/*
 * the stack module
 */
#include <stdio.h>
#include <ctype.h>
#include <string.h>

/*
 * macros
 */
#define MAXSTACK	1024	/* maximum number of elements on stack */

/*
 * globals accessible only within this file
 * note they are declared as static
 */
static int stack[MAXSTACK];	/* the stack itself */
static int topstack = 0;	/* index of above top element of stack */

/*
 * push a value onto the stack
 * exits on stack overflow
 */
int push(int val)
{
	/* check for overflow */
	if (topstack >= MAXSTACK){
		fprintf(stderr, "Stack is full\n");
		return(0);
	}
	/* push onto stack */
	stack[topstack++] = val;
	return(1);
}

/*
 * pop a value from the stack
 * exits on stack underflow
 */
int pop(int *val)
{
	/* check for an empty stack */
	if (topstack <= 0){
		fprintf(stderr, "Stack is empty\n");
		return(0);
	}
	/* pop from stack */
	*val = stack[--topstack];
	return(1);
}


/*
 * print the top of the stack and the elements on the stack
 */
void prstack(void)
{
	int i;			/* used to walk the stack */


	/* check for empty stack */
	if (topstack == 0){
		printf("stack is empty (top = %d\n", topstack);
		return;
	}
	/* print the top index */
	printf("stack contains (top = %d)\n", topstack);

	/* now print the elements of the stack, top down */
	for(i = topstack - 1; i >= 0; i--)
		printf("\t%3d: %10d\n", i, stack[i]);
}

#ifdef DEBUG
/* convert string to integer; return 1 on success, 0 on error */
int stoi(char *s, int *n)
{
	int sgn = 1;	/* sign of input number */

	/* initialize *n */
	*n = 0;

	/* skip white space */
	while (isspace(*s))
		s++;
	/* is it negative? */
	if (*s == '-'){
		sgn = -1;
		s++;
	}
	/* go until there are no more digitss */
	if (!isdigit(*s))
		return(0);
	while(isdigit(*s))
		*n = *n * 10 + (*s++ - '0');
	*n *= sgn;
	/* return if you're at the end */
	return(*s == '\0' || *s == '\n');
}

/*
 * driver
 */
int main(void)
{
	char buf[MAXSTACK];	/* input buffer */
	char *b;		/* pointer to buf */
	int n;			/* input number */


	while(printf("+ for push, - for pop, p for print > "),
		fgets(buf, 1000, stdin) != NULL){
		buf[strlen(buf)-1] = '\n';
		for(b = buf; isspace(*b); b++)
			/* EMPTY */;
		switch(*b){
		case '+':
			if(stoi(b+1, &n) == 0){
				printf("%s: not an integer\n", b+1);
				continue;
			}
			if (push(n))
				printf("\tpushed %d\n", n);
			continue;
		case '-':
			if (pop(&n))
				printf("\tpopped %d\n", n);
			continue;
		case 'p':
			prstack();
			continue;
		case '\0':
			continue;
		default:
			printf("TYPO: %s", buf);
			continue;
		}
	}
	putchar('\n');
	return(0);
}
#endif