SemiFaculty.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 SemiFaculty : UnaryDoubleOperator
    {
        public SemiFaculty(ScriptNode Operand, int Start, int Length, Expression Expression)
            : base(Operand, Start, Length, Expression)
        {
        }
 
        public override IElement Evaluate(double Operand)
        {
            if (Operand < 0 || Math.Truncate(Operand) != Operand)
                throw new ScriptRuntimeException("Operand must be a non-negative integer.", this);
 
            double Result = 1;
 
            while (Operand > 0)
            {
                Result *= Operand;
                if (double.IsInfinity(Result) || double.IsNaN(Result))
                    throw new ScriptRuntimeException("Overflow.", this);
 
                Operand -= 2;
            }
 
            return new DoubleNumber(Result);
        }
    }
}