Added Queue as a Linked List example

Author: Alienpruts

chap-04/queue-as-linked-list.php

@@ -0,0 +1,27 @@
+<?php
+/**
+ * Created by PhpStorm.
+ * User: bert
+ * Date: 3/10/18
+ * Time: 22:33
+ */
+
+use App\Chapter04\AgentQueueList;
+
+require_once __DIR__ . "/../vendor/autoload.php";
+
+try{
+    $agents = new AgentQueueList(10);
+    $agents->enqueue("Fred");
+    $agents->enqueue("John");
+    $agents->enqueue("Keith");
+    $agents->enqueue("Adiyan");
+    $agents->enqueue("Michael");
+
+    echo $agents->dequeue() . PHP_EOL;
+    echo $agents->dequeue() . PHP_EOL;
+    echo $agents->peek() . PHP_EOL;
+
+} catch (Exception $e){
+    echo $e->getMessage();
+}

src/Chapter04/AgentQueueList.php

@@ -0,0 +1,77 @@
+<?php
+/**
+ * Created by PhpStorm.
+ * User: bert
+ * Date: 3/10/18
+ * Time: 22:27
+ */
+
+namespace App\Chapter04;
+
+
+use App\Chapter03\LinkedList;
+use OverflowException;
+use UnderflowException;
+
+class AgentQueueList implements Queue
+{
+    private $limit;
+    private $queue;
+
+    /**
+     * AgentQueueList constructor.
+     * @param $limit
+     */
+    public function __construct(int $limit = 20)
+    {
+        $this->limit = $limit;
+        $this->queue = new LinkedList();
+    }
+
+    /**
+     * Add an item to the rear of the queue.
+     * @param string $item
+     * @return mixed
+     */
+    public function enqueue(string $item)
+    {
+        if ($this->queue->getSize() < $this->limit){
+            $this->queue->insert($item);
+        } else {
+            throw new OverflowException("Queue is full");
+        }
+    }
+
+    /**
+     * Remove an item from the front of the queue.
+     * @return mixed
+     */
+    public function dequeue() : string
+    {
+        if ($this->isEmpty()){
+            throw new UnderflowException("Queue is empty");
+        } else {
+            $lastItem = $this->peek();
+            $this->queue->deleteFirst();
+            return $lastItem;
+        }
+    }
+
+    /**
+     * Returns the front item of the queue, without removing it.
+     * @return mixed
+     */
+    public function peek() : string
+    {
+        return $this->queue->getItemByPosition(1)->data;
+    }
+
+    /**
+     * Check whether the queue is empty.
+     * @return mixed
+     */
+    public function isEmpty() : bool
+    {
+        return $this->queue->getSize() == 0;
+    }
+}