use std::net::SocketAddr;
use std::sync::Arc;

use anyhow::{Context, anyhow};
use metrics::counter;
use quinn::{
    Connection, Endpoint, Incoming, RecvStream, SendStream, ServerConfig, StreamId, TransportConfig,
};
use rustls_pki_types::{CertificateDer, PrivateKeyDer};
use tokio::sync::oneshot;

use crate::config::QuicConfig;
use crate::transport::{QuicMessage, T1Sender, T2Sender, T3Inbound, T3Sender};

/// Datagram receive buffer in bytes. Sized to absorb microbursts at the
/// telemetry rates.
const DATAGRAM_RECV_BUFFER_BYTES: usize = 256 * 1024;

/// Load the cert chain + private key from disk and build a Quinn `ServerConfig`.
pub fn build_server_config(cfg: &QuicConfig) -> anyhow::Result<ServerConfig> {
    let cert_pem = std::fs::read(&cfg.server_cert)
        .with_context(|| format!("read server_cert at {}", cfg.server_cert))?;
    let key_pem = std::fs::read(&cfg.server_key)
        .with_context(|| format!("read server_key at {}", cfg.server_key))?;

    let certs: Vec<CertificateDer<'static>> = rustls_pemfile::certs(&mut cert_pem.as_slice())
        .collect::<Result<_, _>>()
        .with_context(|| format!("parse PEM certs at {}", cfg.server_cert))?;
    if certs.is_empty() {
        return Err(anyhow!("no certificates found in {}", cfg.server_cert));
    }

    let key: PrivateKeyDer<'static> = rustls_pemfile::private_key(&mut key_pem.as_slice())
        .with_context(|| format!("parse PEM key at {}", cfg.server_key))?
        .ok_or_else(|| anyhow!("no private key found in {}", cfg.server_key))?;

    let mut server_config =
        ServerConfig::with_single_cert(certs, key).context("build Quinn ServerConfig")?;

    // Explicit transport config so the values driving evaluation are visible
    // in source and at startup, not buried in Quinn's defaults.
    let mut transport = TransportConfig::default();
    transport.datagram_receive_buffer_size(Some(DATAGRAM_RECV_BUFFER_BYTES));
    server_config.transport = Arc::new(transport);

    tracing::info!(
        datagram_recv_buffer_bytes = DATAGRAM_RECV_BUFFER_BYTES,
        "Quinn TransportConfig tuned"
    );

    Ok(server_config)
}

/// Bind the listener. Must be called from inside a tokio runtime context
/// (Quinn relies on `Handle::current()` internally).
pub fn bind_endpoint(cfg: &QuicConfig) -> anyhow::Result<Endpoint> {
    let server_config = build_server_config(cfg)?;
    let addr: SocketAddr = format!("{}:{}", cfg.server_interface, cfg.server_port)
        .parse()
        .with_context(|| {
            format!(
                "invalid bind address {}:{}",
                cfg.server_interface, cfg.server_port
            )
        })?;
    Endpoint::server(server_config, addr).context("Endpoint::server bind")
}

/// Accept loop: per-connection senders are cloned from the tier handles and
/// shipped into `handle_incoming` for orchestration.
pub async fn accept_loop(endpoint: Endpoint, t1: T1Sender, t2: T2Sender, t3: T3Sender) {
    tracing::info!(local = ?endpoint.local_addr().ok(), "QUIC accept loop running");
    while let Some(incoming) = endpoint.accept().await {
        let t1 = t1.clone();
        let t2 = t2.clone();
        let t3 = t3.clone();
        tokio::spawn(handle_incoming(incoming, t1, t2, t3));
    }
    tracing::info!("QUIC accept loop exited");
}

/// Per-connection orchestrator. Performs the handshake and spawns one reader
/// per tier, then waits for the connection to close and joins the readers.
async fn handle_incoming(incoming: Incoming, t1: T1Sender, t2: T2Sender, t3: T3Sender) {
    let conn = match incoming.await {
        Ok(c) => c,
        Err(e) => {
            tracing::warn!(error = %e, "handshake failed");
            return;
        }
    };
    let remote = conn.remote_address();
    tracing::info!(?remote, "connection established");

    // One task per tier — fully wired across T1/T2/T3.
    let dgram_task = tokio::spawn(read_datagrams(conn.clone(), t1));
    let uni_task = tokio::spawn(read_uni_streams(conn.clone(), t2));
    let bi_task = tokio::spawn(accept_bi_streams(conn.clone(), t3));

    let _ = conn.closed().await;

    if let Err(e) = dgram_task.await {
        tracing::warn!(?remote, error = %e, "T1 datagram task ended unexpectedly");
    }
    if let Err(e) = uni_task.await {
        tracing::warn!(?remote, error = %e, "T2 uni stream task ended unexpectedly");
    }
    if let Err(e) = bi_task.await {
        tracing::warn!(?remote, error = %e, "T3 bi stream task ended unexpectedly");
    }
    tracing::info!(?remote, "connection closed");
}

/// T1 — read QUIC datagrams, decode each as a fixed-size `QuicMessage`, push
/// into the lossy T1 channel.
async fn read_datagrams(conn: Connection, t1: T1Sender) {
    let remote = conn.remote_address();
    let mut received: u64 = 0;
    let mut dropped: u64 = 0;
    let mut decode_errors: u64 = 0;

    loop {
        match conn.read_datagram().await {
            Ok(bytes) => match QuicMessage::decode(&bytes[..]) {
                Ok(msg) => {
                    received += 1;
                    counter!("substrate_received_total", "tier" => "t1").increment(1);
                    if !t1.send_lossy(msg) {
                        dropped += 1;
                        counter!("substrate_dropped_total", "tier" => "t1").increment(1);
                        tracing::trace!(?remote, "T1 channel full, datagram dropped");
                    }
                }
                Err(e) => {
                    decode_errors += 1;
                    counter!("substrate_decode_errors_total", "tier" => "t1").increment(1);
                    tracing::warn!(
                        ?remote,
                        len = bytes.len(),
                        error = %e,
                        "T1 datagram decode failed"
                    );
                }
            },
            Err(e) => {
                tracing::debug!(
                    ?remote,
                    received,
                    dropped,
                    decode_errors,
                    error = %e,
                    "T1 datagram reader ended"
                );
                return;
            }
        }
    }
}

/// T2 — accept unidirectional streams. Each accepted stream gets its own task
/// reading 38-byte chunks until EOF (one stream may carry one event or many).
/// Cross-stream interleaving is allowed; ordering is only guaranteed *within*
/// a stream, matching QUIC's stream semantics.
async fn read_uni_streams(conn: Connection, t2: T2Sender) {
    let remote = conn.remote_address();
    let mut streams_accepted: u64 = 0;

    loop {
        let recv = match conn.accept_uni().await {
            Ok(s) => s,
            Err(e) => {
                tracing::debug!(
                    ?remote,
                    streams_accepted,
                    error = %e,
                    "T2 uni accept loop ended"
                );
                return;
            }
        };
        streams_accepted += 1;
        let t2 = t2.clone();
        tokio::spawn(read_one_uni_stream(remote, recv, t2));
    }
}

/// Per-stream worker for T2. Reads fixed-size `QuicMessage`s back-to-back,
/// awaits backpressure on the T2 channel, and resets the stream on a decode
/// failure (one corrupt stream shouldn't take down the whole connection).
async fn read_one_uni_stream(remote: SocketAddr, mut recv: RecvStream, t2: T2Sender) {
    let stream_id: StreamId = recv.id();
    let mut buf = [0u8; QuicMessage::WIRE_SIZE];
    let mut count: u64 = 0;

    loop {
        match recv.read_exact(&mut buf).await {
            Ok(()) => match QuicMessage::decode(&buf) {
                Ok(msg) => {
                    count += 1;
                    counter!("substrate_received_total", "tier" => "t2").increment(1);
                    if t2.send(msg).await.is_err() {
                        // T2 receiver dropped (substrate shutting down).
                        tracing::warn!(
                            ?remote,
                            ?stream_id,
                            count,
                            "T2 channel closed; abandoning stream"
                        );
                        return;
                    }
                }
                Err(e) => {
                    counter!("substrate_decode_errors_total", "tier" => "t2").increment(1);
                    tracing::warn!(
                        ?remote,
                        ?stream_id,
                        count,
                        error = %e,
                        "T2 decode failed; resetting stream"
                    );
                    let _ = recv.stop(0u32.into());
                    return;
                }
            },
            Err(e) => {
                tracing::trace!(
                    ?remote,
                    ?stream_id,
                    count,
                    error = %e,
                    "T2 uni stream ended"
                );
                return;
            }
        }
    }
}

/// T3 — accept bidirectional streams. Each stream is one command/ack
/// exchange, modeled per the paper's "per-command oneshot channels": the
/// reader pushes a `T3Inbound { command, reply }` to the ECS, awaits the
/// response on `reply_rx`, and writes it back on the same stream.
async fn accept_bi_streams(conn: Connection, t3: T3Sender) {
    let remote = conn.remote_address();
    let mut streams_accepted: u64 = 0;

    loop {
        let (send, recv) = match conn.accept_bi().await {
            Ok(s) => s,
            Err(e) => {
                tracing::debug!(
                    ?remote,
                    streams_accepted,
                    error = %e,
                    "T3 bi accept loop ended"
                );
                return;
            }
        };
        streams_accepted += 1;
        let t3 = t3.clone();
        tokio::spawn(read_one_bi_stream(remote, send, recv, t3));
    }
}

/// Per-stream worker for T3. Reads exactly one command, ships it with a
/// `oneshot::Sender` to the ECS, awaits the reply, writes it back. If the
/// ECS drops the oneshot (no handler installed), the stream is reset so the
/// client sees an explicit reset instead of a half-open stream.
async fn read_one_bi_stream(
    remote: SocketAddr,
    mut send: SendStream,
    mut recv: RecvStream,
    t3: T3Sender,
) {
    let stream_id: StreamId = recv.id();

    let mut buf = [0u8; QuicMessage::WIRE_SIZE];
    if let Err(e) = recv.read_exact(&mut buf).await {
        tracing::trace!(
            ?remote,
            ?stream_id,
            error = %e,
            "T3: incomplete command read; closing"
        );
        return;
    }
    let command = match QuicMessage::decode(&buf) {
        Ok(m) => m,
        Err(e) => {
            counter!("substrate_decode_errors_total", "tier" => "t3").increment(1);
            tracing::warn!(
                ?remote,
                ?stream_id,
                error = %e,
                "T3 command decode failed; resetting stream"
            );
            let _ = recv.stop(0u32.into());
            let _ = send.reset(0u32.into());
            return;
        }
    };
    counter!("substrate_received_total", "tier" => "t3").increment(1);

    let (reply_tx, reply_rx) = oneshot::channel::<QuicMessage>();
    let inbound = T3Inbound {
        command,
        reply: reply_tx,
    };
    if t3.send(inbound).await.is_err() {
        tracing::warn!(?remote, ?stream_id, "T3 channel closed; abandoning command");
        let _ = send.reset(0u32.into());
        return;
    }

    let response = match reply_rx.await {
        Ok(msg) => msg,
        Err(_) => {
            // ECS dropped the oneshot. With M4's handler installed this
            // shouldn't happen normally; if it does, the stream is reset so
            // the client sees a clean signal.
            counter!("substrate_t3_no_handler_total").increment(1);
            tracing::debug!(
                ?remote,
                ?stream_id,
                "T3: no handler for command, resetting stream"
            );
            let _ = send.reset(0u32.into());
            return;
        }
    };

    if let Err(e) = send.write_all(&response.to_bytes()).await {
        tracing::warn!(
            ?remote,
            ?stream_id,
            error = %e,
            "T3 ack write failed"
        );
        return;
    }
    if let Err(e) = send.finish() {
        tracing::warn!(
            ?remote,
            ?stream_id,
            error = %e,
            "T3 ack finish failed"
        );
    }
}