#include <stdio.h>
#include "stack.h"

// ----------------------------------------------------------------------
// The StackNode class implements individual nodes of a stack.  There are
// two member variables -- a value, and a pointer to the next node in the
// stack (NULL if there is no next node.)

class StackNode {
  public: 

    StackNode(Jval, StackNode *);   // Create a new node with given val and next
    ~StackNode();
    Jval val;
    StackNode *next;
};

StackNode::StackNode(Jval v, StackNode *n)
{
  val = v;
  next = n;
}

StackNode::~StackNode()
{
  if (next != NULL) delete next;
}


// ----------------------------------------------------------------------
// Ok -- now, the stack class implementation.  These should all be 
// straightforward to you.


Stack::Stack()                // Create an empty stack
{
  top_node = NULL;
}

Stack::~Stack()               // Destroy a stack
{
  if (top_node != NULL) delete top_node;
}

void Stack::push(Jval v) 
{
  top_node = new StackNode(v, top_node);
}

Jval Stack::pop() 
{
  StackNode *tmp;
  Jval v;
  
  if (top_node == NULL) {
    fprintf(stderr, "ERROR: Trying to pop empty stack\n");
    //    exit(1);
  }

  tmp = top_node;
  v = tmp->val;

  /* Unhook the top node */
  top_node = tmp->next;

  /* Delete the top node */
  tmp->next = NULL;
  delete tmp;

  return v;
}

Jval Stack::top() 
{
  if (top_node == NULL) {
    fprintf(stderr, "ERROR: Trying to get the top of an empty stack\n");
    //    exit(1);
  }

  return top_node->val;
}

bool Stack::isEmpty() 
{
  return (top_node == NULL);
}