Class Fraction

using System;
using System.Collections.Generic;

namespace Fraction
{
    public class Fraction : IComparable
    {
        public Fraction(double numerator, double denominator)
        {
            _numerator = numerator;
            if (denominator == 0) throw new ArgumentException();
            _denominator = denominator;
        }

        public double _numerator { get; private set; }

        public double _denominator { get; private set; }

        public double Value { get { return _numerator / _denominator; } }

        public void Write()
        {
            Console.WriteLine("answer: {0} / {1} = {2}", _numerator, _denominator, Value);
        }

        public Fraction ReduceFraction()
        {
            var gcd = FindTheGreatestCommonDivisor(_numerator, _denominator);
            _numerator /= gcd;
            _denominator /= gcd;
            return new Fraction(_numerator, _denominator);
        }

        public double FindTheGreatestCommonDivisor(double firstNumb, double secondNumb)
        {
            firstNumb = Math.Abs(firstNumb);
            secondNumb = Math.Abs(secondNumb);
            while (firstNumb != secondNumb)
            {
                if (firstNumb > secondNumb)
                    (firstNumb, secondNumb) = (secondNumb, firstNumb);
                secondNumb -= firstNumb;
            }
            return firstNumb;
        }

        public Fraction Sum(Fraction fraction)
        {
            double firstDen = _denominator, secondDen = fraction._denominator;
            double firstNum = _numerator, secondNum = fraction._numerator;
            double finalDen, finalNum;
            Fraction result;

            var gcd = FindTheGreatestCommonDivisor(_denominator, fraction._denominator);

            if (firstDen == secondDen)
            {
                finalNum = firstNum + secondNum;
                finalDen = firstDen;
                result = new Fraction(finalNum, finalDen).ReduceFraction();
            }
            else if (gcd == Math.Min(firstDen, secondDen))
            {
                var factor = Math.Max(firstDen, secondDen) / gcd;
                finalNum = (firstDen < secondDen ? firstNum * factor + secondNum : secondNum * factor + firstNum);
                finalDen = Math.Max(firstDen, secondDen);
                result = new Fraction(finalNum, finalDen).ReduceFraction();
            }
            else
            {
                finalNum = firstNum * secondDen + secondNum * firstDen;
                finalDen = firstDen * secondDen;
                result = new Fraction(finalNum, finalDen).ReduceFraction();
            }
            return result;
        }

        public Fraction Subtraction(Fraction fraction)
        {
            return Sum(new Fraction(-fraction._numerator, fraction._denominator));
        }

        public Fraction Multipy(Fraction fraction)
        {
            var numerator = _numerator * fraction._numerator;
            var denominator = _denominator * fraction._denominator;
            var result = new Fraction(numerator, denominator).ReduceFraction();
            return result;
        }

        public Fraction Divide(Fraction fraction)
        {
            return Multipy(new Fraction(fraction._denominator, fraction._numerator));
        }

        public int CompareTo(object obj)
        {
            var fraction = (Fraction)obj;

            if (_denominator != fraction._denominator)
            {
                _numerator *= fraction._denominator;
                fraction._numerator *= _denominator;
            }

            if (_numerator < fraction._numerator)
                return -1;
            if (_numerator > fraction._numerator)
                return 1;
            return 0;
        }
    }

    internal class Program
    {
        static void Main()
        {
            Console.Write("first numerator: ");
            var firstNumerator = int.Parse(Console.ReadLine());
            Console.Write("\nfirst denominator: ");
            var firstDenominator = int.Parse(Console.ReadLine());

            Console.Write("\nsecond numerator: ");
            var secondNumerator = int.Parse(Console.ReadLine());
            Console.Write("\nsecond denominator: ");
            var secondDenominator = int.Parse(Console.ReadLine());

            var firstFract = new Fraction(firstNumerator, firstDenominator);
            var secondFract = new Fraction(secondNumerator, secondDenominator);

            //firstFract.Sum(secondFract).Write();

            //firstFract.Subtraction(secondFract).Write();

            //firstFract.Multipy(secondFract).Write();

            //firstFract.Divide(secondFract).Write();

            //Console.WriteLine(firstFract.CompareTo(secondFract));

            //firstFract.ReduceFraction().Write();
        }
    }
}