/******************************************************************************
 * K&R 5-10 
 *
 * Reverse Polish Calculator 
 * (command line args)
 *
 * program: rp_calc
 * usage..: rp_calc expression
 *          EX. rp_calc 1 1 +
 * date...: 18 November 2024
 *
 * ~yetimach
 ******************************************************************************/
#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>

#define MAXVAL 100

double val[MAXVAL];
int sp = 0, error = 0;

void push(double f){
    if (sp < MAXVAL)
        val[sp++] = f;
    else{
        printf("error: stack full, can't push %g", f);
        error = 1;
    }
}

double pop(void){
    if (sp > 0)
        return val[--sp];
    else{
        printf("error: stack empty (remember: reverse Polish order)\n");
        error = 1;
        return 0.0;
    }
}

double n_to_power(double num, int p){
    double o_num = num; /*original num*/
    if (p == 0)
        return (num > 0) ? 1 : -1;
    while (p-- > 1)
            num *= o_num;
    return num;
}

double square(double num){
    return num * num;
}

int main(int argc, char *argv[]){
    int type;
    double op2;
    while (--argc > 0){
        if (isdigit((*++argv)[0]) || isdigit((*argv)[1])){
            push(atof(*argv));
        } else {
            type = (*argv)[0];
            switch (type) {
                case '+':
                    push(pop() + pop());
                    break;
                case 'x': case '*':      /*the * must be escaped in bash shell*/
                    push(pop() * pop());
                    break;
                case '-':
                    op2 = pop();
                    push(pop() - op2);
                    break;
                case '/':  
                    op2 = pop();
                    if (op2 != 0)
                        push(pop() / op2);
                    else{
                        printf("error: zero divisor\n");
                        error = 1;
                    }
                    break;
                case '^': case 'p':
                    op2 = pop();
                    push(n_to_power(pop(), op2));
                    break;
                case '%':
                    op2 = pop();
                    push((int) pop() % (int) op2);
                    break;
                case 'a':              /* push last number, so that */
                    push(val[sp-1]);   /* for ex "12 a +" is like "12 12 +" */
                    break;
                case 's':
                    push(square(pop()));
                    break;
                default:
                    printf("error: unkown command %s\n", *argv);
                    error = 1;
                    break;
            }
        }
    }
    if (!error)
        printf("\t%.8g\n", pop());
    return 0;
}