From 138c3d990521bc7a577eefdf0c24775825aa41e0 Mon Sep 17 00:00:00 2001
From: Oleksii Trekhleb <trehleb@gmail.com>
Date: Tue, 3 Apr 2018 18:17:14 +0300
Subject: [PATCH] Refactor MinHeap.

---
 .eslintrc                                     |  3 +-
 README.md                                     |  5 +--
 src/data-structures/heap/MinHeap.js           | 12 +++----
 .../priority-queue/PriorityQueue.js           | 23 +++++++++++++
 .../__test__/PriorityQueue.test.js            | 34 +++++++++++++++++++
 5 files changed, 68 insertions(+), 9 deletions(-)
 create mode 100644 src/data-structures/priority-queue/PriorityQueue.js
 create mode 100644 src/data-structures/priority-queue/__test__/PriorityQueue.test.js

diff --git a/.eslintrc b/.eslintrc
index 29137996..ac7908bd 100644
--- a/.eslintrc
+++ b/.eslintrc
@@ -7,6 +7,7 @@
   },
   "rules": {
     "no-bitwise": "off",
-    "no-lonely-if": "off"
+    "no-lonely-if": "off",
+    "class-methods-use-this": "warn"
   }
 }
diff --git a/README.md b/README.md
index 2e42c91e..4e651196 100644
--- a/README.md
+++ b/README.md
@@ -9,8 +9,9 @@
 3. [Stack](https://github.com/trekhleb/javascript-algorithms/tree/master/src/data-structures/stack)
 4. [Hash Table](https://github.com/trekhleb/javascript-algorithms/tree/master/src/data-structures/hash-table)
 5. [Heap](https://github.com/trekhleb/javascript-algorithms/tree/master/src/data-structures/heap)
-5. [Trie](https://github.com/trekhleb/javascript-algorithms/tree/master/src/data-structures/trie)
-6. [Tree](https://github.com/trekhleb/javascript-algorithms/tree/master/src/data-structures/tree)
+6. [Priority Queue](https://github.com/trekhleb/javascript-algorithms/tree/master/src/data-structures/priority-queue)
+7. [Trie](https://github.com/trekhleb/javascript-algorithms/tree/master/src/data-structures/trie)
+8. [Tree](https://github.com/trekhleb/javascript-algorithms/tree/master/src/data-structures/tree)
     * [Binary Search Tree](https://github.com/trekhleb/javascript-algorithms/tree/master/src/data-structures/tree/binary-search-tree)
 
 ## [Algorithms](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms)
diff --git a/src/data-structures/heap/MinHeap.js b/src/data-structures/heap/MinHeap.js
index d8e92c77..e768c0b5 100644
--- a/src/data-structures/heap/MinHeap.js
+++ b/src/data-structures/heap/MinHeap.js
@@ -85,7 +85,7 @@ export default class MinHeap {
 
     while (
       MinHeap.hasParent(currentIndex) &&
-      MinHeap.lessThen(this.heapContainer[currentIndex], this.parent(currentIndex))
+      this.lessThen(this.heapContainer[currentIndex], this.parent(currentIndex))
     ) {
       this.swap(currentIndex, MinHeap.getParentIndex(currentIndex));
       currentIndex = MinHeap.getParentIndex(currentIndex);
@@ -101,14 +101,14 @@ export default class MinHeap {
     while (this.hasLeftChild(currentIndex)) {
       if (
         this.hasRightChild(currentIndex) &&
-        MinHeap.lessThen(this.rightChild(currentIndex), this.leftChild(currentIndex))
+        this.lessThen(this.rightChild(currentIndex), this.leftChild(currentIndex))
       ) {
         nextIndex = MinHeap.getRightChildIndex(currentIndex);
       } else {
         nextIndex = MinHeap.getLeftChildIndex(currentIndex);
       }
 
-      if (MinHeap.lessThen(this.heapContainer[currentIndex], this.heapContainer[nextIndex])) {
+      if (this.lessThen(this.heapContainer[currentIndex], this.heapContainer[nextIndex])) {
         break;
       }
 
@@ -121,7 +121,7 @@ export default class MinHeap {
     return this.heapContainer.toString();
   }
 
-  static compare(a, b) {
+  compare(a, b) {
     if (a === b) {
       return 0;
     }
@@ -129,7 +129,7 @@ export default class MinHeap {
     return a < b ? -1 : 1;
   }
 
-  static lessThen(a, b) {
-    return MinHeap.compare(a, b) === -1;
+  lessThen(a, b) {
+    return this.compare(a, b) === -1;
   }
 }
diff --git a/src/data-structures/priority-queue/PriorityQueue.js b/src/data-structures/priority-queue/PriorityQueue.js
new file mode 100644
index 00000000..c8a15706
--- /dev/null
+++ b/src/data-structures/priority-queue/PriorityQueue.js
@@ -0,0 +1,23 @@
+import MinHeap from '../heap/MinHeap';
+
+// It is the same as min heap except that when comparing to elements
+// we take into account not element's value but rather its priority.
+export default class PriorityQueue extends MinHeap {
+  constructor() {
+    super();
+    this.priorities = {};
+  }
+
+  add(item, priority = 0) {
+    this.priorities[item] = priority;
+    super.add(item);
+  }
+
+  compare(a, b) {
+    if (this.priorities[a] === this.priorities[b]) {
+      return 0;
+    }
+
+    return this.priorities[a] < this.priorities[b] ? -1 : 1;
+  }
+}
diff --git a/src/data-structures/priority-queue/__test__/PriorityQueue.test.js b/src/data-structures/priority-queue/__test__/PriorityQueue.test.js
new file mode 100644
index 00000000..b628553f
--- /dev/null
+++ b/src/data-structures/priority-queue/__test__/PriorityQueue.test.js
@@ -0,0 +1,34 @@
+import PriorityQueue from '../PriorityQueue';
+
+describe('PriorityQueue', () => {
+  it('should create default priority queue', () => {
+    const priorityQueue = new PriorityQueue();
+
+    expect(priorityQueue).toBeDefined();
+  });
+
+  it('should insert items to the queue and respect priorities', () => {
+    const priorityQueue = new PriorityQueue();
+
+    priorityQueue.add(10, 1);
+    expect(priorityQueue.peek()).toBe(10);
+
+    priorityQueue.add(5, 2);
+    expect(priorityQueue.peek()).toBe(10);
+
+    priorityQueue.add(100, 0);
+    expect(priorityQueue.peek()).toBe(100);
+  });
+
+  it('should poll from queue with respect to priorities', () => {
+    const priorityQueue = new PriorityQueue();
+
+    priorityQueue.add(10, 1);
+    priorityQueue.add(5, 2);
+    priorityQueue.add(100, 0);
+
+    expect(priorityQueue.poll()).toBe(100);
+    expect(priorityQueue.poll()).toBe(10);
+    expect(priorityQueue.poll()).toBe(5);
+  });
+});