odoo.define('web.concurrency', function (require) {
"use strict";

/**
 * Concurrency Utils
 *
 * This file contains a short collection of useful helpers designed to help with
 * everything concurrency related in Odoo.
 *
 * The basic concurrency primitives in Odoo JS are the callback, and the
 * promises.  Promises (promise) are more composable, so we usually use them
 * whenever possible.  We use the jQuery implementation.
 *
 * Those functions are really nothing special, but are simply the result of how
 * we solved some concurrency issues, when we noticed that a pattern emerged.
 */

var Class = require('web.Class');

return {
    /**
     * Returns a promise resolved after 'wait' milliseconds
     *
     * @param {int} [wait=0] the delay in ms
     * @return {Promise}
     */
    delay: function (wait) {
        return new Promise(function (resolve) {
            setTimeout(resolve, wait);
        });
    },
    /**
     * The DropMisordered abstraction is useful for situations where you have
     * a sequence of operations that you want to do, but if one of them
     * completes after a subsequent operation, then its result is obsolete and
     * should be ignored.
     *
     * Note that is is kind of similar to the DropPrevious abstraction, but
     * subtly different.  The DropMisordered operations will all resolves if
     * they complete in the correct order.
     */
    DropMisordered: Class.extend({
        /**
         * @constructor
         *
         * @param {boolean} [failMisordered=false] whether mis-ordered responses
         *   should be failed or just ignored
         */
        init: function (failMisordered) {
            // local sequence number, for requests sent
            this.lsn = 0;
            // remote sequence number, seqnum of last received request
            this.rsn = -1;
            this.failMisordered = failMisordered || false;
        },
        /**
         * Adds a promise (usually an async request) to the sequencer
         *
         * @param {Promise} promise to ensure add
         * @returns {Promise}
         */
        add: function (promise) {
            var self = this;
            var seq = this.lsn++;
            var res = new Promise(function (resolve, reject) {
                promise.then(function (result) {
                    if (seq > self.rsn) {
                        self.rsn = seq;
                        resolve(result);
                    } else if (self.failMisordered) {
                        reject();
                    }
                }).guardedCatch(function (result) {
                    reject(result);
                });
            });
            return res;
        },
    }),
    /**
     * The DropPrevious abstraction is useful when you have a sequence of
     * operations that you want to execute, but you only care of the result of
     * the last operation.
     *
     * For example, let us say that we have a _fetch method on a widget which
     * fetches data.  We want to rerender the widget after.  We could do this::
     *
     *      this._fetch().then(function (result) {
     *          self.state = result;
     *          self.render();
     *      });
     *
     * Now, we have at least two problems:
     *
     * - if this code is called twice and the second _fetch completes before the
     *   first, the end state will be the result of the first _fetch, which is
     *   not what we expect
     * - in any cases, the user interface will rerender twice, which is bad.
     *
     * Now, if we have a DropPrevious::
     *
     *      this.dropPrevious = new DropPrevious();
     *
     * Then we can wrap the _fetch in a DropPrevious and have the expected
     * result::
     *
     *      this.dropPrevious
     *          .add(this._fetch())
     *          .then(function (result) {
     *              self.state = result;
     *              self.render();
     *          });
     */
    DropPrevious: Class.extend({
        /**
         * Registers a new promise and rejects the previous one
         *
         * @param {Promise} promise the new promise
         * @returns {Promise}
         */
        add: function (promise) {
            if (this.currentDef) {
                this.currentDef.reject();
            }
            var rejection;
            var res = new Promise(function (resolve, reject) {
                rejection = reject;
                promise.then(resolve).catch(function (reason) {
                    reject(reason);
                });
            });

            this.currentDef = res;
            this.currentDef.reject = rejection;
            return res;
        }
    }),
    /**
     * A (Odoo) mutex is a primitive for serializing computations.  This is
     * useful to avoid a situation where two computations modify some shared
     * state and cause some corrupted state.
     *
     * Imagine that we have a function to fetch some data _load(), which returns
     * a promise which resolves to something useful. Now, we have some code
     * looking like this::
     *
     *      return this._load().then(function (result) {
     *          this.state = result;
     *      });
     *
     * If this code is run twice, but the second execution ends before the
     * first, then the final state will be the result of the first call to
     * _load.  However, if we have a mutex::
     *
     *      this.mutex = new Mutex();
     *
     * and if we wrap the calls to _load in a mutex::
     *
     *      return this.mutex.exec(function() {
     *          return this._load().then(function (result) {
     *              this.state = result;
     *          });
     *      });
     *
     * Then, it is guaranteed that the final state will be the result of the
     * second execution.
     *
     * A Mutex has to be a class, and not a function, because we have to keep
     * track of some internal state.
     */
    Mutex: Class.extend({
        init: function () {
            this.lock = Promise.resolve();
            this.queueSize = 0;
            this.unlockedProm = undefined;
            this._unlock = undefined;
        },
        /**
         * Add a computation to the queue, it will be executed as soon as the
         * previous computations are completed.
         *
         * @param {function} action a function which may return a Promise
         * @returns {Promise}
         */
        exec: function (action) {
            var self = this;
            var currentLock = this.lock;
            var result;
            this.queueSize++;
            this.unlockedProm = this.unlockedProm || new Promise(function (resolve) {
                self._unlock = resolve;
            });
            this.lock = new Promise(function (unlockCurrent) {
                currentLock.then(function () {
                    result = action();
                    var always = function (returnedResult) {
                        unlockCurrent();
                        self.queueSize--;
                        if (self.queueSize === 0) {
                            self.unlockedProm = undefined;
                            self._unlock();
                        }
                        return returnedResult;
                    };
                    Promise.resolve(result).then(always).guardedCatch(always);
                });
            });
            return this.lock.then(function () {
                return result;
            });
        },
        /**
         * @returns {Promise} resolved as soon as the Mutex is unlocked
         *   (directly if it is currently idle)
         */
        getUnlockedDef: function () {
            return this.unlockedProm || Promise.resolve();
        },
    }),
    /**
     * A MutexedDropPrevious is a primitive for serializing computations while
     * skipping the ones that where executed between a current one and before
     * the execution of a new one. This is useful to avoid useless RPCs.
     *
     * You can read the Mutex description to understand its role ; for the
     * DropPrevious part of this abstraction, imagine the following situation:
     * you have a code that call the server with a fixed argument and a list of
     * operations that only grows after each call and you only care about the
     * RPC result (the server code doesn't do anything). If this code is called
     * three times (A B C) and C is executed before B has started, it's useless
     * to make an extra RPC (B) if you know that it won't have an impact and you
     * won't use its result.
     *
     * Note that the promise returned by the exec call won't be resolved if
     * exec is called before the first exec call resolution ; only the promise
     * returned by the last exec call will be resolved (the other are rejected);
     *
     * A MutexedDropPrevious has to be a class, and not a function, because we
     * have to keep track of some internal state. The exec function takes as
     * argument an action (and not a promise as DropPrevious for example)
     * because it's the MutexedDropPrevious role to trigger the RPC call that
     * returns a promise when it's time.
     */
    MutexedDropPrevious: Class.extend({
        init: function () {
            this.locked = false;
            this.currentProm = null;
            this.pendingAction = null;
            this.pendingProm = null;
        },
        /**
         * @param {function} action a function which may return a promise
         * @returns {Promise}
         */
        exec: function (action) {
            var self = this;
            var resolution;
            var rejection;
            if (this.locked) {
                this.pendingAction = action;
                var oldPendingDef = this.pendingProm;

                this.pendingProm = new Promise(function (resolve, reject) {
                    resolution = resolve;
                    rejection = reject;
                    if (oldPendingDef) {
                        oldPendingDef.reject();
                    }
                    self.currentProm.reject();
                });
                this.pendingProm.resolve = resolution;
                this.pendingProm.reject = rejection;
                return this.pendingProm;
            } else {
                this.locked = true;
                this.currentProm = new Promise(function (resolve, reject) {
                    resolution = resolve;
                    rejection = reject;
                    function unlock() {
                        self.locked = false;
                        if (self.pendingAction) {
                            var action = self.pendingAction;
                            var prom = self.pendingProm;
                            self.pendingAction = null;
                            self.pendingProm = null;
                            self.exec(action)
                                .then(prom.resolve)
                                .guardedCatch(prom.reject);
                        }
                    }
                    Promise.resolve(action())
                        .then(function (result) {
                            resolve(result);
                            unlock();
                        })
                        .guardedCatch(function (reason) {
                            reject(reason);
                            unlock();
                        });
                });
                this.currentProm.resolve = resolution;
                this.currentProm.reject = rejection;
                return this.currentProm;
            }
        }
    }),
    /**
     * Rejects a promise as soon as a reference promise is either resolved or
     * rejected
     *
     * @param {Promise} [target_def] the promise to potentially reject
     * @param {Promise} [reference_def] the reference target
     * @returns {Promise}
     */
    rejectAfter: function (target_def, reference_def) {
        return new Promise(function (resolve, reject) {
            target_def.then(resolve).guardedCatch(reject);
            reference_def.then(reject).guardedCatch(reject);
        });
    }
};

});