import {Encoder, Decoder} from '@msgpack/msgpack';
import {LRUCache} from 'lru-cache';

import {withResolvers} from '@/util/promise.js';
import Script from '@/util/script.js';

import EntityFactory from './entity-factory.js';

const cache = new LRUCache({
  max: 128,
});

const decoder = new Decoder();
const encoder = new Encoder();

const textDecoder = new TextDecoder();

export default class Ecs {

  $$caret = 1;

  Components = {};

  deferredChanges = {}

  $$deindexing = new Set();

  $$destructionDependencies = new Map();

  $$detached = new Set();

  diff = {};

  $$entities = {};

  $$entityFactory = new EntityFactory();

  $$reindexing = new Set();

  Systems = {};

  constructor({Systems, Components} = {}) {
    for (const componentName in Components) {
      this.Components[componentName] = new Components[componentName](this);
    }
    for (const systemName in Systems) {
      this.Systems[systemName] = new Systems[systemName](this);
    }
  }

  addDestructionDependency(id, promise) {
    if (!this.$$destructionDependencies.has(id)) {
      this.$$destructionDependencies.set(id, {promises: new Set()})
    }
    const {promises} = this.$$destructionDependencies.get(id);
    promises.add(promise);
    promise.then(() => {
      promises.delete(promise);
      if (!this.$$destructionDependencies.get(id)?.resolvers) {
        this.$$destructionDependencies.delete(id);
      }
    });
  }

  async apply(patch) {
    const creating = [];
    const destroying = new Set();
    const inserting = [];
    const removing = [];
    const updating = [];
    for (const entityIdString in patch) {
      const entityId = parseInt(entityIdString);
      const components = patch[entityId];
      if (false === components) {
        destroying.add(entityId);
        continue;
      }
      const componentsToRemove = [];
      const componentsToUpdate = {};
      for (const componentName in components) {
        if (false === components[componentName]) {
          componentsToRemove.push(componentName);
        }
        else {
          componentsToUpdate[componentName] = components[componentName];
        }
      }
      if (componentsToRemove.length > 0) {
        removing.push([entityId, componentsToRemove]);
      }
      if (this.$$entities[entityIdString]) {
        const entity = this.$$entities[entityIdString];
        let isInserting = false;
        const entityInserts = {};
        let isUpdating = false;
        const entityUpdates = {};
        for (const componentName in componentsToUpdate) {
          if (entity[componentName]) {
            entityUpdates[componentName] = componentsToUpdate[componentName];
            isUpdating = true;
          }
          else {
            entityInserts[componentName] = componentsToUpdate[componentName];
            isInserting = true;
          }
        }
        if (isInserting) {
          inserting.push([entityId, entityInserts]);
        }
        if (isUpdating) {
          updating.push([entityId, entityUpdates]);
        }
      }
      else {
        creating.push([entityId, componentsToUpdate]);
      }
    }
    const promises = [];
    if (inserting.length > 0) {
      promises.push(this.insertMany(inserting));
    }
    if (updating.length > 0) {
      promises.push(this.updateMany(updating));
    }
    if (creating.length > 0) {
      promises.push(this.createManySpecific(creating));
    }
    await Promise.all(promises);
    if (destroying.size > 0) {
      this.destroyMany(destroying);
    }
    if (removing.length > 0) {
      this.removeMany(removing);
    }
  }

  applyDeferredChanges(entityId) {
    const changes = this.deferredChanges[entityId];
    delete this.deferredChanges[entityId];
    for (const components of changes) {
      this.markChange(entityId, components);
    }
  }

  attach(entityIds) {
    for (const entityId of entityIds) {
      this.$$detached.delete(entityId);
      this.$$reindexing.add(entityId);
      this.applyDeferredChanges(entityId);
    }
  }

  changed(criteria) {
    const it = Object.entries(this.diff).values();
    return {
      [Symbol.iterator]() {
        return this;
      },
      next: () => {
        let result = it.next();
        hasResult: while (!result.done) {
          if (false === result.value[1]) {
            result = it.next();
            continue;
          }
          for (const componentName of criteria) {
            if (!(componentName in result.value[1])) {
              result = it.next();
              continue hasResult;
            }
          }
          break;
        }
        if (result.done) {
          return {done: true, value: undefined};
        }
        return {done: false, value: this.get(result.value[0])};
      },
    };
  }

  async create(components = {}) {
    const [entityId] = await this.createMany([components]);
    return entityId;
  }

  async createDetached(components = {}) {
    const [entityId] = await this.createManyDetached([components]);
    return entityId;
  }

  async createMany(componentsList) {
    const specificsList = [];
    for (const components of componentsList) {
      specificsList.push([this.$$caret, components]);
      this.$$caret += 1;
    }
    return this.createManySpecific(specificsList);
  }

  async createManyDetached(componentsList) {
    const specificsList = [];
    for (const components of componentsList) {
      specificsList.push([this.$$caret, components]);
      this.$$detached.add(this.$$caret);
      this.deferredChanges[this.$$caret] = [];
      this.$$caret += 1;
    }
    return this.createManySpecific(specificsList);
  }

  async createManySpecific(specificsList) {
    if (0 === specificsList.length) {
      return;
    }
    const entityIds = new Set();
    const creating = {};
    for (let i = 0; i < specificsList.length; i++) {
      const [entityId, components] = specificsList[i];
      if (!this.$$detached.has(entityId)) {
        this.deferredChanges[entityId] = [];
      }
      const componentNames = [];
      for (const componentName in components) {
        if (this.Components[componentName]) {
          componentNames.push(componentName);
        }
      }
      entityIds.add(entityId);
      for (const componentName of componentNames) {
        if (!creating[componentName]) {
          creating[componentName] = [];
        }
        creating[componentName].push([entityId, components[componentName]]);
      }
      this.markChange(entityId, components);
    }
    const promises = [];
    for (const i in creating) {
      promises.push(this.Components[i].createMany(creating[i]));
    }
    await Promise.all(promises);
    for (let i = 0; i < specificsList.length; i++) {
      const [entityId, components] = specificsList[i];
      this.$$reindexing.add(entityId);
      this.rebuild(entityId, () => Object.keys(components));
      if (this.$$detached.has(entityId)) {
        continue;
      }
      this.applyDeferredChanges(entityId);
    }
    return entityIds;
  }

  async createSpecific(entityId, components) {
    const [created] = await this.createManySpecific([[entityId, components]]);
    return created;
  }

  deindex(entityIds) {
    // Stage 4 Draft / July 6, 2024
    // const attached = entityIds.difference(this.$$detached);
    const attached = new Set(entityIds);
    for (const detached of this.$$detached) {
      attached.delete(detached);
    }
    if (0 === attached.size) {
      return;
    }
    for (const systemName in this.Systems) {
      const System = this.Systems[systemName];
      if (!System.active) {
        continue;
      }
      System.deindex(attached);
    }
  }

  static async deserialize(ecs, view) {
    const componentNames = Object.keys(ecs.Components);
    const {entities, systems} = decoder.decode(view.buffer);
    for (const system of systems) {
      const System = ecs.system(system);
      if (System) {
        System.active = true;
      }
    }
    const specifics = [];
    let max = 1;
    for (const id in entities) {
      max = Math.max(max, parseInt(id));
      specifics.push([
        parseInt(id),
        Object.fromEntries(
          Object.entries(entities[id])
            .filter(([componentName]) => componentNames.includes(componentName)),
        ),
      ]);
    }
    ecs.$$caret = max + 1;
    await ecs.createManySpecific(specifics);
    return ecs;
  }

  destroy(entityId) {
    if (!this.$$destructionDependencies.has(entityId)) {
      this.$$destructionDependencies.set(entityId, {promises: new Set()});
    }
    const dependencies = this.$$destructionDependencies.get(entityId);
    if (!dependencies.resolvers) {
      dependencies.resolvers = withResolvers();
    }
    return dependencies.resolvers.promise;
  }

  destroyMany(entityIds) {
    const destroying = {};
    // this.deindex(entityIds);
    for (const entityId of entityIds) {
      this.$$deindexing.add(entityId);
      if (!this.$$entities[entityId]) {
        throw new Error(`can't destroy non-existent entity ${entityId}`);
      }
      for (const componentName of this.$$entities[entityId].constructor.componentNames) {
        if (!destroying[componentName]) {
          destroying[componentName] = [];
        }
        destroying[componentName].push(entityId);
      }
    }
    for (const i in destroying) {
      this.Components[i].destroyMany(destroying[i]);
    }
    for (const entityId of entityIds) {
      delete this.$$entities[entityId];
      this.diff[entityId] = false;
    }
  }

  detach(entityIds) {
    for (const entityId of entityIds) {
      this.$$deindexing.add(entityId);
      this.$$detached.add(entityId);
    }
  }

  get entities() {
    const ids = [];
    for (const entity of Object.values(this.$$entities)) {
      ids.push(entity.id);
    }
    return ids;
  }

  get(entityId) {
    return this.$$entities[entityId];
  }

  async insert(entityId, components) {
    return this.insertMany([[entityId, components]]);
  }

  async insertMany(entities) {
    const inserting = {};
    const unique = new Set();
    for (const [entityId, components] of entities) {
      const diff = {};
      for (const componentName in components) {
        if (!inserting[componentName]) {
          inserting[componentName] = [];
        }
        diff[componentName] = {};
        inserting[componentName].push([entityId, components[componentName]]);
      }
      unique.add(entityId);
      this.markChange(entityId, diff);
    }
    const promises = [];
    for (const componentName in inserting) {
      promises.push(this.Components[componentName].insertMany(inserting[componentName]));
    }
    await Promise.all(promises);
    for (const [entityId, components] of entities) {
      this.$$reindexing.add(entityId);
      this.rebuild(entityId, (componentNames) => [...new Set(componentNames.concat(Object.keys(components)))]);
    }
  }

  markChange(entityId, components) {
    if (this.deferredChanges[entityId]) {
      this.deferredChanges[entityId].push(components);
      return;
    }
    // Deleted?
    if (false === components) {
      this.diff[entityId] = false;
    }
    // Created?
    else if (!this.diff[entityId]) {
      this.diff[entityId] = components;
    }
    // Otherwise, merge.
    else {
      for (const componentName in components) {
        this.diff[entityId][componentName] = false === components[componentName]
          ? false
          : this.Components[componentName].mergeDiff(
            this.diff[entityId][componentName] || {},
            components[componentName],
          );
      }
    }
  }

  async readJson(uri) {
    const key = ['$$json', uri].join(':');
    if (!cache.has(key)) {
      const {promise, resolve, reject} = withResolvers();
      cache.set(key, promise);
      this.readAsset(uri)
        .then((chars) => {
          resolve(
            chars.byteLength > 0
              ? JSON.parse(textDecoder.decode(chars))
              : {},
          );
        })
        .catch(reject);
    }
    return cache.get(key);
  }

  async readScript(uriOrCode, context = {}) {
    if (!uriOrCode) {
      return undefined;
    }
    let code = '';
    if (!uriOrCode.startsWith('/')) {
      code = uriOrCode;
    }
    else {
      const buffer = await this.readAsset(uriOrCode);
      if (buffer.byteLength > 0) {
        code = textDecoder.decode(buffer);
      }
    }
    if (!code) {
      return undefined;
    }
    return Script.fromCode(code, context);
  }

  rebuild(entityId, componentNames) {
    let Class;
    if (componentNames) {
      let existing = [];
      if (this.$$entities[entityId]) {
        existing.push(...this.$$entities[entityId].constructor.componentNames);
      }
      Class = this.$$entityFactory.makeClass(componentNames(existing), this.Components);
    }
    else {
      Class = this.$$entities[entityId].constructor;
    }
    if (this.$$entities[entityId] && Class === this.$$entities[entityId].constructor) {
      // Eventually - memoizable.
    }
    return this.$$entities[entityId] = new Class(entityId);
  }

  reindex(entityIds) {
    // Stage 4 Draft / July 6, 2024
    // const attached = entityIds.difference(this.$$detached);
    const attached = new Set(entityIds);
    for (const detached of this.$$detached) {
      attached.delete(detached);
    }
    if (0 === attached.size) {
      return;
    }
    for (const systemName in this.Systems) {
      const System = this.Systems[systemName];
      if (!System.active) {
        continue;
      }
      System.reindex(attached);
    }
  }

  remove(entityId, components) {
    this.removeMany([[entityId, components]]);
  }

  removeMany(entities) {
    const removing = {};
    const unique = new Set();
    for (const [entityId, components] of entities) {
      unique.add(entityId);
      const diff = {};
      for (const componentName of components) {
        diff[componentName] = false;
        if (!removing[componentName]) {
          removing[componentName] = [];
        }
        removing[componentName].push(entityId);
      }
      this.markChange(entityId, diff);
    }
    for (const componentName in removing) {
      this.Components[componentName].destroyMany(removing[componentName]);
    }
    for (const [entityId, components] of entities) {
      this.$$reindexing.add(entityId);
      this.rebuild(entityId, (componentNames) => componentNames.filter((type) => !components.includes(type)));
    }
  }

  static serialize(ecs, view) {
    const buffer = encoder.encode(ecs.toJSON());
    if (!view) {
      view = new DataView(new ArrayBuffer(buffer.length));
    }
    (new Uint8Array(view.buffer)).set(buffer);
    return view;
  }

  setClean() {
    this.diff = {};
  }

  system(systemName) {
    return this.Systems[systemName];
  }

  tick(elapsed) {
    // tick systems
    for (const systemName in this.Systems) {
      const System = this.Systems[systemName];
      if (!System.active) {
        continue;
      }
      if (!System.frequency) {
        System.tick(elapsed);
        continue;
      }
      System.elapsed += elapsed;
      while (System.elapsed >= System.frequency) {
        System.tick(System.frequency);
        System.elapsed -= System.frequency;
      }
    }
    // destroy entities
    const destroying = new Set();
    for (const [entityId, {promises}] of this.$$destructionDependencies) {
      if (0 === promises.size) {
        destroying.add(entityId);
      }
    }
    if (destroying.size > 0) {
      this.destroyMany(destroying);
      for (const entityId of destroying) {
        this.$$destructionDependencies.get(entityId).resolvers.resolve();
        this.$$destructionDependencies.delete(entityId);
      }
    }
    // update indices
    if (this.$$deindexing.size > 0) {
      this.deindex(this.$$deindexing);
      this.$$deindexing.clear();
    }
    if (this.$$reindexing.size > 0) {
      this.reindex(this.$$reindexing);
      this.$$reindexing.clear();
    }
  }

  toJSON() {
    const entities = {};
    for (const id in this.$$entities) {
      if (this.$$entities[id]) {
        entities[id] = this.$$entities[id].toJSON();
      }
    }
    const systems = [];
    for (const systemName in this.Systems) {
      if (this.Systems[systemName].active) {
        systems.push(systemName);
      }
    }
    return {
      entities,
      systems,
    };
  }

  async updateMany(entities) {
    const updating = {};
    const unique = new Set();
    for (const [entityId, components] of entities) {
      this.rebuild(entityId);
      for (const componentName in components) {
        if (!updating[componentName]) {
          updating[componentName] = [];
        }
        updating[componentName].push([entityId, components[componentName]]);
      }
      this.$$reindexing.add(entityId);
      unique.add(entityId);
    }
    const promises = [];
    for (const componentName in updating) {
      promises.push(this.Components[componentName].updateMany(updating[componentName]));
    }
    await Promise.all(promises);
  }

}