#ifndef STACK_H

#define STACK_H
#define printerr(format, ...) printf("error: " format "\n", ##__VA_ARGS__)

/* ERROR MESSAGES */
#define EMPTY_STACK_ERR "stack empty" 
#define SHORT_STACK_ERR "not enough elements on stack"
#define FULL_STACK_ERR  "stack full, can't push %g"

#ifndef MAX_STACK
#define MAX_STACK 100
#endif // MAX_STACK

int sp = 0;
double val[MAX_STACK];

/* length: current length of stack */
#define length() sp

/* push: push f onto value stack */
#define push(x) \
    ((sp < MAX_STACK) ? val[sp++] = x : printerr(FULL_STACK_ERR, (double) (x)))

/* pop: pop and return top value from stack */
double pop(void)
{
    return sp ? val[--sp] : (printerr(EMPTY_STACK_ERR), 0);
}

/* peek: return top value from stack, leave stack unchanged */
#define peek() (sp ? val[sp-1] : (printerr(EMPTY_STACK_ERR), 0))

/* clear: clear stack */
#define clear() (sp = 0)

/* dup: duplicate top element */
#define dup() do { double tmp = peek(); push(tmp); } while (0) 

/* swap: swap top two elements */
#define swap() \
   do { if (sp < 2) printerr(SHORT_STACK_ERR); else { \
            double tmp = peek(); \
            val[sp-1] = val[sp-2], val[sp-2] = tmp; \
        } \
   } while (0) 

#undef printerr
#endif // STACK_H