(operations, ["m", path, order]);
			return;
		}
	}
	diffArrayRegion(before, after, path, operations, 0, before.length, 0, after.length, 0);
};

const diffObject = (
	before: Record<string, JsonValue>,
	after: Record<string, JsonValue>,
	path: Path,
	operations: Op[],
): void => {
	const beforeKeys = Object.keys(before);
	const afterKeys = Object.keys(after);
	if ([...beforeKeys, ...afterKeys].some((key) => RESERVED_SEGMENTS.has(key))) {
		if (!equalJson(before, after)) emitSet(path, after, operations);
		return;
	}
	for (const key of afterKeys) {
		if (overflowedBatches.has(operations)) return;
		if (Object.hasOwn(before, key)) diffValue(before[key]!, after[key]!, [...path, key], operations);
		else emitSet([...path, key], after[key]!, operations);
	}
	for (const key of beforeKeys) {
		if (overflowedBatches.has(operations)) return;
		if (!Object.hasOwn(after, key)) emitOperation(operations, ["d", [...path, key] as unknown as NonEmptyPath]);
	}
};

const diffValue = (before: JsonValue, after: JsonValue, path: Path, operations: Op[]): void => {
	if (before === after || overflowedBatches.has(operations)) return;
	if (typeof before === "string" && typeof after === "string" && path.length > 0) {
		emitString(before, after, path as NonEmptyPath, operations);
		return;
	}
	if (Array.isArray(before) && Array.isArray(after)) {
		diffArray(before, after, path, operations);
		return;
	}
	if (isContainer(before) && isContainer(after) && !Array.isArray(before) && !Array.isArray(after)) {
		diffObject(before, after, path, operations);
		return;
	}
	emitSet(path, after, operations);
};

const jsonCost = (value: JsonValue): number => {
	if (value === null) return 4;
	if (typeof value === "string") return value.length + 2;
	if (typeof value === "number") return String(value).length;
	if (typeof value === "boolean") return value ? 4 : 5;
	if (Array.isArray(value)) {
		let cost = 2;
		for (let index = 0; index < value.length; index++) cost += jsonCost(value[index]!) + (index === 0 ? 0 : 1);
		return cost;
	}
	let cost = 2;
	let index = 0;
	for (const key of Object.keys(value)) {
		cost += key.length + 3 + jsonCost(value[key]!) + (index++ === 0 ? 0 : 1);
	}
	return cost;
};

const pathCost = (path: Path): number => {
	let cost = 2;
	for (let index = 0; index < path.length; index++) {
		const segment: Seg = path[index]!;
		cost += (typeof segment === "string" ? segment.length + 2 : String(segment).length) + (index === 0 ? 0 : 1);
	}
	return cost;
};

const operationCost = (operation: Op): number => {
	switch (operation[0]) {
		case "r":
			return 6 + jsonCost(operation[1]);
		case "s":
			return 7 + pathCost(operation[1]) + jsonCost(operation[2]);
		case "d":
			return 6 + pathCost(operation[1]);
		case "a":
			return 7 + pathCost(operation[1]) + operation[2].length + 2;
		case "t":
			return 7 + pathCost(operation[1]) + String(operation[2]).length;
		case "p":
			return (
				10 +
				pathCost(operation[1]) +
				String(operation[2]).length +
				String(operation[3]).length +
				jsonCost(operation[4])
			);
		case "m":
			return 7 + pathCost(operation[1]) + jsonCost(operation[2]);
	}
};

/** Compute a compact operation batch from two immutable JSON revisions. */
export function diffRevisions(before: JsonValue, after: JsonValue): Op[] {
	const operations: Op[] = [];
	diffValue(before, after, [], operations);
	if (overflowedBatches.has(operations)) return [["r", after]];
	if (operations.length === 0 || operations[0]?.[0] === "r") return operations;
	let deltaCost = 2;
	for (const operation of operations) deltaCost += operationCost(operation) + 1;
	if (deltaCost < 65_536) return operations;
	const snapshotCost = jsonCost(after) + 6;
	return deltaCost >= snapshotCost ? [["r", after]] : operations;
}