Npi-Validator.linq

<Query Kind="Program">
  <Namespace>Xunit</Namespace>
</Query>

#load "xunit"

void Main()
{
	ValidateNpi(Valid).Dump();
	ValidateNpi(Invalid).Dump();
	
	RunTests();
}

//1003347634 - Valid
//1093791627 - Invalid
const string Valid = "1003347634";
const string Invalid = "1093791627";

public bool ValidateNpi(string Npi)
{
	if (!int.TryParse(Npi, out _))
	{
		return false;
	}
	
	//Quick and dirty way to convert a string into IENumerable<int>
	return ValidateNpi(Npi.Select(a => a - '0'));
}

public bool ValidateNpi(int Npi)
{
	return ValidateNpi(Npi.ToString());
}

private bool ValidateNpi(IEnumerable<int> Npi)
{
	if (Npi.Count() != 10)
	{
		return false;
	}

	int index = 1, accumulator = 0, checkDigit = Npi.Last();
	foreach (var item in Npi.SkipLast(1))
	{
		if (index % 2 == 1)
		{
			if ((item * 2) >= 10)
			{
				accumulator = accumulator + item * 2 - 10 + 1;
			}
			else
			{
				accumulator = accumulator + item * 2;
			}
		}
		else
		{
			accumulator = accumulator + item;
		}
		index++;
	}

	return (Ceil(accumulator + 24) - (accumulator + 24) == checkDigit) ? true : false;

	int Ceil(int a)
	{
		//Given an integer, we get the next tenth ie: 66 -> 70, 71 -> 80, etc...
		return (int)Math.Ceiling((double)a / 10) * 10;
	}
}

#region private::Tests

[Fact] void Valid_Npi() => Assert.True (ValidateNpi(Valid));
[Fact] void Invalid_Npi() => Assert.False (ValidateNpi(Invalid));

#endregion