BinomialCoefficient.cs

using System;
using Waher.Script.Abstraction.Elements;
using Waher.Script.Exceptions;
using Waher.Script.Model;
using Waher.Script.Objects;
 
namespace Waher.Script.Operators.Arithmetics
{
    public class BinomialCoefficient : BinaryDoubleOperator
    {
        public BinomialCoefficient(ScriptNode Left, ScriptNode Right, int Start, int Length, Expression Expression)
            : base(Left, Right, Start, Length, Expression)
        {
        }
 
        public override IElement Evaluate(double Left, double Right)
        {
            if (Left < 0 || Left > int.MaxValue || Right < 0 || Right > int.MaxValue || Left != Math.Truncate(Left) || Right != Math.Truncate(Right))
                throw new ScriptRuntimeException("Expected non-negative integers.", this);
 
            int n = (int)Left;
            int k = (int)Right;
            int n_k = n - k;
            if (n_k < 0)
                throw new ScriptRuntimeException("The Denominator in the OVER operator, cannot be larger than the numerator", this);
 
            double Result = 1;
 
            int Max = Math.Max(k, n_k);
            int Min = Math.Min(k, n_k);
 
            while (n > Max)
            {
                Result *= n;
                n--;
 
                while (Result > 1 && Min > 1)
                {
                    Result /= Min;
                    Min--;
                }
            }
 
            while (Min > 1)
            {
                Result /= Min;
                Min--;
            }
 
            return new DoubleNumber(Math.Round(Result));    // Any decimals are a result of rounding errors induced by the algorithm.
        }
 
    }
}