import { writable } from '../store/index.js';
import { loop, now } from '../internal/index.js';
import { is_date } from './utils.js';

/**
 * @template T
 * @param {import('./private.js').TickContext<T>} ctx
 * @param {T} last_value
 * @param {T} current_value
 * @param {T} target_value
 * @returns {T}
 */
function tick_spring(ctx, last_value, current_value, target_value) {
	if (typeof current_value === 'number' || is_date(current_value)) {
		// @ts-ignore
		const delta = target_value - current_value;
		// @ts-ignore
		const velocity = (current_value - last_value) / (ctx.dt || 1 / 60); // guard div by 0
		const spring = ctx.opts.stiffness * delta;
		const damper = ctx.opts.damping * velocity;
		const acceleration = (spring - damper) * ctx.inv_mass;
		const d = (velocity + acceleration) * ctx.dt;
		if (Math.abs(d) < ctx.opts.precision && Math.abs(delta) < ctx.opts.precision) {
			return target_value; // settled
		} else {
			ctx.settled = false; // signal loop to keep ticking
			// @ts-ignore
			return is_date(current_value) ? new Date(current_value.getTime() + d) : current_value + d;
		}
	} else if (Array.isArray(current_value)) {
		// @ts-ignore
		return current_value.map((_, i) =>
			tick_spring(ctx, last_value[i], current_value[i], target_value[i])
		);
	} else if (typeof current_value === 'object') {
		const next_value = {};
		for (const k in current_value) {
			// @ts-ignore
			next_value[k] = tick_spring(ctx, last_value[k], current_value[k], target_value[k]);
		}
		// @ts-ignore
		return next_value;
	} else {
		throw new Error(`Cannot spring ${typeof current_value} values`);
	}
}

/**
 * The spring function in Svelte creates a store whose value is animated, with a motion that simulates the behavior of a spring. This means when the value changes, instead of transitioning at a steady rate, it "bounces" like a spring would, depending on the physics parameters provided. This adds a level of realism to the transitions and can enhance the user experience.
 *
 * https://svelte.dev/docs/svelte-motion#spring
 * @template [T=any]
 * @param {T} [value]
 * @param {import('./private.js').SpringOpts} [opts]
 * @returns {import('./public.js').Spring<T>}
 */
export function spring(value, opts = {}) {
	const store = writable(value);
	const { stiffness = 0.15, damping = 0.8, precision = 0.01 } = opts;
	/** @type {number} */
	let last_time;
	/** @type {import('../internal/private.js').Task} */
	let task;
	/** @type {object} */
	let current_token;
	/** @type {T} */
	let last_value = value;
	/** @type {T} */
	let target_value = value;
	let inv_mass = 1;
	let inv_mass_recovery_rate = 0;
	let cancel_task = false;
	/**
	 * @param {T} new_value
	 * @param {import('./private.js').SpringUpdateOpts} opts
	 * @returns {Promise<void>}
	 */
	function set(new_value, opts = {}) {
		target_value = new_value;
		const token = (current_token = {});
		if (value == null || opts.hard || (spring.stiffness >= 1 && spring.damping >= 1)) {
			cancel_task = true; // cancel any running animation
			last_time = now();
			last_value = new_value;
			store.set((value = target_value));
			return Promise.resolve();
		} else if (opts.soft) {
			const rate = opts.soft === true ? 0.5 : +opts.soft;
			inv_mass_recovery_rate = 1 / (rate * 60);
			inv_mass = 0; // infinite mass, unaffected by spring forces
		}
		if (!task) {
			last_time = now();
			cancel_task = false;
			task = loop((now) => {
				if (cancel_task) {
					cancel_task = false;
					task = null;
					return false;
				}
				inv_mass = Math.min(inv_mass + inv_mass_recovery_rate, 1);
				const ctx = {
					inv_mass,
					opts: spring,
					settled: true,
					dt: ((now - last_time) * 60) / 1000
				};
				const next_value = tick_spring(ctx, last_value, value, target_value);
				last_time = now;
				last_value = value;
				store.set((value = next_value));
				if (ctx.settled) {
					task = null;
				}
				return !ctx.settled;
			});
		}
		return new Promise((fulfil) => {
			task.promise.then(() => {
				if (token === current_token) fulfil();
			});
		});
	}
	/** @type {import('./public.js').Spring<T>} */
	const spring = {
		set,
		update: (fn, opts) => set(fn(target_value, value), opts),
		subscribe: store.subscribe,
		stiffness,
		damping,
		precision
	};
	return spring;
}