Added Circular queue as PHP array example

Author: Alienpruts

chap-04/circular-queue-as-array.php

@@ -0,0 +1,29 @@
+<?php
+/**
+ * Created by PhpStorm.
+ * User: bert
+ * Date: 4/10/18
+ * Time: 9:24
+ */
+
+use App\Chapter04\CircularQueue;
+
+require_once __DIR__ . '/../vendor/autoload.php';
+
+try {
+    $agents = new CircularQueue(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;
+
+
+} catch (Exception $e){
+    echo $e->getMessage();
+}

src/Chapter04/CircularQueue.php

@@ -0,0 +1,113 @@
+<?php
+/**
+ * Created by PhpStorm.
+ * User: bert
+ * Date: 4/10/18
+ * Time: 9:09
+ */
+
+namespace App\Chapter04;
+
+
+use OverflowException;
+use UnderflowException;
+
+class CircularQueue implements Queue
+{
+
+    private $queue;
+    private $limit;
+    private $front = 0;
+    private $rear = 0;
+
+    /**
+     * CircularQueue constructor.
+     * @param $limit
+     */
+    public function __construct(int $limit = 5)
+    {
+        $this->limit = $limit;
+        $this->queue = [];
+    }
+
+    /**
+     * Returns the size of the Circular queue.
+     * This is calculated by either substracting the front position from the rear position or by substracting the sum
+     * of the front and rear positions from the total limit.
+     * @return int
+     */
+    public function size()
+    {
+        if ($this->rear > $this->front){
+            return $this->rear - $this->front;
+        }
+
+        return $this->limit - $this->front + $this->rear;
+    }
+
+    /**
+     * Add an item to the rear of the queue.
+     * @param string $item
+     * @return mixed
+     */
+    public function enqueue(string $item)
+    {
+        if ($this->isFull()){
+            throw new OverflowException("Queue is full.");
+        } else {
+            $this->queue[$this->rear] = $item;
+            // get the last position by using a rest division by the total limit.
+            $this->rear = ($this->rear + 1) % $this->limit;
+        }
+    }
+
+    /**
+     * Remove an item from the front of the queue.
+     * @return mixed|string
+     */
+    public function dequeue()
+    {
+        $item = "";
+        if ($this->isEmpty()){
+            throw new UnderflowException("Queue is empty");
+        } else {
+            $item = $this->queue[$this->front];
+            $this->queue[$this->front] = NULL;
+            $this->front = ($this->front + 1) % $this->limit;
+        }
+
+        return $item;
+    }
+
+    /**
+     * Returns the front item of the queue, without removing it.
+     * @return string
+     */
+    public function peek() : string
+    {
+        return $this->queue[$this->front];
+    }
+
+    /**
+     * Check whether the queue is empty.
+     * @return bool
+     */
+    public function isEmpty() : bool
+    {
+        return $this->front == $this->rear;
+    }
+
+    /**
+     * Check whether the queue is full.
+     * @return bool
+     */
+    public function isFull() : bool
+    {
+        $diff = $this->rear - $this->front;
+        if ($diff == -1 || $diff == ($this->limit - 1)){
+            return true;
+        }
+
+        return false;
+    }
+}