Add new hash table methods.

Author: trekhleb

src/data-structures/hash-table/HashTable.js

@@ -1,14 +1,29 @@
 import LinkedList from '../linked-list/LinkedList';
 
+// Hash table size directly affects on the number of collisions.
+// The bigger the hash table size the less collisions you'll get.
+// For demonstrating purposes hash table size is small to show how collisions
+// are being handled.
 const defaultHashTableSize = 32;
 
 export default class HashTable {
+  /**
+   * @param {number} hashTableSize
+   */
   constructor(hashTableSize = defaultHashTableSize) {
     // Create hash table of certain size and fill each bucket with empty linked list.
     this.buckets = Array(hashTableSize).fill(null).map(() => new LinkedList());
+
+    // Just to keep track of all actual keys in a fast way.
+    this.keys = {};
   }
 
-  // Converts key string to hash number.
+  /**
+   * Converts key string to hash number.
+   *
+   * @param {string} key
+   * @return {number}
+   */
   hash(key) {
     const hash = Array.from(key).reduce(
       (hashAccumulator, keySymbol) => (hashAccumulator + keySymbol.charCodeAt(0)),
@@ -19,8 +34,14 @@ export default class HashTable {
     return hash % this.buckets.length;
   }
 
-  insert(key, value) {
-    const bucketLinkedList = this.buckets[this.hash(key)];
+  /**
+   * @param {string} key
+   * @param {*} value
+   */
+  set(key, value) {
+    const keyHash = this.hash(key);
+    this.keys[key] = keyHash;
+    const bucketLinkedList = this.buckets[keyHash];
     const node = bucketLinkedList.find({ callback: nodeValue => nodeValue.key === key });
 
     if (!node) {
@@ -32,8 +53,14 @@ export default class HashTable {
     }
   }
 
+  /**
+   * @param {string} key
+   * @return {*}
+   */
   delete(key) {
-    const bucketLinkedList = this.buckets[this.hash(key)];
+    const keyHash = this.hash(key);
+    delete this.keys[key];
+    const bucketLinkedList = this.buckets[keyHash];
     const node = bucketLinkedList.find({ callback: nodeValue => nodeValue.key === key });
 
     if (node) {
@@ -43,10 +70,29 @@ export default class HashTable {
     return null;
   }
 
+  /**
+   * @param {string} key
+   * @return {*}
+   */
   get(key) {
     const bucketLinkedList = this.buckets[this.hash(key)];
     const node = bucketLinkedList.find({ callback: nodeValue => nodeValue.key === key });
 
     return node ? node.value.value : null;
   }
+
+  /**
+   * @param {string} key
+   * @return {boolean}
+   */
+  has(key) {
+    return Object.hasOwnProperty.call(this.keys, key);
+  }
+
+  /**
+   * @return {string[]}
+   */
+  getKeys() {
+    return Object.keys(this.keys);
+  }
 }

src/data-structures/hash-table/__test__/HashTable.test.js

@@ -17,19 +17,23 @@ describe('HashTable', () => {
     expect(hashTable.hash('abc')).toBe(6);
   });
 
-  it('should insert, read and delete data with collisions', () => {
+  it('should set, read and delete data with collisions', () => {
     const hashTable = new HashTable(3);
 
     expect(hashTable.hash('a')).toBe(1);
     expect(hashTable.hash('b')).toBe(2);
     expect(hashTable.hash('c')).toBe(0);
     expect(hashTable.hash('d')).toBe(1);
 
-    hashTable.insert('a', 'sky-old');
-    hashTable.insert('a', 'sky');
-    hashTable.insert('b', 'sea');
-    hashTable.insert('c', 'earth');
-    hashTable.insert('d', 'ocean');
+    hashTable.set('a', 'sky-old');
+    hashTable.set('a', 'sky');
+    hashTable.set('b', 'sea');
+    hashTable.set('c', 'earth');
+    hashTable.set('d', 'ocean');
+
+    expect(hashTable.has('x')).toBeFalsy();
+    expect(hashTable.has('b')).toBeTruthy();
+    expect(hashTable.has('c')).toBeTruthy();
 
     const stringifier = value => `${value.key}:${value.value}`;
 
@@ -47,18 +51,38 @@ describe('HashTable', () => {
     expect(hashTable.get('a')).toBeNull();
     expect(hashTable.get('d')).toBe('ocean');
 
-    hashTable.insert('d', 'ocean-new');
+    hashTable.set('d', 'ocean-new');
     expect(hashTable.get('d')).toBe('ocean-new');
   });
 
   it('should be possible to add objects to hash table', () => {
     const hashTable = new HashTable();
 
-    hashTable.insert('objectKey', { prop1: 'a', prop2: 'b' });
+    hashTable.set('objectKey', { prop1: 'a', prop2: 'b' });
 
     const object = hashTable.get('objectKey');
     expect(object).toBeDefined();
     expect(object.prop1).toBe('a');
     expect(object.prop2).toBe('b');
   });
+
+  it('should track actual keys', () => {
+    const hashTable = new HashTable(3);
+
+    hashTable.set('a', 'sky-old');
+    hashTable.set('a', 'sky');
+    hashTable.set('b', 'sea');
+    hashTable.set('c', 'earth');
+    hashTable.set('d', 'ocean');
+
+    expect(hashTable.getKeys()).toEqual(['a', 'b', 'c', 'd']);
+    expect(hashTable.has('a')).toBeTruthy();
+    expect(hashTable.has('x')).toBeFalsy();
+
+    hashTable.delete('a');
+
+    expect(hashTable.has('a')).toBeFalsy();
+    expect(hashTable.has('b')).toBeTruthy();
+    expect(hashTable.has('x')).toBeFalsy();
+  });
 });