import { computed, readonly, type Ref, ref, toRef } from 'vue';

import isEqual from 'dequal';

const clone = <T>(value: T): T => {
	if (value !== null && typeof value === 'object') {
		// nested reactivity means that structuredClone() isn't possible, the next
		// closest thing is to stringify the object into JSON and parse it back.
		return JSON.parse(JSON.stringify(value)) as T;
	}

	return value;
};

const writableObject = <T extends Record<string, unknown>, K extends keyof T>(
	upstream: Ref<T>,
	keys: K[]
) => {
	const scratch = ref(
		((): T => {
			const ro = readonly(upstream);
			const cloned: Record<string, unknown> = {};

			for (const key in upstream.value) {
				if (keys.includes(key as any)) {
					cloned[key] = clone(upstream.value[key]);
				} else {
					cloned[key] = toRef(() => ro.value[key]);
				}
			}

			return cloned as T;
		})()
	) as Ref<T>;

	return {
		scratch: scratch as Readonly<Ref<T>>,
		hasChanges: computed(() => {
			return !isEqual(upstream.value, scratch.value);
		}),
		resetChanges: () => {
			for (const key in upstream.value) {
				scratch.value[key] = clone(upstream.value[key]);
			}
		},
	};
};