XFESystem

using System.Net;
using System.Text;

namespace XFESystem.Browser;

/// <summary>
/// Explicit HttpClient adapter for a runtime with working sockets, TLS, clock
/// and trust roots. It is never selected as an implicit bare-metal fallback.
/// </summary>
public sealed class HttpWebTransport : IWebTransport, IWebTransportCapabilities, IDisposable
{
    public bool IsNative => false;
    public bool SupportsHttp => true;
    public bool SupportsHttps => true;

    private readonly HttpClient client = new(new SocketsHttpHandler
    {
        AllowAutoRedirect = false,
        UseCookies = false,
        AutomaticDecompression = DecompressionMethods.All
    }) { Timeout = Timeout.InfiniteTimeSpan };

    public WebResponse Get(Uri address, BrowserLimits limits)
    {
        ArgumentNullException.ThrowIfNull(address);
        ArgumentNullException.ThrowIfNull(limits);
        limits.Validate();
        BrowserService.ValidateWebAddress(address);
        using var cancellation = new CancellationTokenSource(limits.NetworkTimeout);
        var target = address;
        for (var redirects = 0; ; redirects++)
        {
            using var request = new HttpRequestMessage(HttpMethod.Get, target);
            request.Headers.UserAgent.ParseAdd("XFESystem-Browser/0.3");
            request.Headers.Accept.ParseAdd("text/html,application/xhtml+xml,text/plain,text/markdown");
            using var response = client.Send(request, HttpCompletionOption.ResponseHeadersRead, cancellation.Token);
            var status = (int)response.StatusCode;
            if (status is 301 or 302 or 303 or 307 or 308)
            {
                if (redirects >= limits.MaximumRedirects)
                    throw new InvalidOperationException("The redirect limit was reached.");
                var location = response.Headers.Location ?? throw new InvalidOperationException("The redirect has no Location header.");
                var next = location.IsAbsoluteUri ? location : new Uri(target, location);
                BrowserService.ValidateWebAddress(next);
                if (target.Scheme == "https" && next.Scheme != "https")
                    throw new InvalidOperationException("HTTPS cannot redirect to HTTP.");
                target = next;
                continue;
            }
            if (response.Content.Headers.ContentLength > (long)limits.MaximumHtmlCharacters * 4)
                throw new BrowserLimitException("The HTTP body exceeds its byte limit.");
            var charset = response.Content.Headers.ContentType?.CharSet?.Trim('"') ?? "utf-8";
            Encoding encoding;
            try { encoding = Encoding.GetEncoding(charset); }
            catch (ArgumentException) { throw new NotSupportedException("The response charset is unsupported: " + charset); }
            using var stream = response.Content.ReadAsStream(cancellation.Token);
            using var reader = new StreamReader(stream, encoding, true, 4_096);
            var text = new BoundedText(limits.MaximumHtmlCharacters);
            var buffer = new char[4_096];
            while (true)
            {
                var read = reader.ReadAsync(buffer.AsMemory(), cancellation.Token).AsTask().GetAwaiter().GetResult();
                if (read == 0) break;
                text.Append(new string(buffer, 0, read));
            }
            // HttpClient's default TLS handler validates trust chain, hostname and validity.
            // No custom certificate callback is installed by this adapter.
            return new(target, status, response.Content.Headers.ContentType?.ToString() ?? string.Empty,
                text.ToString(), target.Scheme == "https", response.Headers.TryGetValues("Content-Security-Policy", out var policies)
                    ? string.Join("; ", policies) : null);
        }
    }

    public void Dispose() => client.Dispose();
}