const abortRegistry = new FinalizationRegistry(controller => controller.abort())

const camelToKebab = string => string.replace(/([a-z])([A-Z])/g, (_, a, b) => `${a}-${b.toLowerCase()}`)
const kebabToCamel = string => string.replace(/([a-z])-([a-z])/g, (_, a, b) => a+b.toUpperCase())

export class ChangeEvent extends Event {
	#final
	constructor(...changes) {
		super('change')
		this.changes = changes
	}
	get final() {
		if (!this.#final) {
			this.#final = new Map(this.changes)
		}
		return this.#final
	}
}

export class SimpleState extends EventTarget {}

export class MapStorage extends Storage {
	#map = new Map()
	key(index) {
		return [...this.#map.keys()][index]
	}
	getItem(keyName) {
		if (this.#map.has(keyName))
			return this.#map.get(keyName)
		else
			return null
	}
	setItem(keyName, keyValue) {
		this.#map.set(keyName, String(keyValue))
	}
	removeItem(keyName) {
		this.#map.delete(keyName)
	}
	clear() {
		this.#map.clear()
	}
}

export class State extends SimpleState {
	#target
	#options
	#queue
	#forwardCache
	#abortController
	#nested = new Map()
	#weakRef = new WeakRef(this)

	static isState(object) { return SimpleState.prototype.isPrototypeOf(object) }

	constructor(target={}, options={}) {
		super()

		this.#abortController = new AbortController
		abortRegistry.register(this, this.#abortController)

		this.#options = options
		this.#target = target
		this.values = new Proxy(target, {
			set: (_target, prop, value) => {
				const old = this.get(prop)
				if (old !== value) {
					this.emit(prop, value)
					if (this.#options.shallow) {
						if (State.isState(old)) this.disown(prop, old)
						if (State.isState(value)) this.adopt(prop, value)
					}
					this.set(prop, value)
				}
				return true
			},
			get: (_target, prop) => this.get(prop),
		})

		this.addEventListener

		// Try running a "<name>Changed" method for every changed property
		// Can be disabled to maybe squeeze out some performance
		if (options.methods ?? true) {
			this.addEventListener("change", ({final}) => {
				final.forEach((value, prop) => {
					if (`${prop}Changed` in this) this[`${prop}Changed`](value)
				})
			})
		}
	}

	// When you only need one value, you can skip the proxy.
	set value(value) { this.values.value = value }
	get value() { return this.values.value }

	adopt(prop, state) {
		let handlers = this.#nested.get(state)
		if (!handlers) {
			// Actual adoption
			handlers = new Map()
			this.#nested.set(state, handlers)
		}
		const ref = this.#weakRef
		const handler = () => ref.deref()?.emit(prop, state)

		handlers.set(prop, handler)
		state.addEventListener("change", handler, {signal: this.#abortController.signal})
	}
	disown(prop, state) {
		const handlers = this.#nested.get(state)
		const handler = handlers.get(prop)
		state.removeEventListener("change", handler)
		handlers.delete(prop)
		if (handlers.size == 0) {
			this.#nested.delete(state)
		}
	}

	// Anounces that a prop has changed
	emit(prop, value) {
		if (this.#options.defer ?? true) {
			if (!this.#queue) {
				this.#queue = []
				queueMicrotask(() => {
					this.dispatchEvent(new ChangeEvent(...this.#queue))
					this.#queue = undefined
				})
			}
			this.#queue.push([prop, value])
		} else {
			this.dispatchEvent(new ChangeEvent([prop, value]))
		}
	}

	forward(property="value", fallback) {
		if (!this.#forwardCache) this.#forwardCache = new Map()
		const cached = this.#forwardCache.get(property)?.deref()
		if (cached) {
			return cached
		} else {
			const forwarded = new ForwardState(this, property, fallback)
			const ref = new WeakRef(forwarded)
			this.#forwardCache.set(property, ref)
			forwardFinalizationRegistry.register(forwarded, [this.#forwardCache, property])
			return forwarded
		}
	}

	set(...args) {
		if (args.length === 1) return this.set("value", ...args)

		const [prop, value] = args
		this.#target[prop] = value
	}

	get(...args) {
		if (args.length === 0) return this.get("value")

		const prop = args[0]
		return this.#target[prop]
	}

	subscribe(prop, callback) {
		if (!callback) return this.subscribe("value", prop)

		const controller = new AbortController()
		this.addEventListener("change", ({final}) => {
			if (final.has(prop)) return callback(final.get(prop))
		}, {signal: controller.signal})
		callback(this.value)
		return () => controller.abort()
	}

	// Backwards compatibility
	get proxy() { return this.values }
}

const forwardFinalizationRegistry = new FinalizationRegistry(([cache, name]) => {
	cache.remove(name)
})

export class ForwardState extends SimpleState {
	#backend
	#property
	#fallback

	constructor(backend, property, fallback) {
		super()
		this.#backend = backend
		this.#property = property
		this.#fallback = fallback
		const ref = new WeakRef(this)
		const abortController = new AbortController()
		backend.addEventListener("change", event => {
			const state = ref.deref()
			if (state) {
				const relevantChanges = event.changes
					.filter(([name]) => name === property)
					.map(([_, value]) => ["value", value])
				if (relevantChanges.length > 0)
					state.dispatchEvent(new ChangeEvent(...relevantChanges))
			} else {
				abortController.abort()
			}
		}, {signal: abortController.signal})
	}

	get value() { return this.#backend.values[this.#property] ?? this.#fallback }
	set value(value) { this.#backend.values[this.#property] = value }
}

class StorageChangeEvent extends Event {
	constructor(storage, key, value, targetState) {
		super("storagechange")
		this.storageArea = storage
		this.key = key
		this.newValue = value
		this.targetState = targetState
	}
}

export class StoredState extends State {
	#storage

	constructor(init, options={}) {
		super({}, options)
		this.#storage = options.storage ?? localStorage ?? new MapStorage()

		// Initialise storage from defaults
		for (const [prop, value] of Object.entries(init)) {
			if (this.#storage[prop] === undefined)
				this.set(prop, value)
		}

		// Emit change events for any changed keys
		for (let i=0; i<this.#storage.length; i++) {
			const key = this.#storage.key(i)
			const value = this.#storage[key]
			if (value !== JSON.stringify(init[key]))
				this.emit(key, value)
		}

		// Listen for changes from other windows
		const handler = event => {
			if (event.targetState !== this && event.storageArea == this.#storage) {
				this.emit(event.key, JSON.parse(event.newValue))
			}
		}
		addEventListener("storage", handler)
		addEventListener("storagechange", handler)
	}

	set(prop, value) {
		const json = JSON.stringify(value)
		dispatchEvent(new StorageChangeEvent(this.#storage, prop, json, this))
		this.#storage[prop] = json
	}

	get(prop) {
		const value = this.#storage[prop]
		return value && JSON.parse(value)
	}
}

const attributeObserver = new MutationObserver(mutations => {
	for (const {type, target, attributeName: name} of mutations) {
		if (type == "attributes") {
			const next = target.getAttribute(name)
			const camelName = kebabToCamel(name)
			if (String(target.state.values[camelName]) !== next)
				target.state.values[camelName] = next
		}
	}
})

export const component = (generator, name) => {
	name = name ?? camelToKebab(generator.name)
	const Element = class extends HTMLElement{
		constructor() {
			super()
			this.state = new State(Object.fromEntries([...this.attributes].map(attribute => [kebabToCamel(attribute.name), attribute.value])))
			this.state.addEventListener("change", event => {
				for (const [name, value] of event.changes) {
					const kebabName = camelToKebab(name)
					if (this.getAttribute(kebabName) !== String(value))
						this.setAttribute(kebabName, value)
				}
			})
			attributeObserver.observe(this, {attributes: true})
			this.replaceChildren(generator(this))
		}
	}
	customElements.define(name, Element)
	return Element;
}

class ComposedState extends SimpleState {
	#func
	#states
	#options

	constructor(func, options, ...states) {
		super()

		this.#func = func
		this.#states = states
		this.#options = options

		const abortController = new AbortController()
		abortRegistry.register(this, abortController)
		const ref = new WeakRef(this)

		states.forEach(state => {
			state.addEventListener("change", event => {
				const value = event.final.get("value")
				if (value) ref.deref()?.scheduleUpdate()
			}, {signal: abortController.signal})
		})

		this.update()
	}

	#microtaskQueued
	scheduleUpdate() {
		if (this.#options.defer) {
			if (!this.#microtaskQueued) {
				queueMicrotask(() => {
					this.#microtaskQueued = false
					this.update()
				})
			}
			this.#microtaskQueued = true
		} else {
			this.update()
		}
	}

	update() {
		this.value = this.#func(...this.#states.map(state => state.value))
		this.dispatchEvent(new ChangeEvent([["value", this.value]]))
	}
}

export const compose = func => (...states) => new ComposedState(func, {defer: true}, ...states)

const eventName = "mutation"

class MutationEvent extends Event {
	constructor() {
		super(eventName, {bubbles: true})
	}
}

const mutationObserver = new MutationObserver(mutations => {
	for (const mutation of mutations) {
		mutation.target.dispatchEvent(new MutationEvent())
	}
})

export class DOMState extends SimpleState {
	#target
	#defer
	#getValue
	#equal

	#old
	#changedValue = false

	constructor(target, options) {
		super()
		this.#defer = options.defer ?? false
		this.#target = target
		this.#getValue = options.get ?? (target => target.value)
		this.#equal = options.equal ?? ((a, b) => a===b)

		this.#old = this.#getValue(target)

		const controller = new AbortController()
		target.addEventListener(eventName, event=>{this.update(event)}, {signal: controller.signal})

		abortRegistry.register(this, controller)
		mutationObserver.observe(target, {
			attributes: true,
			childList: true,
			characterData: true,
			subtree: true,
		})
	}

	get value() { return this.#old }

	update() {
		const current = this.#getValue(this.#target)

		if (this.#equal(this.#old, current)) return

		this.#old = current

		if (this.#defer) {
			if (!this.#changedValue) {
				queueMicrotask(() => {
					this.#changedValue = false
					this.dispatchEvent(new ChangeEvent(["value", this.#changedValue]))
				})
				this.#changedValue = current
			}
		} else {
			this.dispatchEvent(new ChangeEvent(["value", current]))
		}
	}
}

export default State