Does not generate correct values when using custom hash length and alphabet

[brutaldev]

If you remove the salt and use a custom alphabet with min hash length the resulting hash does not match other languages/libraries.

Expected SQL settings

DECLARE
  @salt nvarchar(255) = N'',
  @alphabet nvarchar(255) = N'ABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890',
  @seps nvarchar(255) = N'cfhistuCFHISTU',  -- Match .NET
  @guards nvarchar(255) = N'',
  @minHashLength int = 7;

This breaks with a divide by zero error because @guards is blank but it actually needs to be calculated. All the code that changes the alphabet and calculates the guard characters is missing:

https://github.com/ullmark/hashids.net/blob/master/src/Hashids.net/Hashids.cs#L85 https://github.com/niieani/hashids.js/blob/master/src/hashids.ts#L80

The guard characters in this scenario should get calculated to DEG and the alphabet should be reduced to JKLMNOPQRVWXYZ1234567890. So this following unintuitive settings actually result in the same values as other languages with the expected alphabet and calculated guards:

DECLARE
  @salt nvarchar(255) = N'',
  @alphabet nvarchar(255) = N'JKLMNOPQRVWXYZ1234567890',
  @seps nvarchar(255) = N'CFHISTU',
  @guards nvarchar(255) = N'DEG',
  @minHashLength int = 7;

-- LG0WPG8

Working examples in C# and TS.

JavaScript

var hashids = new Hashids("", 7, "ABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890");

var id = hashids.encode(123);
var numbers = hashids.decode(id);

console.log(id)
// LG0WPG8

.NET

using System;
using HashidsNet;
					
public class Program
{
	public static void Main()
	{
		const long testId = 123;
		var hashids = new Hashids(minHashLength: 7, alphabet: "ABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890");
		var id = hashids.EncodeLong(testId);
		
		Console.WriteLine(id);
                // LG0WPG8
	}
}