#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <errno.h>

#include "lifo.h"

#define LMAGIC 0xA5A5A5A5

// an element in a lifo
struct erec {
	void *data;
	struct erec * next;
};

// a lifo
struct lrec {
	int magic; // used to mark a valid lifo
	struct erec * head; // last element added to the lifo
	int nume;  // number of elements in the lifo
};
	
// initializes an existing lifo
struct lrec * Lnew()
{
	struct lrec * lp;
	lp = (struct lrec *)malloc(sizeof(struct lrec));
	if (lp)
	{
		lp->magic = LMAGIC;
		lp->head = NULL;
		lp->nume = 0;
	}
	else // lp is NULL
	{
		errno = ENOMEM;
		return NULL;
	}
	return lp;
}

// add an item to the lifo
int Ladd(struct lrec * lp, void *data)
{
	struct erec * ep;
	if (!lp || lp->magic != LMAGIC)
	{
		errno = EBADL;
		return -1;
	}
	ep = (struct erec *)malloc(sizeof(struct erec)); // allocate new element
	if (ep)
	{
		ep->data = data;
		ep->next = lp->head;
		lp->head = ep;
		lp->nume++;
	}
	else // ep is NULL, malloc failed
	{
		errno = ENOMEM;
		return -1;
	}
	return 0;
}

// remove an item from the lifo
void * Lremove(struct lrec * lp)
{
	if (!lp || lp->magic != LMAGIC)
	{
		errno = EBADL;
		return NULL;
	}
	if (lp->nume >= 1) // first be sure there is data in the lifo
	{
		void *data;
		struct erec *ep;
		ep = lp->head;
		lp->head = lp->head->next;
		lp->nume--;
		data = ep->data;
		free(ep);
		return data;
	}
	else // no data in the lifo
	{
		errno = ENODATA;
		return NULL;
	}
}

// return the head item of the lifo
void * Lshow(struct lrec * lp)
{
	if (!lp || lp->magic != LMAGIC)
	{
		errno = EBADL;
		return NULL;
	}
	if (lp->nume >= 1) // make sure there is data in the lifo
	{
		return lp->head->data;	
	}
	else // no data in the lifo
	{
		errno = ENODATA;
		return NULL;
	}
}

// move the head item to the tail of the lifo
// and return the new head item
int Lfree(struct lrec * lp)
{
	if (!lp || lp->magic != LMAGIC)
	{
		errno = EBADL;
		return -1;
	}
	while (lp->nume >= 1) // make sure there is data in the lifo
	{
		struct erec *ep;
		ep = lp->head;
		lp->head = lp->head->next;
		lp->nume--;
		free(ep);
	}
	return 0;
}