SOURCE FILE: QueueP.cpp

// *********************************************************
// Implementation file QueueP.cpp for the ADT queue.
// Pointer-based implementation.
// *********************************************************

#include "QueueP.h"  // header file
#include <cstddef>  // for NULL

// The queue is implemented as a circular linked list
// with one external pointer to the back of the queue.
struct queueNode
{  queueItemType Item;
   QptrType       Next;
};  // end struct

queueClass::queueClass() : BackPtr(NULL)
{
}  // end default constructor

queueClass::queueClass(const queueClass& Q)
{  // Implementation left as an exercise (Exercise 4).
}  // end copy constructor

queueClass::~queueClass()
{
   bool Success;

   while (!QueueIsEmpty())
      QueueDelete(Success);
   // Assertion: BackPtr == NULL
}  // end destructor

bool queueClass::QueueIsEmpty() const
{
   return bool(BackPtr == NULL);
}  // end QueueIsEmpty

void queueClass::QueueInsert(queueItemType NewItem,
                             bool& Success)
{
   // create a new node
   QptrType NewPtr = new queueNode;

   Success = bool(NewPtr != NULL);  // check allocation
   if (Success)
   {  // allocation successful; set data portion of new node
      NewPtr->Item = NewItem;

      // insert the new node
      if (QueueIsEmpty())
         // insertion into empty queue
         NewPtr->Next = NewPtr;

      else
      {  // insertion into nonempty queue
         NewPtr->Next = BackPtr->Next;
         BackPtr->Next = NewPtr;
      }  // end if

      BackPtr = NewPtr;  // new node is at back
   }  // end if
}  // end QueueInsert

void queueClass::QueueDelete(bool& Success)
{
   Success = bool(!QueueIsEmpty());

   if (Success)
   {  // queue is not empty; remove front
      QptrType FrontPtr = BackPtr->Next;
      if (FrontPtr == BackPtr)   // special case?
         BackPtr = NULL;         // yes, one node in queue
      else
         BackPtr->Next = FrontPtr->Next;

      FrontPtr->Next = NULL;  // defensive strategy
      delete FrontPtr;
   }  // end if
}  // end QueueDelete

void queueClass::QueueDelete(queueItemType& QueueFront, 
                             bool& Success)
{
   Success = bool(!QueueIsEmpty());

   if (Success)
   {  // queue is not empty; retrieve front
      QptrType FrontPtr = BackPtr->Next;
      QueueFront = FrontPtr->Item;

      QueueDelete(Success);  // delete front
   }  // end if
}  // end QueueDelete

void queueClass::GetQueueFront(queueItemType& QueueFront, 
                               bool& Success) const
{
   Success = bool(!QueueIsEmpty());

   if (Success)
   {  // queue is not empty; retrieve front
      QptrType FrontPtr = BackPtr->Next;
      QueueFront = FrontPtr->Item;
   }   // end if
}  // end GetQueueFront
// End of implementation file.