/**

* Use this to create a "patch" object in the worker thread.

* @template {any} T

* @template {any} U

* @param {T} obj

* @param {U} old

* @returns {T | (Partial<T> | Partial<U>) | undefined}

* @example

* patch = diff(newObject, oldObject);

*/

export function diff(obj, old) {

if (typeof obj === 'object') {

const isArray = Array.isArray(obj);

if (!old || typeof old !== 'object' || isArray !== Array.isArray(old)) {

return obj;

}

if (isArray) {

let out;

let i = 0;

const max = Math.min(obj.length, old.length);

for (; i < max; i++) {

const differs = different(obj[i], old[i]);

if (differs) break;

}

// for previously-empty arrays, hint at newness by using an Array

const useArray = old.length === 0;

const offset = obj.length - old.length;

for (let j = obj.length; j-- > i; ) {

const oldJ = j - offset;

if (oldJ >= 0) {

const differs = different(obj[j], old[oldJ]);

if (differs) {

if (!out) out = useArray ? [] : {};

out[j] = diff(obj[j], old[oldJ]);

}

} else {

if (!out) out = useArray ? [] : {};

out[j] = obj[j];

}

}

return out;

}

let out;

for (let key in obj) {

if (!(key in old) || obj[key] !== old[key]) {

if (!out) out = {};

// `undefined` means removed, missing means unchanged.

const r = diff(obj[key], old[key]);

if (r !== undefined) {

out[key] = r;

}

}

}

for (let key in old) {

if (obj == null || !(key in obj)) {

if (!out) out = {};

// `undefined` means removed, missing means unchanged.

out[key] = undefined;

}

}

return out;

} else if (obj != old) {

return obj;

}

}

function different(obj, old) {

for (let key in obj) {

if (old == null || !(key in old) || obj[key] !== old[key]) {

return true;

}

}

for (let key in old) {

if (obj == null || !(key in obj)) {

return true;

}

}

return false;

}