/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this file,
 * You can obtain one at http://mozilla.org/MPL/2.0/. */
"use strict";

const { assert } = require("resource://devtools/shared/DevToolsUtils.js");

/**
 * The `TaskCache` allows for re-using active tasks when spawning a second task
 * would simply duplicate work and is unnecessary. It maps from a task's unique
 * key to the promise of its result.
 */
const TaskCache = (module.exports = class TaskCache {
  constructor() {
    this._cache = new Map();
  }

  /**
   * Get the promise keyed by the given unique `key`, if one exists.
   *
   * @param {Any} key
   * @returns {Promise<Any> | undefined}
   */
  get(key) {
    return this._cache.get(key);
  }

  /**
   * Put the task result promise in the cache and associate it with the given
   * `key` which must not already have an entry in the cache.
   *
   * @param {Any} key
   * @param {Promise<Any>} promise
   */
  put(key, promise) {
    assert(!this._cache.has(key), "We should not override extant entries");

    this._cache.set(key, promise);
  }

  /**
   * Remove the cache entry with the given key.
   *
   * @param {Any} key
   */
  remove(key) {
    assert(
      this._cache.has(key),
      `Should have an extant entry for key = ${key}`
    );

    this._cache.delete(key);
  }
});

/**
 * Create a new action-orchestrating task that is automatically cached. The
 * tasks themselves are responsible from removing themselves from the cache.
 *
 * @param {Function(...args) -> Any} getCacheKey
 * @param {Generator(...args) -> Any} task
 *
 * @returns Cacheable, Action-Creating Task
 */
TaskCache.declareCacheableTask = function ({ getCacheKey, task }) {
  const cache = new TaskCache();

  return function (...args) {
    return async function ({ dispatch, getState }) {
      const key = getCacheKey(...args);

      const extantResult = cache.get(key);
      if (extantResult) {
        return extantResult;
      }

      // Ensure that we have our new entry in the cache *before* dispatching the
      // task!
      let resolve;
      cache.put(
        key,
        new Promise(r => {
          resolve = r;
        })
      );

      resolve(
        dispatch(async function () {
          try {
            args.push(() => cache.remove(key), dispatch, getState);
            return await task(...args);
          } catch (error) {
            // Don't perma-cache errors.
            if (cache.get(key)) {
              cache.remove(key);
            }
            throw error;
          }
        })
      );

      return cache.get(key);
    };
  };
};
