namespace DataTransfer.Client.Services;

/// <summary>
///     Plaintext messages of the commit-then-reveal key exchange (see <see cref="SecureChannel" />). They are relayed
///     like frames, but before the channel is encrypted. Layout: version (byte) | type (byte) | payload.
/// </summary>
internal static class HandshakeCodec
{
    public const int CommitmentBytes = 32;

    // A P-256 SPKI key has 91 bytes; anything much larger is not a key.
    private const int MaxPublicKeyBytes = 512;
    private const byte Version = 1;

    private enum MessageType : byte
    {
        Commitment = 1,
        PublicKey = 2
    }

    public static byte[] EncodeCommitment(byte[] commitment) => Encode(MessageType.Commitment, commitment);

    public static byte[] EncodePublicKey(byte[] publicKey) => Encode(MessageType.PublicKey, publicKey);

    /// <exception cref="InvalidDataException">The message is not a commitment.</exception>
    public static byte[] DecodeCommitment(byte[] message) =>
        Decode(message, MessageType.Commitment, CommitmentBytes, CommitmentBytes);

    /// <exception cref="InvalidDataException">The message is not a public key.</exception>
    public static byte[] DecodePublicKey(byte[] message) => Decode(message, MessageType.PublicKey, 1, MaxPublicKeyBytes);

    private static byte[] Encode(MessageType type, byte[] payload)
    {
        var message = new byte[payload.Length + 2];
        message[0] = Version;
        message[1] = (byte)type;
        payload.CopyTo(message, 2);
        return message;
    }

    private static byte[] Decode(byte[] message, MessageType type, int minPayloadBytes, int maxPayloadBytes)
    {
        ArgumentNullException.ThrowIfNull(message);

        var payloadBytes = message.Length - 2;
        if (payloadBytes < minPayloadBytes || payloadBytes > maxPayloadBytes || message[0] != Version || message[1] != (byte)type)
        {
            throw new InvalidDataException($"Expected a valid key-exchange message of type {type}.");
        }

        return message[2..];
    }
}