/******************************************************************************
 * 
 * Name       : Scott Kristjanson
 * Number     : 123456789
 * email      : skristja@sfu.ca 
 * Course     : Cmpt135
 * Assignment : 1 
 * Question   : B4
 *
 * File  : PostFix.cpp
 *
 * This implements a postfix calculator by Toby Donaldson. 
 * Modified slightly by Scott Kristjanson for Cmpt135 Spring 2016 Class
 * 
 * This program implements a postfix calculator and has been enhanced to support 
 * the following additional features as part of assignment 1:
 *    % operator - must convert doubles to ints, then compute (a % b)
 *    dup        - duplicates  current element on the tos
 *    inc        - adds one to current element on the tos
 *    dec        - subtracts one from  element on the tos
 *    ^2         - squares the element on the tos
 * 
 * Implementing this new functionality required that the student modify the
 * procedure process_token to support this new functionality.
 *
 ******************************************************************************/

#include "error.h"
#include <iostream>
#include <string>
#include <vector>

using namespace std;


////////////////////////////////////////////////
//
// Global variables.
//
////////////////////////////////////////////////
const string prompt = "--> ";
string token;          // global variable
vector<double> stack;  // global variable
bool Error = false;


////////////////////////////////////////////////
//
// Helper functions.
//
////////////////////////////////////////////////
// Converts string s to a double.
double string_to_double(const string& s) {
    return stod(s);
}

void report_error(const string& s)
{
    cout << "Error: " << s << endl;
    if (!Error) {
       cout << "Sorry, errors not handled yet!" << endl;
       cout << "We will look at error handling in Chapter 16" << endl;
    }
    Error = true;
}


////////////////////////////////////////////////
//
// Stack functions.
//
////////////////////////////////////////////////

// return top of stack without removing it
double peek() {
    if (stack.size() == 0) {
        report_error("tried to peek at an empty stack");
        return 0;  // never called: needed to satisfy compiler
    } else {
        return stack.back();
    }
}

// remove and return the element of the stack
double pop() {
  double tos = 0;
  if (stack.size() == 0) {
    report_error("tried to pop  an empty stack");
    } else {
    tos = stack.back();
    stack.pop_back();
  }
  return tos;
}

// put x on the top of the stack
void push(double x) {
	stack.push_back(x);
}

////////////////////////////////////////////////
//
// Token processing functions.
//
////////////////////////////////////////////////

void process_token() {
    if (token == "+") {
        double b = pop();
        double a = pop();
        push(a + b);
    } else if (token == "-") {
        double b = pop();
        double a = pop();
        push(a - b);
    } else if (token == "*") {
        double b = pop();
        double a = pop();
        push(a * b);
    } else if (token == "/") {
        double b = pop();
        double a = pop();
        push(a / b);
    } else if (token == "%") {
        int b = pop();
        int a = pop();
        push(a % b);
    } else if (token == "inc") {
        double a = pop();
        push(a + 1); 
    } else if (token == "dec") {
        double a = pop();
        push(a - 1);         
    } else if (token == "^2") {
        double a = pop();
        push(a * a);    
    } else if (token == "dup") {
        double a = peek();
        push(a);
    } else if (token == "=") {
        cout << "tos = " << peek() << "\n";
        cout << prompt;
    } else {
        push(string_to_double(token));
    } // if
}



////////////////////////////////////////////////
//
// Main function.
//
////////////////////////////////////////////////
int main() {
    cout << "Postfix expression evaluator\n";
    cout << prompt;
    while ((!Error) && (cin >> token) && (token != "Q")) {
       process_token();
    } 
    cout << "Bye!" << endl;
}