//! WebTransport server endpoint for browser clients.
//!
//! Accepts HTTP/3 WebTransport sessions and dispatches RPC requests through the
//! same v2 handler registry as the native QUIC endpoint. Browsers connect via:
//!
//! ```js
//! const wt = new WebTransport("https://server:7443");
//! ```
//!
//! Each WebTransport bidirectional stream carries a single RPC request/response
//! using the same wire format as the native QUIC transport:
//!
//! ```text
//! [method_id: u16][request_id: u32][payload_len: u32][protobuf bytes]
//! ```

use std::sync::Arc;

use bytes::BytesMut;
use h3_quinn::quinn;
use h3_webtransport::server::AcceptedBi;
use tokio::io::{AsyncReadExt, AsyncWriteExt};
use tracing::{debug, info, warn};

use crate::v2_handlers::ServerState;
use quicproquo_rpc::error::RpcStatus;
use quicproquo_rpc::framing::{RequestFrame, ResponseFrame};
use quicproquo_rpc::method::{HandlerResult, MethodRegistry, RequestContext};

/// Concrete H3 connection type.
type H3Conn = h3::server::Connection<h3_quinn::Connection, bytes::Bytes>;

/// Concrete request resolver type.
type H3Resolver = h3::server::RequestResolver<h3_quinn::Connection, bytes::Bytes>;

/// Type alias for the concrete WebTransport session type used by this server.
type WtSession =
    h3_webtransport::server::WebTransportSession<h3_quinn::Connection, bytes::Bytes>;

/// Type alias for the concrete WebTransport bidi stream.
type WtBidiStream = h3_webtransport::stream::BidiStream<
    h3_quinn::BidiStream<bytes::Bytes>,
    bytes::Bytes,
>;

/// Start the WebTransport listener in a background task.
///
/// The endpoint uses the provided quinn `ServerConfig` (with "h3" ALPN) and
/// binds to `listen_addr`. Incoming HTTP/3 CONNECT requests are upgraded to
/// WebTransport sessions.
pub fn spawn_webtransport_listener(
    listen_addr: std::net::SocketAddr,
    server_config: quinn::ServerConfig,
    state: Arc<ServerState>,
    registry: Arc<MethodRegistry<ServerState>>,
) -> anyhow::Result<()> {
    let endpoint = quinn::Endpoint::server(server_config, listen_addr)
        .map_err(|e| anyhow::anyhow!("bind WebTransport endpoint {listen_addr}: {e}"))?;

    info!(addr = %listen_addr, "WebTransport endpoint listening");

    tokio::spawn(async move {
        accept_loop(endpoint, state, registry).await;
    });

    Ok(())
}

/// Accept QUIC connections and upgrade them to HTTP/3 + WebTransport.
async fn accept_loop(
    endpoint: quinn::Endpoint,
    state: Arc<ServerState>,
    registry: Arc<MethodRegistry<ServerState>>,
) {
    while let Some(incoming) = endpoint.accept().await {
        let state = Arc::clone(&state);
        let registry = Arc::clone(&registry);

        tokio::spawn(async move {
            let connection = match incoming.await {
                Ok(c) => c,
                Err(e) => {
                    warn!(error = %e, "WebTransport: QUIC accept failed");
                    return;
                }
            };

            let remote = connection.remote_address();
            debug!(remote = %remote, "WebTransport: new QUIC connection");
            metrics::counter!("webtransport_connections_total").increment(1);
            metrics::gauge!("webtransport_active_connections").increment(1.0);

            if let Err(e) = handle_h3_connection(connection, state, registry).await {
                debug!(remote = %remote, error = %e, "WebTransport: session error");
            }

            metrics::gauge!("webtransport_active_connections").decrement(1.0);
        });
    }
}

/// Handle an HTTP/3 connection: accept the WebTransport CONNECT request and
/// process bidirectional streams as RPC calls.
async fn handle_h3_connection(
    connection: quinn::Connection,
    state: Arc<ServerState>,
    registry: Arc<MethodRegistry<ServerState>>,
) -> anyhow::Result<()> {
    let h3_quinn_conn = h3_quinn::Connection::new(connection);
    let mut h3_conn: H3Conn = h3::server::builder()
        .enable_webtransport(true)
        .enable_extended_connect(true)
        .enable_datagram(true)
        .build::<h3_quinn::Connection, bytes::Bytes>(h3_quinn_conn)
        .await
        .map_err(|e| anyhow::anyhow!("H3 connection setup: {e}"))?;

    // Accept HTTP/3 requests until we get a CONNECT for WebTransport.
    loop {
        let resolver: H3Resolver = match h3_conn.accept().await {
            Ok(Some(r)) => r,
            Ok(None) => {
                debug!("WebTransport: H3 connection closed");
                return Ok(());
            }
            Err(e) => {
                return Err(anyhow::anyhow!("WebTransport: H3 accept error: {e}"));
            }
        };

        let (request, stream) = resolver
            .resolve_request()
            .await
            .map_err(|e| anyhow::anyhow!("resolve request: {e}"))?;

        let method = request.method().clone();
        let uri = request.uri().clone();

        if method == http::Method::CONNECT {
            debug!(uri = %uri, "WebTransport: CONNECT request");

            let wt_session = h3_webtransport::server::WebTransportSession::accept(
                request, stream, h3_conn,
            )
            .await
            .map_err(|e| anyhow::anyhow!("WebTransport session accept: {e}"))?;

            info!("WebTransport: session established");
            metrics::counter!("webtransport_sessions_total").increment(1);

            serve_wt_streams(wt_session, state, registry).await;
            return Ok(());
        }

        debug!(method = %method, uri = %uri, "WebTransport: non-CONNECT request ignored");
    }
}

/// Per-connection state from the WebTransport auth handshake.
#[derive(Debug, Clone, Default)]
struct ConnectionState {
    session_token: Option<Vec<u8>>,
    identity_key: Option<Vec<u8>>,
}

/// Accept bidirectional streams from a WebTransport session and dispatch
/// each as an RPC request.
async fn serve_wt_streams(
    session: WtSession,
    state: Arc<ServerState>,
    registry: Arc<MethodRegistry<ServerState>>,
) {
    // Auth handshake: the first bidi stream carries the session token.
    let conn_state: Arc<ConnectionState> = match accept_auth_stream(&session).await {
        Ok(cs) => Arc::new(cs),
        Err(e) => {
            warn!(error = %e, "WebTransport: auth handshake failed");
            return;
        }
    };

    loop {
        match session.accept_bi().await {
            Ok(Some(AcceptedBi::BidiStream(_session_id, stream))) => {
                let state = Arc::clone(&state);
                let registry = Arc::clone(&registry);
                let conn_state = Arc::clone(&conn_state);
                tokio::spawn(async move {
                    if let Err(e) =
                        handle_wt_bidi_stream(stream, state, registry, &conn_state).await
                    {
                        debug!(error = %e, "WebTransport: stream error");
                    }
                });
            }
            Ok(Some(AcceptedBi::Request(_req, _stream))) => {
                debug!("WebTransport: ignoring nested HTTP/3 request");
            }
            Ok(None) => {
                debug!("WebTransport: no more bidi streams");
                break;
            }
            Err(e) => {
                debug!(error = %e, "WebTransport: accept_bi error");
                break;
            }
        }
    }
}

/// Accept the first bidirectional stream as an auth init handshake.
///
/// The client sends a raw session token (length-prefixed: `u32 BE + token bytes`).
/// The server reads it and sends a 1-byte ack (0x00).
async fn accept_auth_stream(session: &WtSession) -> anyhow::Result<ConnectionState> {
    let accepted = session
        .accept_bi()
        .await
        .map_err(|e| anyhow::anyhow!("auth stream accept: {e}"))?
        .ok_or_else(|| anyhow::anyhow!("session closed before auth handshake"))?;

    let mut stream: WtBidiStream = match accepted {
        AcceptedBi::BidiStream(_session_id, stream) => stream,
        AcceptedBi::Request(_, _) => {
            anyhow::bail!("expected bidi stream for auth, got HTTP/3 request")
        }
    };

    // Read the token: [len: u32 BE][token bytes]
    let mut header = [0u8; 4];
    AsyncReadExt::read_exact(&mut stream, &mut header)
        .await
        .map_err(|e| anyhow::anyhow!("auth read header: {e}"))?;
    let len = u32::from_be_bytes(header) as usize;

    if len > 4096 {
        anyhow::bail!("auth token too large: {len} bytes");
    }

    let mut token = vec![0u8; len];
    if len > 0 {
        AsyncReadExt::read_exact(&mut stream, &mut token)
            .await
            .map_err(|e| anyhow::anyhow!("auth read token: {e}"))?;
    }

    // Send ack: single zero byte.
    AsyncWriteExt::write_all(&mut stream, &[0u8])
        .await
        .map_err(|e| anyhow::anyhow!("auth ack send: {e}"))?;

    debug!(token_len = token.len(), "WebTransport: auth init received");

    Ok(ConnectionState {
        session_token: Some(token),
        identity_key: None,
    })
}

/// Handle a single WebTransport bidirectional stream: read request, dispatch,
/// write response. Uses the same framing as native QUIC.
async fn handle_wt_bidi_stream(
    mut stream: WtBidiStream,
    state: Arc<ServerState>,
    registry: Arc<MethodRegistry<ServerState>>,
    conn_state: &ConnectionState,
) -> anyhow::Result<()> {
    // Read the complete request from the stream.
    let max_size =
        quicproquo_rpc::framing::MAX_PAYLOAD_SIZE + quicproquo_rpc::framing::REQUEST_HEADER_SIZE;

    let mut buf = Vec::with_capacity(1024);
    let mut tmp = [0u8; 8192];
    loop {
        let n = AsyncReadExt::read(&mut stream, &mut tmp)
            .await
            .map_err(|e| anyhow::anyhow!("recv: {e}"))?;

        if n == 0 {
            break;
        }

        buf.extend_from_slice(&tmp[..n]);

        if buf.len() > max_size {
            anyhow::bail!("payload too large");
        }
    }

    let mut bytes = BytesMut::from(buf.as_slice());

    let frame = match RequestFrame::decode(&mut bytes)
        .map_err(|e| anyhow::anyhow!("decode: {e}"))?
    {
        Some(f) => f,
        None => anyhow::bail!("incomplete request frame"),
    };

    let trace_id = format!(
        "wt-{:016x}",
        std::time::SystemTime::now()
            .duration_since(std::time::UNIX_EPOCH)
            .unwrap_or_default()
            .as_nanos() as u64
    );

    let result = match registry.get(frame.method_id) {
        Some((handler, name, timeout)) => {
            let span = tracing::info_span!(
                "wt_rpc",
                trace_id = %trace_id,
                method_id = frame.method_id,
                method = name,
                req_id = frame.request_id,
            );
            let _guard = span.enter();
            debug!("dispatching");

            let deadline = timeout.map(|d| tokio::time::Instant::now() + d);
            let start = std::time::Instant::now();

            let ctx = RequestContext {
                identity_key: conn_state.identity_key.clone(),
                session_token: conn_state.session_token.clone(),
                payload: frame.payload,
                trace_id: trace_id.clone(),
                deadline,
            };

            let result = if let Some(dur) = timeout {
                match tokio::time::timeout(dur, handler(Arc::clone(&state), ctx)).await {
                    Ok(r) => r,
                    Err(_) => {
                        warn!(method = name, "WebTransport: request deadline exceeded");
                        HandlerResult::err(RpcStatus::DeadlineExceeded, "request deadline exceeded")
                    }
                }
            } else {
                handler(Arc::clone(&state), ctx).await
            };

            let elapsed = start.elapsed();
            metrics::histogram!("webtransport_request_duration_seconds", "method" => name)
                .record(elapsed.as_secs_f64());
            metrics::counter!("webtransport_requests_total", "method" => name).increment(1);

            result
        }
        None => {
            warn!(method_id = frame.method_id, "WebTransport: unknown method");
            HandlerResult::err(RpcStatus::UnknownMethod, "unknown method")
        }
    };

    let response = ResponseFrame {
        status: result.status as u8,
        request_id: frame.request_id,
        payload: result.payload,
    };

    let encoded = response.encode();
    AsyncWriteExt::write_all(&mut stream, &encoded)
        .await
        .map_err(|e| anyhow::anyhow!("send response: {e}"))?;
    AsyncWriteExt::shutdown(&mut stream)
        .await
        .map_err(|e| anyhow::anyhow!("shutdown: {e}"))?;

    Ok(())
}

/// Build a quinn `ServerConfig` for the WebTransport endpoint.
///
/// Uses the same TLS cert/key as the main server but with "h3" ALPN.
pub fn build_webtransport_server_config(
    cert_path: &std::path::Path,
    key_path: &std::path::Path,
) -> anyhow::Result<quinn::ServerConfig> {
    use anyhow::Context;
    use rustls::pki_types::{CertificateDer, PrivateKeyDer};
    use rustls::version::TLS13;

    let cert_bytes = std::fs::read(cert_path).context("read WebTransport cert")?;
    let key_bytes = std::fs::read(key_path).context("read WebTransport key")?;

    let cert_chain = vec![CertificateDer::from(cert_bytes)];
    let key = PrivateKeyDer::try_from(key_bytes)
        .map_err(|_| anyhow::anyhow!("invalid WebTransport private key"))?;

    let mut tls = rustls::ServerConfig::builder_with_protocol_versions(&[&TLS13])
        .with_no_client_auth()
        .with_single_cert(cert_chain, key)?;
    tls.alpn_protocols = vec![b"h3".to_vec()];

    let crypto = quinn_proto::crypto::rustls::QuicServerConfig::try_from(tls)
        .map_err(|e| anyhow::anyhow!("invalid WebTransport TLS config: {e}"))?;

    let mut transport = quinn::TransportConfig::default();
    transport.max_idle_timeout(Some(
        std::time::Duration::from_secs(300)
            .try_into()
            .map_err(|e| anyhow::anyhow!("idle timeout: {e}"))?,
    ));
    // WebTransport sessions may have multiple simultaneous streams.
    transport.max_concurrent_bidi_streams(64u32.into());
    transport.max_concurrent_uni_streams(16u32.into());

    let mut server_config = quinn::ServerConfig::with_crypto(Arc::new(crypto));
    server_config.transport_config(Arc::new(transport));

    Ok(server_config)
}

#[cfg(test)]
mod tests {
    #[test]
    fn webtransport_module_compiles() {
        assert!(true);
    }
}