//! HTTP/3 proxy service.
//!
//! Accepts QUIC connections via quinn, runs h3 server to handle HTTP/3 requests,
//! and forwards them to backends using the same routing and pool infrastructure
//! as the HTTP/1+2 proxy.

use std::net::SocketAddr;
use std::pin::Pin;
use std::sync::Arc;
use std::task::{Context, Poll};
use std::time::Duration;

use arc_swap::ArcSwap;
use bytes::{Buf, Bytes};
use http_body::Frame;
use tracing::{debug, warn};

use rustproxy_config::{RouteConfig, TransportProtocol};
use rustproxy_metrics::MetricsCollector;
use rustproxy_routing::{MatchContext, RouteManager};

use crate::connection_pool::ConnectionPool;
use crate::protocol_cache::ProtocolCache;
use crate::upstream_selector::UpstreamSelector;

/// HTTP/3 proxy service.
///
/// Handles QUIC connections with the h3 crate, parses HTTP/3 requests,
/// and forwards them to backends using per-request route matching and
/// shared connection pooling.
pub struct H3ProxyService {
    route_manager: Arc<ArcSwap<RouteManager>>,
    metrics: Arc<MetricsCollector>,
    connection_pool: Arc<ConnectionPool>,
    #[allow(dead_code)]
    protocol_cache: Arc<ProtocolCache>,
    #[allow(dead_code)]
    upstream_selector: UpstreamSelector,
    backend_tls_config: Arc<rustls::ClientConfig>,
    connect_timeout: Duration,
}

impl H3ProxyService {
    pub fn new(
        route_manager: Arc<ArcSwap<RouteManager>>,
        metrics: Arc<MetricsCollector>,
        connection_pool: Arc<ConnectionPool>,
        protocol_cache: Arc<ProtocolCache>,
        backend_tls_config: Arc<rustls::ClientConfig>,
        connect_timeout: Duration,
    ) -> Self {
        Self {
            route_manager: Arc::clone(&route_manager),
            metrics: Arc::clone(&metrics),
            connection_pool,
            protocol_cache,
            upstream_selector: UpstreamSelector::new(),
            backend_tls_config,
            connect_timeout,
        }
    }

    /// Handle an accepted QUIC connection as HTTP/3.
    ///
    /// If `real_client_addr` is provided (from PROXY protocol), it overrides
    /// `connection.remote_address()` for client IP attribution.
    pub async fn handle_connection(
        &self,
        connection: quinn::Connection,
        _fallback_route: &RouteConfig,
        port: u16,
        real_client_addr: Option<SocketAddr>,
    ) -> anyhow::Result<()> {
        let remote_addr = real_client_addr.unwrap_or_else(|| connection.remote_address());
        debug!("HTTP/3 connection from {} on port {}", remote_addr, port);

        let mut h3_conn: h3::server::Connection<h3_quinn::Connection, Bytes> =
            h3::server::builder()
                .send_grease(false)
                .build(h3_quinn::Connection::new(connection))
                .await
                .map_err(|e| anyhow::anyhow!("H3 connection setup failed: {}", e))?;

        let client_ip = remote_addr.ip().to_string();

        loop {
            match h3_conn.accept().await {
                Ok(Some(resolver)) => {
                    let (request, stream) = match resolver.resolve_request().await {
                        Ok(pair) => pair,
                        Err(e) => {
                            debug!("HTTP/3 request resolve error: {}", e);
                            continue;
                        }
                    };

                    self.metrics.record_http_request();

                    let rm = self.route_manager.load();
                    let pool = Arc::clone(&self.connection_pool);
                    let metrics = Arc::clone(&self.metrics);
                    let backend_tls = Arc::clone(&self.backend_tls_config);
                    let connect_timeout = self.connect_timeout;
                    let client_ip = client_ip.clone();

                    tokio::spawn(async move {
                        if let Err(e) = handle_h3_request(
                            request, stream, port, &client_ip, &rm, &pool, &metrics,
                            &backend_tls, connect_timeout,
                        ).await {
                            debug!("HTTP/3 request error from {}: {}", client_ip, e);
                        }
                    });
                }
                Ok(None) => {
                    debug!("HTTP/3 connection from {} closed", remote_addr);
                    break;
                }
                Err(e) => {
                    debug!("HTTP/3 accept error from {}: {}", remote_addr, e);
                    break;
                }
            }
        }

        Ok(())
    }
}

/// Handle a single HTTP/3 request with per-request route matching.
async fn handle_h3_request(
    request: hyper::Request<()>,
    mut stream: h3::server::RequestStream<h3_quinn::BidiStream<Bytes>, Bytes>,
    port: u16,
    client_ip: &str,
    route_manager: &RouteManager,
    _connection_pool: &ConnectionPool,
    metrics: &MetricsCollector,
    backend_tls_config: &Arc<rustls::ClientConfig>,
    connect_timeout: Duration,
) -> anyhow::Result<()> {
    let method = request.method().clone();
    let uri = request.uri().clone();
    let path = uri.path().to_string();

    // Extract host from :authority or Host header
    let host = request.uri().authority()
        .map(|a| a.as_str().to_string())
        .or_else(|| request.headers().get("host").and_then(|v| v.to_str().ok()).map(|s| s.to_string()))
        .unwrap_or_default();

    debug!("HTTP/3 {} {} (host: {}, client: {})", method, path, host, client_ip);

    // Per-request route matching
    let ctx = MatchContext {
        port,
        domain: if host.is_empty() { None } else { Some(&host) },
        path: Some(&path),
        client_ip: Some(client_ip),
        tls_version: Some("TLSv1.3"),
        headers: None,
        is_tls: true,
        protocol: Some("http"),
        transport: Some(TransportProtocol::Udp),
    };

    let route_match = route_manager.find_route(&ctx)
        .ok_or_else(|| anyhow::anyhow!("No route matched for HTTP/3 request to {}{}", host, path))?;
    let route = route_match.route;

    // Resolve backend target (use matched target or first target)
    let target = route_match.target
        .or_else(|| route.action.targets.as_ref().and_then(|t| t.first()))
        .ok_or_else(|| anyhow::anyhow!("No target for HTTP/3 route"))?;

    let backend_host = target.host.first();
    let backend_port = target.port.resolve(port);
    let backend_addr = format!("{}:{}", backend_host, backend_port);

    // Determine if backend requires TLS (same logic as proxy_service.rs)
    let mut use_tls = target.tls.is_some();
    if let Some(ref tls) = route.action.tls {
        if tls.mode == rustproxy_config::TlsMode::TerminateAndReencrypt {
            use_tls = true;
        }
    }

    // Connect to backend via TCP with timeout
    let tcp_stream = tokio::time::timeout(
        connect_timeout,
        tokio::net::TcpStream::connect(&backend_addr),
    ).await
        .map_err(|_| anyhow::anyhow!("Backend connect timeout to {}", backend_addr))?
        .map_err(|e| anyhow::anyhow!("Backend connect to {} failed: {}", backend_addr, e))?;

    let _ = tcp_stream.set_nodelay(true);

    // Branch: wrap in TLS if backend requires it, then HTTP/1.1 handshake.
    // hyper's SendRequest<B> is NOT generic over the IO type, so both branches
    // produce the same type and can be unified.
    let mut sender = if use_tls {
        let connector = tokio_rustls::TlsConnector::from(Arc::clone(backend_tls_config));
        let server_name = rustls::pki_types::ServerName::try_from(backend_host.to_string())
            .map_err(|e| anyhow::anyhow!("Invalid backend SNI '{}': {}", backend_host, e))?;
        let tls_stream = connector.connect(server_name, tcp_stream).await
            .map_err(|e| anyhow::anyhow!("Backend TLS handshake to {} failed: {}", backend_addr, e))?;
        let io = hyper_util::rt::TokioIo::new(tls_stream);
        let (sender, conn) = hyper::client::conn::http1::handshake(io).await
            .map_err(|e| anyhow::anyhow!("Backend handshake failed: {}", e))?;
        tokio::spawn(async move { let _ = conn.await; });
        sender
    } else {
        let io = hyper_util::rt::TokioIo::new(tcp_stream);
        let (sender, conn) = hyper::client::conn::http1::handshake(io).await
            .map_err(|e| anyhow::anyhow!("Backend handshake failed: {}", e))?;
        tokio::spawn(async move { let _ = conn.await; });
        sender
    };

    // Stream request body from H3 client to backend via an mpsc channel.
    // This avoids buffering the entire request body in memory.
    let (body_tx, body_rx) = tokio::sync::mpsc::channel::<Bytes>(4);
    let total_bytes_in = Arc::new(std::sync::atomic::AtomicU64::new(0));
    let total_bytes_in_writer = Arc::clone(&total_bytes_in);

    // Spawn the H3 body reader task
    let body_reader = tokio::spawn(async move {
        while let Ok(Some(mut chunk)) = stream.recv_data().await {
            let data = Bytes::copy_from_slice(chunk.chunk());
            total_bytes_in_writer.fetch_add(data.len() as u64, std::sync::atomic::Ordering::Relaxed);
            chunk.advance(chunk.remaining());
            if body_tx.send(data).await.is_err() {
                break;
            }
        }
        stream
    });

    // Create a body that polls from the mpsc receiver
    let body = H3RequestBody { receiver: body_rx };
    let backend_req = build_backend_request(&method, &backend_addr, &path, &host, &request, body, use_tls)?;

    let response = sender.send_request(backend_req).await
        .map_err(|e| anyhow::anyhow!("Backend request failed: {}", e))?;

    // Await the body reader to get the stream back
    let mut stream = body_reader.await
        .map_err(|e| anyhow::anyhow!("Body reader task failed: {}", e))?;
    let total_bytes_in = total_bytes_in.load(std::sync::atomic::Ordering::Relaxed);

    // Build H3 response
    let status = response.status();
    let mut h3_response = hyper::Response::builder().status(status);

    // Copy response headers (skip hop-by-hop)
    for (name, value) in response.headers() {
        let n = name.as_str().to_lowercase();
        if n == "transfer-encoding" || n == "connection" || n == "keep-alive" || n == "upgrade" {
            continue;
        }
        h3_response = h3_response.header(name, value);
    }

    // Extract content-length for body loop termination (must be before into_body())
    let content_length: Option<u64> = response.headers()
        .get(hyper::header::CONTENT_LENGTH)
        .and_then(|v| v.to_str().ok())
        .and_then(|s| s.parse().ok());

    // Add Alt-Svc for HTTP/3 advertisement
    let alt_svc = route.action.udp.as_ref()
        .and_then(|u| u.quic.as_ref())
        .map(|q| {
            let p = q.alt_svc_port.unwrap_or(port);
            let ma = q.alt_svc_max_age.unwrap_or(86400);
            format!("h3=\":{}\"; ma={}", p, ma)
        })
        .unwrap_or_else(|| format!("h3=\":{}\"; ma=86400", port));
    h3_response = h3_response.header("alt-svc", alt_svc);

    let h3_response = h3_response.body(())
        .map_err(|e| anyhow::anyhow!("Failed to build H3 response: {}", e))?;

    // Send response headers
    stream.send_response(h3_response).await
        .map_err(|e| anyhow::anyhow!("Failed to send H3 response: {}", e))?;

    // Stream response body back
    use http_body_util::BodyExt;
    use http_body::Body as _;
    let mut body = response.into_body();
    let mut total_bytes_out: u64 = 0;

    // Per-frame idle timeout: if no frame arrives within this duration, assume
    // the body is complete (or the backend has stalled). This prevents indefinite
    // hangs on close-delimited bodies or when hyper's internal trailers oneshot
    // never resolves after all data has been received.
    const FRAME_IDLE_TIMEOUT: Duration = Duration::from_secs(30);

    loop {
        // Layer 1: If the body already knows it is finished (Content-Length
        // bodies track remaining bytes internally), break immediately to
        // avoid blocking on hyper's internal trailers oneshot.
        if body.is_end_stream() {
            break;
        }

        // Layer 3: Per-frame idle timeout safety net
        match tokio::time::timeout(FRAME_IDLE_TIMEOUT, body.frame()).await {
            Ok(Some(Ok(frame))) => {
                if let Some(data) = frame.data_ref() {
                    total_bytes_out += data.len() as u64;
                    stream.send_data(Bytes::copy_from_slice(data)).await
                        .map_err(|e| anyhow::anyhow!("Failed to send H3 data: {}", e))?;

                    // Layer 2: Content-Length byte count check
                    if let Some(cl) = content_length {
                        if total_bytes_out >= cl {
                            break;
                        }
                    }
                }
            }
            Ok(Some(Err(e))) => {
                warn!("Backend body read error: {}", e);
                break;
            }
            Ok(None) => break, // Body ended naturally
            Err(_) => {
                debug!(
                    "H3 body frame idle timeout ({:?}) after {} bytes; finishing stream",
                    FRAME_IDLE_TIMEOUT, total_bytes_out
                );
                break;
            }
        }
    }

    // Record metrics
    let route_id = route.name.as_deref().or(route.id.as_deref());
    metrics.record_bytes(total_bytes_in, total_bytes_out, route_id, Some(client_ip));

    // Finish the stream
    stream.finish().await
        .map_err(|e| anyhow::anyhow!("Failed to finish H3 stream: {}", e))?;

    Ok(())
}

/// Build an HTTP/1.1 backend request from the H3 frontend request.
fn build_backend_request<B>(
    method: &hyper::Method,
    backend_addr: &str,
    path: &str,
    host: &str,
    original_request: &hyper::Request<()>,
    body: B,
    use_tls: bool,
) -> anyhow::Result<hyper::Request<B>> {
    let scheme = if use_tls { "https" } else { "http" };
    let mut req = hyper::Request::builder()
        .method(method)
        .uri(format!("{}://{}{}", scheme, backend_addr, path))
        .header("host", host);

    // Forward non-pseudo headers
    for (name, value) in original_request.headers() {
        let n = name.as_str();
        if !n.starts_with(':') && n != "host" {
            req = req.header(name, value);
        }
    }

    req.body(body)
        .map_err(|e| anyhow::anyhow!("Failed to build backend request: {}", e))
}

/// A streaming request body backed by an mpsc channel receiver.
///
/// Implements `http_body::Body` so hyper can poll chunks as they arrive
/// from the H3 client, avoiding buffering the entire request body in memory.
struct H3RequestBody {
    receiver: tokio::sync::mpsc::Receiver<Bytes>,
}

impl http_body::Body for H3RequestBody {
    type Data = Bytes;
    type Error = hyper::Error;

    fn poll_frame(
        mut self: Pin<&mut Self>,
        cx: &mut Context<'_>,
    ) -> Poll<Option<Result<Frame<Self::Data>, Self::Error>>> {
        match self.receiver.poll_recv(cx) {
            Poll::Ready(Some(data)) => Poll::Ready(Some(Ok(Frame::data(data)))),
            Poll::Ready(None) => Poll::Ready(None),
            Poll::Pending => Poll::Pending,
        }
    }
}