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Filename : priority-queue.js
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var _ = require("../lodash");

module.exports = PriorityQueue;

/**
 * A min-priority queue data structure. This algorithm is derived from Cormen,
 * et al., "Introduction to Algorithms". The basic idea of a min-priority
 * queue is that you can efficiently (in O(1) time) get the smallest key in
 * the queue. Adding and removing elements takes O(log n) time. A key can
 * have its priority decreased in O(log n) time.
 */
function PriorityQueue() {
  this._arr = [];
  this._keyIndices = {};
}

/**
 * Returns the number of elements in the queue. Takes `O(1)` time.
 */
PriorityQueue.prototype.size = function() {
  return this._arr.length;
};

/**
 * Returns the keys that are in the queue. Takes `O(n)` time.
 */
PriorityQueue.prototype.keys = function() {
  return this._arr.map(function(x) { return x.key; });
};

/**
 * Returns `true` if **key** is in the queue and `false` if not.
 */
PriorityQueue.prototype.has = function(key) {
  return _.has(this._keyIndices, key);
};

/**
 * Returns the priority for **key**. If **key** is not present in the queue
 * then this function returns `undefined`. Takes `O(1)` time.
 *
 * @param {Object} key
 */
PriorityQueue.prototype.priority = function(key) {
  var index = this._keyIndices[key];
  if (index !== undefined) {
    return this._arr[index].priority;
  }
};

/**
 * Returns the key for the minimum element in this queue. If the queue is
 * empty this function throws an Error. Takes `O(1)` time.
 */
PriorityQueue.prototype.min = function() {
  if (this.size() === 0) {
    throw new Error("Queue underflow");
  }
  return this._arr[0].key;
};

/**
 * Inserts a new key into the priority queue. If the key already exists in
 * the queue this function returns `false`; otherwise it will return `true`.
 * Takes `O(n)` time.
 *
 * @param {Object} key the key to add
 * @param {Number} priority the initial priority for the key
 */
PriorityQueue.prototype.add = function(key, priority) {
  var keyIndices = this._keyIndices;
  key = String(key);
  if (!_.has(keyIndices, key)) {
    var arr = this._arr;
    var index = arr.length;
    keyIndices[key] = index;
    arr.push({key: key, priority: priority});
    this._decrease(index);
    return true;
  }
  return false;
};

/**
 * Removes and returns the smallest key in the queue. Takes `O(log n)` time.
 */
PriorityQueue.prototype.removeMin = function() {
  this._swap(0, this._arr.length - 1);
  var min = this._arr.pop();
  delete this._keyIndices[min.key];
  this._heapify(0);
  return min.key;
};

/**
 * Decreases the priority for **key** to **priority**. If the new priority is
 * greater than the previous priority, this function will throw an Error.
 *
 * @param {Object} key the key for which to raise priority
 * @param {Number} priority the new priority for the key
 */
PriorityQueue.prototype.decrease = function(key, priority) {
  var index = this._keyIndices[key];
  if (priority > this._arr[index].priority) {
    throw new Error("New priority is greater than current priority. " +
        "Key: " + key + " Old: " + this._arr[index].priority + " New: " + priority);
  }
  this._arr[index].priority = priority;
  this._decrease(index);
};

PriorityQueue.prototype._heapify = function(i) {
  var arr = this._arr;
  var l = 2 * i;
  var r = l + 1;
  var largest = i;
  if (l < arr.length) {
    largest = arr[l].priority < arr[largest].priority ? l : largest;
    if (r < arr.length) {
      largest = arr[r].priority < arr[largest].priority ? r : largest;
    }
    if (largest !== i) {
      this._swap(i, largest);
      this._heapify(largest);
    }
  }
};

PriorityQueue.prototype._decrease = function(index) {
  var arr = this._arr;
  var priority = arr[index].priority;
  var parent;
  while (index !== 0) {
    parent = index >> 1;
    if (arr[parent].priority < priority) {
      break;
    }
    this._swap(index, parent);
    index = parent;
  }
};

PriorityQueue.prototype._swap = function(i, j) {
  var arr = this._arr;
  var keyIndices = this._keyIndices;
  var origArrI = arr[i];
  var origArrJ = arr[j];
  arr[i] = origArrJ;
  arr[j] = origArrI;
  keyIndices[origArrJ.key] = i;
  keyIndices[origArrI.key] = j;
};