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

typedef struct stacknode {
  Jval val;
  struct stacknode *link;
} StackNode;

typedef struct {
  StackNode *top;
  int size;
} TrueStack;


/*  allocates an empty stack. */
Stack new_stack()
{
    TrueStack *ts;

    ts = (TrueStack *) malloc(sizeof(TrueStack));
    ts->top = NULL;
    ts->size = 0;
    return (Stack) ts;
}

/* pushes a new piece of data onto the stack */
void stack_push(Stack s, Jval val) {
    StackNode *sn;
    TrueStack *ts;
		     
    ts = (TrueStack *) s;

    /* Create the new node */
    sn = (StackNode *) malloc(sizeof(StackNode));
    sn->val = val;
    sn->link = ts->top;

    /* Put the new node at the beginning of the stack */
    ts->top = sn;
    ts->size++;
}

/* returns the top value on the stack without popping it */
Jval stack_top(Stack s)
{ 
    TrueStack *ts;
					 
    if (stack_empty(s)) {
	fprintf(stderr, "Error: Stack_top called on an empty stack\n");
	exit(1);
							  }
    ts = (TrueStack *) s;
    return ts->top->val;
}

/* pops a value off the stack and returns it */
Jval stack_pop(Stack s) {
    Jval val;
    StackNode *sn;
    TrueStack *ts;

    val = stack_top(s);
    ts = (TrueStack *) s;
    sn = ts->top;
    ts->top = sn->link;
    ts->size--;
    free(sn);
    return val;
}
  
int stack_size(Stack s) {
    TrueStack *ts;

    ts = (TrueStack *) s;
    return ts->size;
}

int stack_empty(Stack s) {
    return (stack_size(s) == 0);
}

void free_stack(Stack s)
{
    TrueStack *ts;

    while (!stack_empty(s)) (void) stack_pop(s);
    ts = (TrueStack *) s;
    free(ts);
}