Added Priority Queue as a Linked List example + changed property access modifiers

Author: Alienpruts

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

@@ -0,0 +1,30 @@
+<?php
+/**
+ * Created by PhpStorm.
+ * User: bert
+ * Date: 3/10/18
+ * Time: 23:33
+ */
+
+use App\Chapter04\AgentPriorityQueueList;
+
+require_once __DIR__ . "/../vendor/autoload.php";
+
+try {
+    $agents = new AgentPriorityQueueList(10);
+
+    $agents->enqueueWithPriority("Fred", 1);
+    $agents->enqueueWithPriority("John", 2);
+    $agents->enqueueWithPriority("Keith", 3);
+    $agents->enqueueWithPriority("Adiyan", 4);
+    $agents->enqueueWithPriority("Michael", 2);
+
+    $agents->display();
+
+    echo $agents->dequeue() . PHP_EOL;
+    echo $agents->dequeue() . PHP_EOL;
+
+
+} catch (Exception $e){
+    echo $e->getMessage();
+}

src/Chapter03/LinkedList.php

@@ -11,10 +11,10 @@
 
 class LinkedList implements \Iterator
 {
-    private $_firstNode = NULL;
-    private $_totalNodes = 0;
-    private $_currentNode = NULL;
-    private $_currentPosition = 0;
+    protected $_firstNode = NULL;
+    protected $_totalNodes = 0;
+    protected $_currentNode = NULL;
+    protected $_currentPosition = 0;
 
     public function insert(string $data = NULL)
     {

src/Chapter04/AgentPriorityQueueList.php

@@ -0,0 +1,39 @@
+<?php
+/**
+ * Created by PhpStorm.
+ * User: bert
+ * Date: 3/10/18
+ * Time: 23:26
+ */
+
+namespace App\Chapter04;
+
+
+use OverflowException;
+
+class AgentPriorityQueueList extends AgentQueueList
+{
+    public function __construct(int $limit = 20)
+    {
+        $this->limit = $limit;
+        $this->queue = new PriorityQueueLinkedList();
+    }
+
+    public function enqueueWithPriority(string $item, int $priority)
+    {
+        if ($this->queue->getSize() < $this->limit){
+            $this->queue->insert($item, $priority);
+        } else {
+            throw new OverflowException("Queue is full");
+        }
+    }
+
+    public function display()
+    {
+        // TODO : this will show the 'Total books line', because of the implementation of display in LinkedList!
+        // This is in fact a weak way of using display method, but so be it!
+        $this->queue->display();
+    }
+
+
+}

src/Chapter04/AgentQueueList.php

@@ -15,8 +15,8 @@
 
 class AgentQueueList implements Queue
 {
-    private $limit;
-    private $queue;
+    protected $limit;
+    protected $queue;
 
     /**
      * AgentQueueList constructor.

src/Chapter04/PriorityQueueLinkedList.php

@@ -0,0 +1,49 @@
+<?php
+/**
+ * Created by PhpStorm.
+ * User: bert
+ * Date: 3/10/18
+ * Time: 23:13
+ */
+
+namespace App\Chapter04;
+
+
+use App\Chapter03\LinkedList;
+
+class PriorityQueueLinkedList extends LinkedList
+{
+    public function insert(string $data = null, int $priority = NULL)
+    {
+        $newNode = new PriorityQueueListNode($data, $priority);
+        $this->_totalNodes++;
+
+        // List is empty - newly added node is the first.
+        if ($this->_firstNode === NULL){
+            $this->_firstNode = $newNode;
+        } else {
+            $previous = $this->_firstNode;
+            $currentNode = $this->_firstNode;
+            while ($currentNode !== NULL){
+                // List is not emptu, but new item has highest priority
+                if ($currentNode->priority < $priority){
+                    if ($currentNode == $this->_firstNode){
+                        $previous = $this->_firstNode;
+                        $this->_firstNode = $newNode;
+                        $newNode->next = $previous;
+                        return;
+                    }
+
+                    $newNode->next = $currentNode;
+                    $previous->next = $newNode;
+                    return;
+                }
+                $previous = $currentNode;
+                $currentNode = $currentNode->next;
+            }
+        }
+    return true;
+    }
+
+
+}

src/Chapter04/PriorityQueueListNode.php

@@ -0,0 +1,32 @@
+<?php
+/**
+ * Created by PhpStorm.
+ * User: bert
+ * Date: 3/10/18
+ * Time: 23:04
+ */
+
+namespace App\Chapter04;
+
+
+use App\Chapter03\ListNode;
+
+class PriorityQueueListNode extends ListNode
+{
+
+    public $data = NULL;
+    public $next = NULL;
+    public $priority = NULL;
+
+    /**
+     * PriorityQueueListNode constructor.
+     * @param null $data
+     * @param null $priority
+     */
+    public function __construct(string $data = NULL, int $priority = NULL)
+    {
+        $this->data = $data;
+        $this->priority = $priority;
+    }
+    
+}