var PriorityQueue = function(size) {
  var me = {}, slots, i, total = null;

  // initialize arrays to hold queue elements
  size = Math.max(+size | 0, 1);
  slots = [];
  for (i = 0; i < size; i += 1) {
    slots.push([]);
  }

  //  Public methods
  me.size = function () {
    var i;
    if (total === null) {
      total = 0;
      for (i = 0; i < size; i += 1) {
        total += slots[i].length;
      }
    }
    return total;
  };

  me.enqueue = function (obj, priority) {
    var priorityOrig;

    // Convert to integer with a default value of 0.
    priority = priority && + priority | 0 || 0;

    // Clear cache for total.
    total = null;
    if (priority) {
      priorityOrig = priority;
      if (priority < 0 || priority >= size) {
        priority = (size - 1);
        // put obj at the end of the line
        console.error("invalid priority: " + priorityOrig + " must be between 0 and " + priority);
      }
    }

    slots[priority].push(obj);
  };

  me.dequeue = function (callback) {
    var obj = null, i, sl = slots.length;

    // Clear cache for total.
    total = null;
    for (i = 0; i < sl; i += 1) {
      if (slots[i].length) {
        obj = slots[i].shift();
        break;
      }
    }
    return obj;
  };

  return me;
};

/**
 * Generate an Object pool with a specified `factory`.
 *
 * @param {Object} factory
 *   Factory to be used for generating and destorying the items.
 * @param {String} factory.name
 *   Name of the factory. Serves only logging purposes.
 * @param {Function} factory.create
 *   Should create the item to be acquired,
 *   and call it's first callback argument with the generated item as it's argument.
 * @param {Function} factory.destroy
 *   Should gently close any resources that the item is using.
 *   Called before the items is destroyed.
 * @param {Function} factory.validate
 *   Should return true if connection is still valid and false
 *   If it should be removed from pool. Called before item is
 *   acquired from pool.
 * @param {Number} factory.max
 *   Maximum number of items that can exist at the same time.  Default: 1.
 *   Any further acquire requests will be pushed to the waiting list.
 * @param {Number} factory.min
 *   Minimum number of items in pool (including in-use). Default: 0.
 *   When the pool is created, or a resource destroyed, this minimum will
 *   be checked. If the pool resource count is below the minimum, a new
 *   resource will be created and added to the pool.
 * @param {Number} factory.idleTimeoutMillis
 *   Delay in milliseconds after the idle items in the pool will be destroyed.
 *   And idle item is that is not acquired yet. Waiting items doesn't count here.
 * @param {Number} factory.reapIntervalMillis
 *   Cleanup is scheduled in every `factory.reapIntervalMillis` milliseconds.
 * @param {Boolean|Function} factory.log
 *   Whether the pool should log activity. If function is specified,
 *   that will be used instead. The function expects the arguments msg, loglevel
 * @param {Number} factory.priorityRange
 *   The range from 1 to be treated as a valid priority
 * @param {RefreshIdle} factory.refreshIdle
 *   Should idle resources be destroyed and recreated every idleTimeoutMillis? Default: true.
 * @returns {Object} An Object pool that works with the supplied `factory`.
 */
exports.Pool = function (factory) {
  var me = {},

      idleTimeoutMillis = factory.idleTimeoutMillis || 30000,
      reapInterval = factory.reapIntervalMillis || 1000,
      refreshIdle = ('refreshIdle' in factory) ? factory.refreshIdle : true,
      availableObjects = [],
      waitingClients = new PriorityQueue(factory.priorityRange || 1),
      count = 0,
      removeIdleScheduled = false,
      removeIdleTimer = null,
      draining = false,

      // Prepare a logger function.
      log = factory.log ?
        (function (str, level) {
           if (typeof factory.log === 'function') {
             factory.log(str, level);
           }
           else {
             console.log(level.toUpperCase() + " pool " + factory.name + " - " + str);
           }
         }
        ) :
        function () {};

  factory.validate = factory.validate || function() { return true; };
        
  factory.max = parseInt(factory.max, 10);
  factory.min = parseInt(factory.min, 10);
  
  factory.max = Math.max(isNaN(factory.max) ? 1 : factory.max, 1);
  factory.min = Math.min(isNaN(factory.min) ? 0 : factory.min, factory.max-1);
  
  ///////////////

  /**
   * Request the client to be destroyed. The factory's destroy handler
   * will also be called.
   *
   * This should be called within an acquire() block as an alternative to release().
   *
   * @param {Object} obj
   *   The acquired item to be destoyed.
   */
  me.destroy = function(obj) {
    count -= 1;
    availableObjects = availableObjects.filter(function(objWithTimeout) {
              return (objWithTimeout.obj !== obj);
    });
    factory.destroy(obj);
    
    ensureMinimum();
  };

  /**
   * Checks and removes the available (idle) clients that have timed out.
   */
  function removeIdle() {
    var toRemove = [],
        now = new Date().getTime(),
        i,
        al, tr,
        timeout;

    removeIdleScheduled = false;

    // Go through the available (idle) items,
    // check if they have timed out
    for (i = 0, al = availableObjects.length; i < al && (refreshIdle || (count - factory.min)) > toRemove.length ; i += 1) {
      timeout = availableObjects[i].timeout;
      if (now >= timeout) {
        // Client timed out, so destroy it.
        log("removeIdle() destroying obj - now:" + now + " timeout:" + timeout, 'verbose');
        toRemove.push(availableObjects[i].obj);
      } 
    }

    for (i = 0, tr = toRemove.length; i < tr; i += 1) {
      me.destroy(toRemove[i]);
    }

    // Replace the available items with the ones to keep.
    al = availableObjects.length;

    if (al > 0) {
      log("availableObjects.length=" + al, 'verbose');
      scheduleRemoveIdle();
    } else {
      log("removeIdle() all objects removed", 'verbose');
    }
  }


  /**
   * Schedule removal of idle items in the pool.
   *
   * More schedules cannot run concurrently.
   */
  function scheduleRemoveIdle() {
    if (!removeIdleScheduled) {
      removeIdleScheduled = true;
      removeIdleTimer = setTimeout(removeIdle, reapInterval);
    }
  }

  /**
   * Handle callbacks with either the [obj] or [err, obj] arguments in an
   * adaptive manner. Uses the `cb.length` property to determine the number
   * of arguments expected by `cb`.
   */
  function adjustCallback(cb, err, obj) {
    if (!cb) return;
    if (cb.length <= 1) {
      cb(obj);
    } else {
      cb(err, obj);
    }
  }

  /**
   * Try to get a new client to work, and clean up pool unused (idle) items.
   *
   *  - If there are available clients waiting, shift the first one out (LIFO),
   *    and call its callback.
   *  - If there are no waiting clients, try to create one if it won't exceed
   *    the maximum number of clients.
   *  - If creating a new client would exceed the maximum, add the client to
   *    the wait list.
   */
  function dispense() {
    var obj = null,
        objWithTimeout = null,
        err = null,
        clientCb = null,
        waitingCount = waitingClients.size();
        
    log("dispense() clients=" + waitingCount + " available=" + availableObjects.length, 'info');
    if (waitingCount > 0) {
      while (availableObjects.length > 0) {
        log("dispense() - reusing obj", 'verbose');
        objWithTimeout = availableObjects[0];
        if (!factory.validate(objWithTimeout.obj)) {
          me.destroy(objWithTimeout.obj);
          continue;
        }
        availableObjects.shift();
        clientCb = waitingClients.dequeue();
        return clientCb(err, objWithTimeout.obj);
      }
      if (count < factory.max) {
        createResource();
      }
    }
  }
  
  function createResource() {
    count += 1;
    log("createResource() - creating obj - count=" + count + " min=" + factory.min + " max=" + factory.max, 'verbose');
    factory.create(function () {
      var err, obj;
      var clientCb = waitingClients.dequeue();
      if (arguments.length > 1) {
        err = arguments[0];
        obj = arguments[1];
      } else {
        err = (arguments[0] instanceof Error) ? arguments[0] : null;
        obj = (arguments[0] instanceof Error) ? null : arguments[0];
      }
      if (err) {
        count -= 1;
        if (clientCb) {
          clientCb(err, obj);
        }
        process.nextTick(function(){
          dispense();
        });
      } else {
        if (clientCb) {
          clientCb(err, obj);
        } else {
          me.release(obj);
        }
      }
    });
  }
  
  function ensureMinimum() {
    var i, diff;
    if (!draining && (count < factory.min)) {
      diff = factory.min - count;
      for (i = 0; i < diff; i++) {
        createResource();
      }
    }
  }

  /**
   * Request a new client. The callback will be called,
   * when a new client will be availabe, passing the client to it.
   *
   * @param {Function} callback
   *   Callback function to be called after the acquire is successful.
   *   The function will receive the acquired item as the first parameter.
   *
   * @param {Number} priority
   *   Optional.  Integer between 0 and (priorityRange - 1).  Specifies the priority
   *   of the caller if there are no available resources.  Lower numbers mean higher
   *   priority.
   *
   * @returns {Object} `true` if the pool is not fully utilized, `false` otherwise.
   */
  me.acquire = function (callback, priority) {
    if (draining) {
      throw new Error("pool is draining and cannot accept work");
    }
    waitingClients.enqueue(callback, priority);
    dispense();
    return (count < factory.max);
  };

  me.borrow = function (callback, priority) {
    log("borrow() is deprecated. use acquire() instead", 'warn');
    me.acquire(callback, priority);
  };

  /**
   * Return the client to the pool, in case it is no longer required.
   *
   * @param {Object} obj
   *   The acquired object to be put back to the pool.
   */
  me.release = function (obj) {
	// check to see if this object has already been released (i.e., is back in the pool of availableObjects)
    if (availableObjects.some(function(objWithTimeout) { return (objWithTimeout.obj === obj); })) {
      log("release called twice for the same resource: " + (new Error().stack), 'error');
      return;
    }
    //log("return to pool");
    var objWithTimeout = { obj: obj, timeout: (new Date().getTime() + idleTimeoutMillis) };
    availableObjects.push(objWithTimeout);
    log("timeout: " + objWithTimeout.timeout, 'verbose');
    dispense();
    scheduleRemoveIdle();
  };

  me.returnToPool = function (obj) {
    log("returnToPool() is deprecated. use release() instead", 'warn');
    me.release(obj);
  };

  /**
   * Disallow any new requests and let the request backlog dissapate.
   *
   * @param {Function} callback
   *   Optional. Callback invoked when all work is done and all clients have been
   *   released.
   */
  me.drain = function(callback) {
    log("draining", 'info');

    // disable the ability to put more work on the queue.
    draining = true;

    var check = function() {
      if (waitingClients.size() > 0) {
        // wait until all client requests have been satisfied.
        setTimeout(check, 100);
      } else if (availableObjects.length != count) {
        // wait until all objects have been released.
        setTimeout(check, 100);
      } else {
        if (callback) {
          callback();
        }
      }
    };
    check();
  };

  /**
   * Forcibly destroys all clients regardless of timeout.  Intended to be
   * invoked as part of a drain.  Does not prevent the creation of new
   * clients as a result of subsequent calls to acquire.
   *
   * Note that if factory.min > 0, the pool will destroy all idle resources
   * in the pool, but replace them with newly created resources up to the
   * specified factory.min value.  If this is not desired, set factory.min
   * to zero before calling destroyAllNow()
   *
   * @param {Function} callback
   *   Optional. Callback invoked after all existing clients are destroyed.
   */
  me.destroyAllNow = function(callback) {
    log("force destroying all objects", 'info');
    var willDie = availableObjects;
    availableObjects = [];
    var obj = willDie.shift();
    while (obj !== null && obj !== undefined) {
      me.destroy(obj.obj);
      obj = willDie.shift();
    }
    removeIdleScheduled = false;
    clearTimeout(removeIdleTimer);
    if (callback) {
      callback();
    }
  };

  /**
   * Decorates a function to use a acquired client from the object pool when called.
   *
   * @param {Function} decorated
   *   The decorated function, accepting a client as the first argument and 
   *   (optionally) a callback as the final argument.
   *
   * @param {Number} priority
   *   Optional.  Integer between 0 and (priorityRange - 1).  Specifies the priority
   *   of the caller if there are no available resources.  Lower numbers mean higher
   *   priority.
   */
  me.pooled = function(decorated, priority) {
    return function() {
      var callerArgs = arguments;
      var callerCallback = callerArgs[callerArgs.length - 1];
      var callerHasCallback = typeof callerCallback === 'function';
      me.acquire(function(err, client) {
        if(err) {
          if(callerHasCallback) {
            callerCallback(err);
          }
          return;
        }

        var args = [client].concat(Array.prototype.slice.call(callerArgs, 0, callerHasCallback ? -1 : undefined));
        args.push(function() {
          me.release(client);
          if(callerHasCallback) {
            callerCallback.apply(null, arguments);
          }
        });
        
        decorated.apply(null, args);
      }, priority);
    };
  };

  me.getPoolSize = function() {
    return count;
  };

  me.getName = function() {
    return factory.name;
  };

  me.availableObjectsCount = function() {
    return availableObjects.length;
  };

  me.waitingClientsCount = function() {
    return waitingClients.size();
  };


  // create initial resources (if factory.min > 0)
  ensureMinimum();

  return me;
};
