Rendezvous/src/FinalFactory.Rendezvous.Contracts/Udp/NatPunchRequestTokenCodec.cs

namespace FinalFactory.Rendezvous.Contracts;

public enum NatPunchPeerRole
{
    HostPresence = 1,
    Host = 2,
    Client = 3,
}

public sealed class NatPunchRequestToken
{
    public NatPunchPeerRole Role { get; set; }
    public MediationHandle MediationHandle { get; set; }
    public string Capability { get; set; } = string.Empty;

    public override string ToString() => "[NatPunchRequestToken: capability redacted]";
}

public static class NatPunchRequestTokenCodec
{
    public const int EncodedLength = ContractLimits.NatPunchRequestTokenCharacters;

    private const string VersionPrefix = "rv1:";
    private const int HandleLength = 32;
    private const int CapabilityLength = ContractLimits.DerivedCredentialCharacters;
    private const char Separator = ':';
    private const char Padding = '.';

    public static string Encode(
        NatPunchPeerRole role,
        MediationHandle mediationHandle,
        string capability)
    {
        if (!TryGetRoleCode(role, out char roleCode)
            || mediationHandle.Value == Guid.Empty
            || capability is null
            || capability.Length != CapabilityLength
            || !ContractValidation.IsCapabilityValid(capability))
        {
            throw new ArgumentException("The NAT punch request token fields are invalid.");
        }

        string payload = string.Concat(
            VersionPrefix,
            roleCode,
            Separator,
            mediationHandle.Value.ToString("N"),
            Separator,
            capability);
        return payload.PadRight(EncodedLength, Padding);
    }

    public static bool TryDecode(string? encoded, out NatPunchRequestToken? token)
    {
        token = null;
        if (encoded is null
            || encoded.Length != EncodedLength
            || !encoded.StartsWith(VersionPrefix, StringComparison.Ordinal)
            || !TryParseRole(encoded[VersionPrefix.Length], out NatPunchPeerRole role))
        {
            return false;
        }

        int roleSeparator = VersionPrefix.Length + 1;
        int handleOffset = roleSeparator + 1;
        int capabilitySeparator = handleOffset + HandleLength;
        int capabilityOffset = capabilitySeparator + 1;
        int paddingOffset = capabilityOffset + CapabilityLength;
        string handleText = encoded.Substring(handleOffset, HandleLength);
        if (encoded[roleSeparator] != Separator
            || encoded[capabilitySeparator] != Separator
            || !Guid.TryParseExact(handleText, "N", out Guid handle)
            || handle == Guid.Empty
            || !string.Equals(handleText, handle.ToString("N"), StringComparison.Ordinal)
            || !ContainsOnlyPadding(encoded, paddingOffset))
        {
            return false;
        }

        string capability = encoded.Substring(capabilityOffset, CapabilityLength);
        if (!ContractValidation.IsCapabilityValid(capability))
        {
            return false;
        }

        token = new NatPunchRequestToken
        {
            Role = role,
            MediationHandle = new MediationHandle(handle),
            Capability = capability,
        };
        return true;
    }

    private static bool TryGetRoleCode(NatPunchPeerRole role, out char code)
    {
        code = role switch
        {
            NatPunchPeerRole.HostPresence => 'p',
            NatPunchPeerRole.Host => 'h',
            NatPunchPeerRole.Client => 'c',
            _ => default,
        };
        return code != default;
    }

    private static bool TryParseRole(char code, out NatPunchPeerRole role)
    {
        role = code switch
        {
            'p' => NatPunchPeerRole.HostPresence,
            'h' => NatPunchPeerRole.Host,
            'c' => NatPunchPeerRole.Client,
            _ => default,
        };
        return role != default;
    }

    private static bool ContainsOnlyPadding(string value, int offset)
    {
        for (int index = offset; index < value.Length; index++)
        {
            if (value[index] != Padding)
            {
                return false;
            }
        }

        return true;
    }
}