/* Exercise 4-8. Suppose that there will never be more than one character of
    pushback. Modify getch and ungetch accordingly. */
#include <stdio.h>
#include <stdlib.h> /* for atof() */
#include <ctype.h>

#define MAXOP   100 /* max size of operand or operator */
#define NUMBER  '0' /* signal that a number was found */
#define MAXVAL  100 /* maximum depth of val stack */

enum boolean { EMPTY, FULL };

int getop(char []);
void push(double);
double pop(void);
int getch(void);
void ungetch(int);

int sp = 0;         /* next free stack position */
double val[MAXVAL]; /* value stack */
char buf;           /* buffer for ungetch */
int state = EMPTY;  /* whether buffer is full or empty */

/* reverse Polish calculator */
main()
{
    int type;
    double op1, op2;
    char s[MAXOP];

    while ((type = getop(s)) != EOF) {
        switch (type) {
        case NUMBER:
            push(atof(s));
            break;
        case '+':
            push(pop() + pop());
            break;
        case '*':
            push(pop() * pop());
            break;
        case '-':
            op2 = pop();
            push(pop() - op2);
            break;
        case '/':
            op2 = pop();
            if (op2 != 0.0)
                push(pop() / op2);
            else
                printf("error: zero divisor\n");
            break;
        case '%':
            op2 = pop();
            if (op2 != 0.0) {
                for (op1 = pop(); op1 >= op2; op1 -= op2)
                    ;
                push(op1);
            } else
                printf("error: zero divisor\n");
            break;
        case '\n':
            printf("\t%.8g\n", pop());
            break;
        default:
            printf("error: unknown command %s\n", s);
            break;
        }
    }
    return 0;
}

/* push:  push f onto value stack */
void push(double f)
{
    if (sp < MAXVAL)
        val[sp++] = f;
    else
        printf("error: stack full, can't push %g\n", f);
}

/* pop:  pop and return top value from stack */
double pop(void)
{
    if (sp > 0)
        return val[--sp];
    else {
        printf("error: stack empty\n");
        return 0.0;
    }
}

/* getop:  get next operator or numeric operand */
int getop(char s[])
{
    int i, c;

    while ((s[0] = c = getch()) == ' ' || c == '\t')
        ;
    s[1] = '\0';
    if (!isdigit(c) && c != '.' && c != '+' && c != '-')
        return c;   /* not a number */
    i = 0;
    if (isdigit(c) || c == '+' || c == '-') /* collect integer part */
        while (isdigit(s[++i] = c = getch()))
            ;
    if (c == '.')   /* collect fraction part */
        while (isdigit(s[++i] = c = getch()))
            ;
    s[i] = '\0';
    if (c != EOF)
        ungetch(c);
    if ((s[0] == '+' || s[0] == '-') && i == 1)
        return s[0];    /* return operator if no digits afterwards */
    return NUMBER;
}

int getch(void) /* get a (possibly pushed back) character */
{
    if (state == FULL) {
        state = EMPTY;
        return buf;
    }
    return getchar();
}

void ungetch(int c) /* push character back on input */
{
    if (state == FULL)
        printf("ungetch: buffer is full\n");
    else {
        buf = c;
        state = FULL;
    }
}