Add HashTable.

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

@@ -0,0 +1,36 @@
+import LinkedList from '../linked-list/LinkedList';
+
+const defaultHashTableSize = 32;
+
+export default class HashTable {
+  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());
+  }
+
+  // Converts key string to hash number.
+  hash(key) {
+    const hash = Array.from(key).reduce(
+      (hashAccumulator, keySymbol) => (hashAccumulator + keySymbol.charCodeAt(0)),
+      0,
+    );
+
+    // Reduce hash number so it would fit hash table size.
+    return hash % this.buckets.length;
+  }
+
+  insert(key, value) {
+    const bucketLinkedList = this.buckets[this.hash(key)];
+    bucketLinkedList.appendUnique({ key, value });
+  }
+
+  delete(key) {
+    const bucketLinkedList = this.buckets[this.hash(key)];
+    return bucketLinkedList.deleteByKey(key);
+  }
+
+  get(key) {
+    const bucketLinkedList = this.buckets[this.hash(key)];
+    return bucketLinkedList.findByKey(key);
+  }
+}

src/data-structures/hash-table/README.md

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+# Hashed Table
+
+|Operation                  |Complexity         |
+|---------------------------|-------------------|
+|Find                       |O(1)*              |
+|Insert                     |O(1)*              |
+|Delete                     |O(1)*              |
+
+* - assuming that we have "good" hash function and big enough hash table size so that collisions are rare. 

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

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+import HashTable from '../HashTable';
+
+describe('HashTable', () => {
+  it('should create hash table of certain size', () => {
+    const defaultHashTable = new HashTable();
+    expect(defaultHashTable.buckets.length).toBe(32);
+
+    const biggerHashTable = new HashTable(64);
+    expect(biggerHashTable.buckets.length).toBe(64);
+  });
+
+  it('should generate proper hash for specified keys', () => {
+    const hashTable = new HashTable();
+
+    expect(hashTable.hash('a')).toBe(1);
+    expect(hashTable.hash('b')).toBe(2);
+    expect(hashTable.hash('abc')).toBe(6);
+  });
+
+  it('should insert, 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');
+
+    expect(hashTable.buckets[0].toString()).toBe('c:earth');
+    expect(hashTable.buckets[1].toString()).toBe('a:sky,d:ocean');
+    expect(hashTable.buckets[2].toString()).toBe('b:sea');
+
+    expect(hashTable.get('a').value).toBe('sky');
+    expect(hashTable.get('d').value).toBe('ocean');
+
+    hashTable.delete('a');
+
+    expect(hashTable.get('a')).toBeNull();
+    expect(hashTable.get('d').value).toBe('ocean');
+
+    hashTable.insert('d', 'ocean-new');
+    expect(hashTable.get('d').value).toBe('ocean-new');
+  });
+});