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

namespace JobTrackerApi.Services.JobImport;

internal static class JobImportHttpTransport
{
    // The optional dial function is a socket-boundary test seam, not a configurable policy.
    internal static SocketsHttpHandler CreateHandler(
        IHostAddressResolver resolver,
        Func<IPEndPoint, CancellationToken, ValueTask<Stream>>? connect = null) => new()
    {
        AutomaticDecompression = DecompressionMethods.All,
        AllowAutoRedirect = false,
        // A proxy would resolve/connect the origin outside this boundary.
        UseProxy = false,
        ConnectCallback = (context, cancellationToken) =>
            ConnectAsync(context.DnsEndPoint, resolver, connect ?? ConnectSocketAsync, cancellationToken)
        // Return plain TCP; SocketsHttpHandler retains the original URI hostname for TLS/SNI.
    };

    internal static bool IsBlockedAddress(IPAddress address)
    {
        if (address.AddressFamily == AddressFamily.InterNetworkV6 && address.ScopeId != 0) return true;
        if (address.IsIPv4MappedToIPv6) address = address.MapToIPv4();

        if (IPAddress.IsLoopback(address)) return true;
        var bytes = address.GetAddressBytes();
        if (address.AddressFamily == AddressFamily.InterNetwork)
        {
            return bytes[0] is 0 or 10 or 127 or >= 224 ||
                   (bytes[0] == 100 && bytes[1] is >= 64 and <= 127) ||
                   (bytes[0] == 169 && bytes[1] == 254) ||
                   (bytes[0] == 172 && bytes[1] is >= 16 and <= 31) ||
                   (bytes[0] == 192 && bytes[1] == 168) ||
                   (bytes[0] == 198 && bytes[1] is 18 or 19);
        }

        if (address.AddressFamily != AddressFamily.InterNetworkV6) return true;
        if (address.Equals(IPAddress.IPv6Any) || address.IsIPv6LinkLocal ||
            address.IsIPv6SiteLocal || address.IsIPv6Multicast || address.IsIPv6Teredo) return true;
        if ((bytes[0] & 0xfe) == 0xfc) return true; // Unique local fc00::/7.

        // Reject legacy IPv4-compatible, well-known NAT64 and 6to4 destinations: their
        // eventual IPv4 peer is not represented by a native IPv6 socket destination.
        return bytes.AsSpan(0, 12).IndexOfAnyExcept((byte)0) < 0 ||
               (bytes[0] == 0x00 && bytes[1] == 0x64 && bytes[2] == 0xff && bytes[3] == 0x9b &&
                ((bytes[4] == 0 && bytes[5] == 1) || bytes.AsSpan(4, 8).IndexOfAnyExcept((byte)0) < 0)) ||
               (bytes[0] == 0x20 && bytes[1] == 0x02);
    }

    private static async ValueTask<Stream> ConnectAsync(
        DnsEndPoint endpoint,
        IHostAddressResolver resolver,
        Func<IPEndPoint, CancellationToken, ValueTask<Stream>> connect,
        CancellationToken cancellationToken)
    {
        cancellationToken.ThrowIfCancellationRequested();
        var addresses = IPAddress.TryParse(endpoint.Host, out var literal)
            ? [literal]
            : await resolver.ResolveAsync(endpoint.Host, cancellationToken);

        // Validate the entire answer before attempting any connection, including mixed answers.
        if (addresses.Length == 0 || addresses.Any(IsBlockedAddress))
            throw new HttpRequestException("Local or private network URLs are not allowed.");

        SocketException? lastError = null;
        foreach (var address in addresses)
        {
            cancellationToken.ThrowIfCancellationRequested();
            var normalized = address.IsIPv4MappedToIPv6 ? address.MapToIPv4() : address;
            try
            {
                // No hostname is passed to the socket, so there is no second DNS lookup.
                return await connect(new IPEndPoint(normalized, endpoint.Port), cancellationToken);
            }
            catch (SocketException error)
            {
                lastError = error;
            }
        }

        throw new HttpRequestException("Could not connect to the import destination.", lastError);
    }

    internal static async ValueTask<Stream> ConnectSocketAsync(IPEndPoint endpoint, CancellationToken cancellationToken)
    {
        var socket = new Socket(endpoint.AddressFamily, SocketType.Stream, ProtocolType.Tcp) { NoDelay = true };
        try
        {
            await socket.ConnectAsync(endpoint, cancellationToken);
            return new NetworkStream(socket, ownsSocket: true);
        }
        catch
        {
            socket.Dispose();
            throw;
        }
    }
}