added Hash Table

README.md

@@ -26,6 +26,15 @@ This repo contains the following sections implemented in **JavaScript**
 
 Find the detailed contents and problem list here: [Table Of Contents](TOC.md)
 
+## Contributors
+
+| Name                                      | Twitter                                     | LinkedIn                                      | Website                                    |
+| ----------------------------------------- | ------------------------------------------- | --------------------------------------------- | ------------------------------------------ |
+| [Ashok Dey](https://github.com/ashokdey)  | [ashokdey\_](https://twitter.com/ashokdey_) | [Ashok Dey](https://linkedin.com/in/ashokdey) | [https://ashokdey.in](https://ashokdey.in) |
+| [Ashu Deshwal](https://github.com/TheSTL) | [\_TheSTL\_](https://twitter.com/_TheSTL_)  | -                                             | -                                          |
+
+[Detailed list of contributors](https://github.com/knaxus/problem-solving-javascript/graphs/contributors)
+
 ## Contribution Guide
 
 It's great to know that you want to contribute to this repo. Thanks for taking interest. please fing the [guide here](https://github.com/knaxus/problem-solving-javascript/blob/master/CONTRIBUTING.md)

TOC.md

@@ -49,6 +49,7 @@
   - Problems
     - [K Largest Elements](src/_DataStructures_/Heaps/k-largest-in-array)
     - [K Smallest Elements](src/_DataStructures_/Heaps/k-smallest-in-array)
+- [Hash Table](src/_DataStructures_/HashTable)
 
 ### Logical Problems
 

src/_DataStructures_/HashTable/HashEntry.js

@@ -0,0 +1,9 @@
+class HashEntry {
+  constructor({ key, value }) {
+    this.key = key;
+    this.value = value;
+    this.next = null;
+  }
+}
+
+module.exports = HashEntry;

src/_DataStructures_/HashTable/index.js

@@ -0,0 +1,225 @@
+const HashEntry = require('./HashEntry');
+
+class HashTable {
+  constructor(slots, { allowResize = true, strongHash = true, custonHash = null }) {
+    // init with a default set of slots
+    this.slot = slots || 19;
+    // size to hold the current size
+    // and help to resize when the table is half filled
+    this.size = 0;
+    // threshold (let it be 70%)
+    this.threshold = 0.7;
+    // the main bucket
+    this.bucket = new Array(this.slot);
+    this.allowResize = allowResize;
+    this.strongHash = strongHash;
+    if (custonHash) {
+      // eslint-disable-next-line no-underscore-dangle
+      this._hash = custonHash;
+    }
+
+    // fill the bucket with null
+    for (let i = 0; i < this.slot; i += 1) this.bucket[i] = null;
+  }
+
+  _hash(key) {
+    // convert the key to String;
+    const stringKey = String(key);
+
+    let index = 0;
+    const PRIME_MULTIPLIER = 1801; // Random prime number
+    const PRIME_ADDER = 2029; // Random prime number
+
+    // loop till the length of the key or max 100
+    const loopTill = Math.min(stringKey.length, 100);
+
+    for (let i = 0; i < loopTill; i += 1) {
+      const char = stringKey[i];
+      const value = char.charCodeAt(0) - 96;
+      index = (index * PRIME_MULTIPLIER + value) % this.bucket.length;
+      if (this.strongHash) {
+        index = (index + PRIME_ADDER) % this.bucket.length;
+      }
+    }
+    return index;
+  }
+
+  _resize() {
+    const oldSlot = this.slot;
+    const oldBucket = this.bucket;
+
+    this.slot = oldSlot * 2;
+    const newBucket = new Array(this.slot);
+    // fill the new bucket with nulls
+    for (let i = 0; i < this.slot; i += 1) newBucket[i] = null;
+
+    this.bucket = newBucket;
+
+    for (let i = 0; i < oldSlot; i += 1) {
+      if (oldBucket[i]) {
+        let head = oldBucket[i];
+
+        while (head !== null) {
+          const { key, value } = head;
+          // eslint-disable-next-line no-underscore-dangle
+          const newIndex = this._hash(key);
+          // eslint-disable-next-line no-underscore-dangle
+          this._push(newIndex, { key, value });
+          head = head.next;
+        }
+      }
+    }
+  }
+
+  _push(index, value) {
+    /**
+     * Utility to add a SLL to the index in case of more than one
+     * key hashes to the same index
+     */
+    const node = new HashEntry(value);
+    if (!this.bucket[index]) {
+      this.bucket[index] = node;
+      this.size += 1;
+      return index;
+    }
+
+    let head = this.bucket[index];
+    // extract the key and see if it already exists
+    const { key, value: newValue } = value;
+
+    // traverse to the end
+    while (head.next !== null) {
+      if (head.key === key) {
+        // overridet the value with the new value
+        head.value = newValue;
+        return index;
+      }
+      head = head.next;
+    }
+    // if the key was not found
+    head.next = node;
+    this.size += 1;
+    return index;
+  }
+
+  _value(index, key) {
+    let head = this.bucket[index];
+    while (head !== null) {
+      if (head.key === key) {
+        return head.value;
+      }
+      head = head.next;
+    }
+    return null;
+  }
+
+  // eslint-disable-next-line class-methods-use-this
+  _convertNodesToSLL(nodeCollection) {
+    // convert collection of nodes into a SLL
+    let head = nodeCollection[0];
+    const start = head;
+    let i = 1;
+    while (i < nodeCollection.length) {
+      head.next = nodeCollection[i];
+      i += 1;
+      head = head.next;
+    }
+
+    return start;
+  }
+
+  set(key, value) {
+    // eslint-disable-next-line no-underscore-dangle
+    const index = this._hash(key);
+    // storing value as an key-value pair
+    // eslint-disable-next-line no-underscore-dangle
+    this._push(index, { key, value });
+
+    /**
+     * calculate the load factor, if it's greater than threshold
+     * resize the hash table
+     */
+    const loadFactor = Number((this.size / this.slot).toFixed(1));
+    if (loadFactor > this.threshold && this.allowResize) {
+      // console.log('Resizing hash table');
+      // eslint-disable-next-line no-underscore-dangle
+      this._resize();
+    }
+  }
+
+  get(key) {
+    // get the index for the given key
+    // eslint-disable-next-line no-underscore-dangle
+    const index = this._hash(key);
+    if (!this.bucket[index]) return null;
+    // eslint-disable-next-line no-underscore-dangle
+    return this._value(index, key);
+  }
+
+  remove(key) {
+    // get the index
+    // eslint-disable-next-line no-underscore-dangle
+    const index = this._hash(key);
+
+    // get the SLL using the index
+    let head = this.bucket[index];
+    // return null if the head is null
+    if (!head) {
+      return null;
+    }
+
+    if (head.key === key) {
+      let node = head;
+      this.bucket[index] = head.next;
+      const val = { key, value: node.value };
+      node = null;
+      this.size -= 1;
+      return val;
+    }
+
+    let previous = null;
+
+    while (head !== null) {
+      if (head.key === key) {
+        let node = head;
+        previous.next = head.next;
+        this.size -= 1;
+        const res = { key, value: node.value };
+        node = null;
+        return res;
+      }
+      previous = head;
+      head = head.next;
+    }
+    return null;
+  }
+
+  getSize() {
+    return this.size;
+  }
+
+  isEmpty() {
+    return this.size === 0;
+  }
+}
+
+// const ht = new HashTable(5, { allowResize: false, strongHash: false });
+// ht.set('maroon', 'I maroon');
+// ht.set('hello', 'I am a new value');
+// console.log(ht.bucket);
+// ht.set('hell', 'Bad value');
+// ht.set('hello', 'I am a yet another value');
+// ht.set('yellow', 'I am yellow');
+
+// console.log(ht.get('hello'));
+// console.log(ht.get('maroon'));
+// console.log(ht.bucket);
+
+// console.log('deleting hello........');
+// console.log(ht.remove('hello'));
+// console.log(ht.bucket);
+
+// console.log(ht.remove('yellow'));
+// console.log(ht.bucket);
+
+module.exports = HashTable;

src/_DataStructures_/LinkedList/index.js

@@ -1,8 +1,8 @@
-// do not change the node class, you never know how many things it caan break! :) 
+// do not change the node class, you never know how many things it caan break! :)
 class Node {
   constructor(data, next) {
     this.data = data;
-    this.next = next;
+    this.next = next || null;
   }
 }