Greg Young Probability kata proposed solution #1

gistfile1.cs

using System;
using NUnit.Framework;

namespace ProbabilityKata
{
    [TestFixture]
    public class ProbabilityTests
    {
        [Test]
        public void CanCompareTwoEqualProbabilities()
        {
            var p1 = new Probability(1);
            var p2 = new Probability(1);

            Assert.AreEqual(p1,p2);
        }

        [Test]
        public void CanCompareTwoNotEqualProbabilities()
        {
            var p1 = new Probability(1);
            var p2 = new Probability(0.9f);

            Assert.AreNotEqual(p1, p2);
        }

        [Test]
        public void CanCombineProbabilities()
        {
            var p1 = new Probability(0.9f);
            var p2 = new Probability(0.9f);

            Assert.AreEqual(p1.CombinedWith(p2), new Probability(0.81f));
        }

        [Test]
        public void CanInverseOfProbabilities()
        {
            var p1 = new Probability(0.9f);

            Assert.AreEqual(p1.InverseOf(p1), new Probability(0.1f));
        }

        [Test]
        public void CanEitherOfProbabilities()
        {
            var p1 = new Probability(0.9f);
            var p2 = new Probability(0.5f);

            Assert.AreEqual(p1.Either(p2), new Probability(0.95f));
        }
    }

    public class Probability
    {
        private readonly float _probability;
        private const double Epsilon = 0.001;

        public Probability(float probability)
        {
            _probability = probability;
        }

        public Probability CombinedWith(Probability p)
        {
            return new Probability(p._probability * _probability);
        }

        public Probability InverseOf(Probability p)
        {
            return new Probability(1 - p._probability);
        }

        public Probability Either(Probability p)
        {
            return new Probability(_probability+p._probability - _probability * p._probability);
        }

        public override string ToString()
        {
            return "P(" + _probability + ")";
        }

        public bool Equals(Probability other)
        {
            if (ReferenceEquals(null, other)) return false;
            if (ReferenceEquals(this, other)) return true;
            return Math.Abs(other._probability - _probability) < Epsilon;
        }

        public override bool Equals(object obj)
        {
            if (obj == null || !(obj is Probability))
                return false;

            return Equals((Probability) obj);
        }

        public override int GetHashCode()
        {
            return _probability.GetHashCode();
        }
    }
}