LumaTunnel

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

namespace LumaTunnel.Client.Core.Routing;

public readonly record struct IpCidr(IPAddress Network, int PrefixLength)
{
    public static bool TryParse(string value, out IpCidr cidr)
    {
        cidr = default;
        var parts = value.Split('/', 2, StringSplitOptions.TrimEntries);
        if (parts.Length != 2 || !IPAddress.TryParse(parts[0], out var address) || !int.TryParse(parts[1], out var prefix))
            return false;
        var max = address.AddressFamily == AddressFamily.InterNetwork ? 32 : 128;
        if (prefix < 0 || prefix > max)
            return false;

        cidr = new IpCidr(address, prefix);
        return true;
    }

    public bool Contains(IPAddress address)
    {
        if (address.AddressFamily != Network.AddressFamily)
            return false;

        var network = Network.GetAddressBytes();
        var candidate = address.GetAddressBytes();
        var wholeBytes = PrefixLength / 8;
        var remainingBits = PrefixLength % 8;
        if (!network.AsSpan(0, wholeBytes).SequenceEqual(candidate.AsSpan(0, wholeBytes)))
            return false;
        if (remainingBits == 0)
            return true;

        var mask = (byte)(0xFF << (8 - remainingBits));
        return (network[wholeBytes] & mask) == (candidate[wholeBytes] & mask);
    }
}