using System.Net;
using System.Net.Sockets;

namespace DodoSSH.Api.Features.Relay;

/// <summary>
/// The two things the relay asks of the network: what a name resolves to, and a connection to an address.
/// </summary>
/// <remarks>
/// <para>
/// A seam for tests, and a narrow one on purpose. <see cref="RelayAddressPolicy"/> is not behind it:
/// whatever this answers, the policy judges, and a test that replaces this still runs every address
/// through the real rules. What a test cannot do without the seam is dial anything, because every
/// address a test machine can reach on its own — loopback — is one the policy refuses outright.
/// </para>
/// </remarks>
internal interface IRelayNetwork
{
    /// <summary>Resolves a stored hostname, which may be an address literal.</summary>
    Task<IPAddress[]> ResolveAsync(string host, CancellationToken cancellationToken);

    /// <summary>Opens a TCP connection to one address.</summary>
    /// <returns>A stream that owns the connection.</returns>
    Task<Stream> ConnectAsync(IPAddress address, int port, CancellationToken cancellationToken);
}

/// <summary>The operating system's resolver and sockets.</summary>
internal sealed class SystemRelayNetwork : IRelayNetwork
{
    /// <inheritdoc />
    public Task<IPAddress[]> ResolveAsync(string host, CancellationToken cancellationToken) =>

        // A literal parses to itself without a lookup, which is what an address typed into a host
        // editor should do — and what makes "10.0.0.5" mean the same thing on every node.
        IPAddress.TryParse(host, out var literal)
            ? Task.FromResult<IPAddress[]>([literal])
            : Dns.GetHostAddressesAsync(host, cancellationToken);

    /// <inheritdoc />
    public async Task<Stream> ConnectAsync(IPAddress address, int port, CancellationToken cancellationToken)
    {
        var socket = new Socket(address.AddressFamily, SocketType.Stream, ProtocolType.Tcp)
        {
            // An interactive shell is a stream of keystrokes; Nagle would hold each one back for the
            // acknowledgement of the last, which is the latency the user feels as a sticky terminal.
            NoDelay = true,
        };

        try
        {
            await socket.ConnectAsync(new IPEndPoint(address, port), cancellationToken).ConfigureAwait(false);

            return new NetworkStream(socket, ownsSocket: true);
        }
        catch
        {
            socket.Dispose();
            throw;
        }
    }
}