// ReSharper disable IdentifierTypo

using System.Text;

namespace IRCStates;

public static class Casemap
{
    public enum CaseMapping
    {
        Rfc1459,
        Ascii
    }

    private const string AsciiUpperChars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
    private const string AsciiLowerChars = "abcdefghijklmnopqrstuvwxyz";
    private const string Rfc1459UpperChars = AsciiUpperChars + @"[]~\";
    private const string Rfc1459LowerChars = AsciiLowerChars + "{}^|";

    private static string Replace(string s, string upperSet, string lowerSet)
    {
        StringBuilder sb = new(s);
        for (var i = 0; i < upperSet.Length; i++)
        {
            sb.Replace(upperSet[i], lowerSet[i]);
        }

        return sb.ToString();
    }

    public static string CaseFold(CaseMapping mapping, string s)
    {
        return s == null
            ? string.Empty
            : mapping switch
            {
                CaseMapping.Rfc1459 => Replace(s, Rfc1459UpperChars, Rfc1459LowerChars),
                CaseMapping.Ascii   => Replace(s, AsciiUpperChars, AsciiLowerChars),
                _                   => throw new ArgumentOutOfRangeException(nameof(mapping), mapping, null)
            };
    }
}