using System.Text;

namespace StellaOps.Graph.Indexer.Schema;

/// <summary>
/// Base32 encoder using the Crockford alphabet (0-9A-HJKMNPQRSTVWXYZ).
/// </summary>
internal static class Base32Crockford
{
    private const string Alphabet = "0123456789ABCDEFGHJKMNPQRSTVWXYZ";

    public static string Encode(ReadOnlySpan<byte> data)
    {
        if (data.IsEmpty)
        {
            return string.Empty;
        }

        var output = new StringBuilder((data.Length * 8 + 4) / 5);
        var buffer = 0;
        var bitsLeft = 0;

        foreach (var b in data)
        {
            buffer = (buffer << 8) | b;
            bitsLeft += 8;

            while (bitsLeft >= 5)
            {
                bitsLeft -= 5;
                var index = (buffer >> bitsLeft) & 0x1F;
                output.Append(Alphabet[index]);
            }
        }

        if (bitsLeft > 0)
        {
            var index = (buffer << (5 - bitsLeft)) & 0x1F;
            output.Append(Alphabet[index]);
        }

        return output.ToString();
    }
}