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

#define NUMBER '0'
#define MAXLINE 1000
#define MAXVAL 100

int getop(char[]);

void push(double);
double pop(void);
void clear(void);
double peek(void);

// Naïve implementation of remainder operation
double frem(double f1, double f2) {
    return f1 - (f2 * (int)(f1 / f2));
}

/**
 * Reverse polish notation calculator
 */
int main(int argc, char* argv[]) {
    if (argc <= 1) {
        fprintf(stderr, "Usage: rpol EXPR...");
        return 1;
    }
    argv++; // Skip program name
    double op2;
    for (; *argv != NULL; argv++) {
        char* tok = *argv;
        int type = getop(tok);
        switch (type) {
            case NUMBER:
                push(atof(tok));
                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 division attempted!\n");
                break;
            case '%':
                op2 = pop();
                if (op2 != 0.0) 
                    push(frem(pop(), op2));
                else 
                    printf("error: zero division attempted!\n");
                break;
            case '~':
                clear();
                break;
            default:
                printf("unknown command %s\n", tok); // Just ignore it?
                break;
        }
    }

    printf("\t%.8g\n", pop());
    return 0;
}

int sp; // Stack position
double val[MAXVAL]; // Value stack
void push(double f) {
    if (sp < MAXVAL) {
        val[sp++] = f;
    } else {
        printf("error, stack full\n");
    }
}

double pop(void) {
    if (sp >= 0) {
        return val[--sp];
    } else {
        printf("error, empty stack\n");
        return 0.0;
    }
}

double peek(void) {
    if (sp >= 0) {
        return val[sp-1];
    } else {
        printf("error, empty stack\n");
        return 0.0;
    }   
}

void clear(void) {
    sp = 0;
}

int getop(char s[]) {
    char c = s[0];
    if (!isdigit(c) && c != '.') 
        return c;
    return NUMBER;
}