using System;
using System.Security.Cryptography;
using System.Text;

namespace StellaOps.Auth.Security.Dpop;

internal static class DpopNonceUtilities
{
    private static readonly char[] Base64Padding = { '=' };

    internal static string GenerateNonce()
    {
        Span<byte> buffer = stackalloc byte[32];
        RandomNumberGenerator.Fill(buffer);

        return Convert.ToBase64String(buffer)
            .TrimEnd(Base64Padding)
            .Replace('+', '-')
            .Replace('/', '_');
    }

    internal static byte[] ComputeNonceHash(string nonce)
    {
        ArgumentException.ThrowIfNullOrWhiteSpace(nonce);
        var bytes = Encoding.UTF8.GetBytes(nonce);
        return SHA256.HashData(bytes);
    }

    internal static string EncodeHash(ReadOnlySpan<byte> hash)
        => Convert.ToHexString(hash);

    internal static string ComputeStorageKey(string audience, string clientId, string keyThumbprint)
    {
        ArgumentException.ThrowIfNullOrWhiteSpace(audience);
        ArgumentException.ThrowIfNullOrWhiteSpace(clientId);
        ArgumentException.ThrowIfNullOrWhiteSpace(keyThumbprint);

        return string.Create(
            "dpop-nonce:".Length + audience.Length + clientId.Length + keyThumbprint.Length + 2,
            (audience.Trim(), clientId.Trim(), keyThumbprint.Trim()),
            static (span, parts) =>
            {
                var index = 0;
                const string Prefix = "dpop-nonce:";
                Prefix.CopyTo(span);
                index += Prefix.Length;

                index = Append(span, index, parts.Item1);
                span[index++] = ':';
                index = Append(span, index, parts.Item2);
                span[index++] = ':';
                _ = Append(span, index, parts.Item3);
            });

        static int Append(Span<char> span, int index, string value)
        {
            if (value.Length == 0)
            {
                throw new ArgumentException("Value must not be empty after trimming.");
            }

            value.AsSpan().CopyTo(span[index..]);
            return index + value.Length;
        }
    }
}