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

#include "llist.h"
#include "stack.h"


void init_stack(stack *S)
{
    printf("initializing\n");
    S -> L.head = (elist*) 0;
    printf("initialized\n");
}

int stack_empty(stack *S)
{
    if (S -> L.head == (elist*) 0) return 1;
    else return 0;
}

/* The following will not work because the memory used for storing e
   will be released as soon as we exit the function */

/* void push(stack S, int x) */
/* { */
/*     elist e; */
/*     e.key = x; */
/*     e.next = S.L.head; */
/*     S.L.head = &e; */
/* } */

void push(stack *S, int x)
{
    elist *e = malloc(sizeof(elist));
    if (e == NULL) { 
	printf("malloc failed\n");
	exit(1);
    }
    e -> key = x; /* equivalent to (*e).key = x; */
    e -> next = S -> L.head;
    S -> L.head = e;
}

int pop(stack *S)
{
    elist *e;
    int k;
    if (stack_empty(S)) {
	printf("Error: stack underflow\n");
	exit(1);
    }
    e = S -> L.head;
    S -> L.head = e -> next;
    k = e -> key;
    free(e);
    return k;
}

void print_stack(stack *S) 
{
    elist *e = S -> L.head;
    printf("S: ");
    while (e != (elist*) 0) {
	printf("%d ", e -> key);
	e = e -> next;
    }
    printf("\n");
}



/* int main()  */
/* { */
/*     stack T; */
/*     stack *S = &T; */
/*     init_stack(S); */
/*     print_stack(S); */

/*     push(S, 10); print_stack(S); */
/*     push(S, 9); print_stack(S); */
/*     push(S, 6); print_stack(S); */
/*     pop(S); print_stack(S); */
/*     pop(S); print_stack(S); */
/*     push(S, 8); print_stack(S); */
/*     push(S, 9); print_stack(S); */
/*     pop(S); print_stack(S); */

/*     return 0; */
/* } */