AP.NET/Utils/Base36.cs

namespace ActivityPub.Utils;

/// <summary>
/// Helper class for converting numbers to and from Base36 strings
/// https://stackoverflow.com/a/35004409
/// </summary>
public class Base36
{
    private static readonly char[] BaseChars =
         "0123456789abcdefghijklmnopqrstuvwxyz".ToCharArray();
    private static readonly Dictionary<char, int> CharValues = BaseChars
               .Select((c, i) => new { Char = c, Index = i })
               .ToDictionary(c => c.Char, c => c.Index);

    /// <summary>
    /// Convert a long to a Base36 string
    /// </summary>
    /// <param name="value">The number to convert</param>
    /// <returns>A string representing the number in Base36</returns>
    public static string ToString(long value)
    {
        long targetBase = BaseChars.Length;
        // Determine exact number of characters to use.
        char[] buffer = new char[Math.Max(
                   (int)Math.Ceiling(Math.Log(value + 1, targetBase)), 1)];

        var i = buffer.Length;
        do
        {
            buffer[--i] = BaseChars[value % targetBase];
            value = value / targetBase;
        }
        while (value > 0);

        return new string(buffer, i, buffer.Length - i);
    }

    /// <summary>
    /// Converts a Base64 string back into a number
    /// </summary>
    /// <param name="number">The string to convert</param>
    /// <returns>The number resulting from converting the string</returns>
    public static long FromString(string number)
    {
        char[] chrs = number.ToLower().ToCharArray();
        int m = chrs.Length - 1;
        int n = BaseChars.Length, x;
        long result = 0;
        for (int i = 0; i < chrs.Length; i++)
        {
            x = CharValues[chrs[i]];
            result += x * (long)Math.Pow(n, m--);
        }
        return result;
    }
}