'use strict';

// ============================================================================
// Settings Contexts
// ============================================================================

import {EventEmitter} from 'utilities/events';
import {has, get as getter, array_equals} from 'utilities/object';


/**
 * The SettingsContext class provides a context through which to read
 * settings values in addition to emitting events when settings values
 * are changed.
 * @extends EventEmitter
 */
export default class SettingsContext extends EventEmitter {
	constructor(manager, context) {
		super();

		if ( manager instanceof SettingsContext ) {
			this.parent = manager;
			this.manager = manager.manager;

			this.parent.on('context_changed', this._rebuildContext, this);

		} else {
			this.parent = null;
			this.manager = manager;
		}

		this.manager.__contexts.push(this);
		this._context = context || {};

		this.__cache = new Map;
		this.__meta = new Map;
		this.__profiles = [];
		this.order = [];

		this._rebuildContext();
	}

	destroy() {
		if ( this.parent )
			this.parent.off('context_changed', this._rebuildContext, this);

		for(const profile of this.__profiles)
			profile.off('changed', this._onChanged, this);

		const contexts = this.manager.__contexts,
			idx = contexts.indexOf(this);

		if ( idx !== -1 )
			contexts.splice(idx, 1);
	}


	// ========================================================================
	// State Construction
	// ========================================================================

	_rebuildContext() {
		this.__context = this.parent ?
			Object.assign({}, this.parent._context, this._context) :
			this._context;

		// Make sure we re-build the cache. Dependency hell.
		if ( ! this.selectProfiles() )
			this.rebuildCache();

		this.emit('context_changed');
	}


	selectProfiles() {
		const new_profiles = [],
			order = this.order = [];
		for(const profile of this.manager.__profiles)
			if ( profile.matches(this.__context) ) {
				new_profiles.push(profile);
				order.push(profile.id);
			}

		if ( array_equals(this.__profiles, new_profiles) )
			return false;

		const changed_ids = new Set;

		for(const profile of this.__profiles)
			if ( ! new_profiles.includes(profile) ) {
				profile.off('changed', this._onChanged, this);
				changed_ids.add(profile.id);
			}

		for(const profile of new_profiles)
			if ( ! this.__profiles.includes(profile) ) {
				profile.on('changed', this._onChanged, this);
				changed_ids.add(profile.id);
			}

		this.__profiles = new_profiles;
		this.emit('profiles_changed');
		this.rebuildCache(changed_ids);
		return true;
	}


	rebuildCache() {
		const old_cache = this.__cache,
			old_meta = this.__meta,
			meta = this.__meta = new Map;

		this.__cache = new Map;

		// TODO: Limit the values we recalculate to ones affected by the change
		// that happened to the profiles. This is harder because of setting
		// dependencies.

		for(const [key, old_value] of old_cache) {
			const new_value = this.get(key),
				new_m = meta.get(key),
				old_m = old_meta.get(key),
				new_uses = new_m ? new_m.uses : null,
				old_uses = old_m ? old_m.uses : null;

			if ( new_value !== old_value ) {
				this.emit('changed', key, new_value, old_value);
				this.emit(`changed:${key}`, new_value, old_value);
			}

			if ( new_uses !== old_uses ) {
				this.emit('uses_changed', key, new_uses, old_uses);
				this.emit(`uses_changed:${key}`, new_uses, old_uses);
			}
		}
	}


	// ========================================================================
	// Context Control
	// ========================================================================

	context(context) {
		return new SettingsContext(this, context);
	}


	updateContext(context) {
		let changed = false;

		for(const key in context)
			if ( has(context, key) && context[key] !== this._context[key] ) {
				this._context[key] = context[key];
				changed = true;
			}

		if ( changed )
			this._rebuildContext();
	}


	setContext(context) {
		this._context = context;
		this._rebuildContext();
	}


	// ========================================================================
	// Data Access
	// ========================================================================

	_onChanged(key) {
		this._update(key, key, []);
	}

	_update(key, initial, visited) {
		if ( ! this.__cache.has(key) )
			return;

		else if ( visited.includes(key) )
			throw new Error(`cyclic dependent chain when updating setting "${initial}"`);

		visited.push(key);

		const old_value = this.__cache.get(key),
			old_meta = this.__meta.get(key),
			new_value = this._get(key, key, []),
			new_meta = this.__meta.get(key),

			old_uses = old_meta ? old_meta.uses : null,
			new_uses = new_meta ? new_meta.uses : null;

		if ( old_uses !== new_uses ) {
			this.emit('uses_changed', key, new_uses, old_uses);
			this.emit(`uses_changed:${key}`, new_uses, old_uses);
		}

		if ( old_value === new_value )
			return;

		this.emit('changed', key, new_value, old_value);
		this.emit(`changed:${key}`, new_value, old_value);

		const definition = this.manager.definitions.get(key);
		if ( definition && definition.required_by )
			for(const req_key of definition.required_by)
				if ( ! req_key.startsWith('context.') )
					this._update(req_key, initial, Array.from(visited));
	}


	_get(key, initial, visited) {
		if ( visited.includes(key) )
			throw new Error(`cyclic dependency when resolving setting "${initial}"`);

		visited.push(key);

		const definition = this.manager.definitions.get(key),
			raw_value = this._getRaw(key),
			meta = {
				uses: raw_value ? raw_value[1].id : null
			};

		let value = raw_value ? raw_value[0] : undefined;

		if ( definition ) {
			if ( Array.isArray(definition) )
				throw new Error(`non-existent setting "${key}" required when resolving setting "${initial}"`);

			if ( meta.uses === null ) {
				const def_default = definition.default;
				if ( typeof def_default === 'function' )
					value = def_default(this);
				else
					value = def_default;
			}

			if ( definition.requires )
				for(const req_key of definition.requires)
					if ( ! req_key.startsWith('context.') && ! this.__cache.has(req_key) )
						this._get(req_key, initial, Array.from(visited));

			if ( definition.process )
				value = definition.process(this, value, meta);
		}

		this.__cache.set(key, value);
		this.__meta.set(key, meta);
		return value;
	}


	_getRaw(key) {
		for(const profile of this.__profiles)
			if ( profile.has(key) )
				return [profile.get(key), profile]
	}


	// ========================================================================
	// Data Access
	// ========================================================================

	update(key) {
		this._update(key, key, []);
	}

	get(key) {
		if ( key.startsWith('context.') )
			return getter(key.slice(8), this.__context);

		if ( this.__cache.has(key) )
			return this.__cache.get(key);

		return this._get(key, key, []);
	}

	uses(key) {
		if ( key.startsWith('context.') )
			return null;

		if ( ! this.__meta.has(key) )
			this._get(key, key, []);

		return this.__meta.get(key).uses;
	}
}