// ==========================================================================
// Reverse Polish Notation Calculator
// ==========================================================================
// RPN Calculator in C
//
// Inf2C-CS Coursework 1. Task B 
// SOLUTION

// Paul Jackson
// 10 Oct 2012

//---------------------------------------------------------------------------
// C definitions for SPIM system calls
//---------------------------------------------------------------------------
#include <stdio.h>

int read_char() { return getchar(); }
int read_int()
{
    int i;
    scanf("%i", &i);
    return i;
}
void read_string(char* s, int size) { fgets(s, size, stdin); }

void print_char(int c)     { putchar(c); }   
void print_int(int i)      { printf("%i", i); }
void print_string(char* s) { printf("%s", s); }

//---------------------------------------------------------------------------
// Global variables
//---------------------------------------------------------------------------

#define STACK_SIZE 5
int stack[STACK_SIZE];  // Stack always has 0 .. STACK_SIZE - 1 entries.
int input_char;         // Input character

int *sp = &stack[0];    // Pointer to 1st empty location on stack.


//---------------------------------------------------------------------------
// PUSH function
//---------------------------------------------------------------------------
// Push onto stack.  No-op if stack full.

void push(int i) {
    if (sp < stack + STACK_SIZE) {
        *sp = i;
        sp++;
    }
    return;
}

//---------------------------------------------------------------------------
// POP function
//---------------------------------------------------------------------------
// Pop from stack.  Returns 0 if stack empty.

int pop() {
    if (stack < sp) {
        sp--;
        return *sp;
    }
    else
        return 0;
}

//---------------------------------------------------------------------------
// MAIN function
//---------------------------------------------------------------------------

int main (void)
{

    print_string("> ");

    while (1) {

        input_char = read_char();


        if (input_char == EOF) {          // Case not needed for student solution
            print_string("\nERROR\n");
            return 1;
        }
        else if (input_char == '\n') break;

        else if ('0' <= input_char && input_char <= '9') {
            int cval = input_char - '0';
            push(pop() * 10 + cval);
        }
        else if (input_char == 'e') { // Enter new number
            push(0);
        }
        else if (input_char == 'n') { // Negate
            push(-pop());
        }
        else if (input_char == '+') {
            // C standard doesn't say which pop() is evaluated first.
            // Doesn't matter here, since + is commutative.
            push(pop() + pop());
        }
        else if (input_char == '-') {
            int arg2 = pop();
            int arg1 = pop();
            push(arg1 - arg2);
        }
        else if (input_char == '*') {
            push(pop() * pop()) ;
        }
    } // END while (1)

    // Print stack

    print_string("\n");

    // Extra code if one wants to print a stack empty message.
    // This code was not required in submitted solutions.
    
    // if (sp == stack) {
    //        print_string("EMPTY\n");
    //    } else {

    int *pp = &stack[0];  // pp = print pointer
    while (pp != sp) {
      print_int(pp - stack);
      print_string(": ");
      print_int(*pp);
      print_string("\n");
      pp++;
    }
    //    }

    return 0;
}


//---------------------------------------------------------------------------
// End of file
//---------------------------------------------------------------------------