use anyhow::anyhow;
use bytes::Bytes;
use h2::{RecvStream, client, server};
use http::Request;
use log::{debug, error, info};
use serde::{Deserialize, Serialize};
use std::collections::{HashMap, HashSet};
use std::error::Error;
use std::fs::File;
use std::io::BufReader;
use std::sync::Arc;
use std::sync::atomic::{AtomicU64, Ordering};
use tokio::io::{AsyncReadExt, AsyncWriteExt};
use tokio::net::{TcpListener, TcpStream};
use tokio::sync::{Mutex, Notify};
use tokio::time::{Duration, sleep, timeout};
use tokio_rustls::rustls;
use tokio_rustls::rustls::pki_types::{CertificateDer, ServerName};
use tokio_rustls::{TlsAcceptor, TlsConnector};

#[derive(Debug, Copy, Clone, PartialEq, Eq, Serialize, Deserialize)]
pub enum Identity {
    Server,
    Client,
}

impl Identity {
    pub fn as_u8(&self) -> u8 {
        match self {
            Identity::Server => b'S',
            Identity::Client => b'C',
        }
    }
}

#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct ClientTunnelConfig {
    pub host: Option<String>,
    pub url: String,
    pub token: String,
    pub identity: Identity,
}

impl Default for ClientTunnelConfig {
    fn default() -> Self {
        Self {
            host: None,
            url: "127.0.0.1:3333".to_string(),
            token: "super_secret_magic_token".to_string(),
            identity: Identity::Client,
        }
    }
}

unsafe impl Send for ClientTunnelConfig {}

#[derive(Debug, Clone, Default, Serialize, Deserialize)]
pub struct SslConfig {
    pub cert: String,
    pub key: String,
}

#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct ServerTunnelConfig {
    pub url: String,
    pub cert: Option<SslConfig>,
    pub token: String,
    pub identity: Identity,
}

impl Default for ServerTunnelConfig {
    fn default() -> Self {
        Self {
            url: "127.0.0.1:3333".to_string(),
            cert: None,
            token: "super_secret_magic_token".to_string(),
            identity: Identity::Client,
        }
    }
}

static CLIENT_HELLO: &[u8] = b"Sekai unpacker client hello";
static SERVER_HELLO: &[u8] = b"Sekai unpacker server hello";
static TLS_BOOTSTRAP_MAGIC: &[u8] = b"SPTLSP1";
static RESUME_MAGIC: &[u8] = b"SPRESM1";

pub enum TunnelEndpointRaw {
    Client(Box<client::SendRequest<Bytes>>),
    Server(Box<server::Connection<TcpStream, Bytes>>),
}

#[derive(Clone)]
pub enum TunnelEndpoint {
    Client(Arc<ClientManager>),
    Server(Arc<ServerManager>),
}

pub struct ClientManager {
    pub session_id: AtomicU64,
    pub current_client: Mutex<Option<client::SendRequest<Bytes>>>,
    pub config: Option<ClientTunnelConfig>,
    pub notify: Arc<Notify>,
}

impl ClientManager {
    pub async fn get_client(&self) -> anyhow::Result<client::SendRequest<Bytes>> {
        loop {
            let cached = { self.current_client.lock().await.clone() };
            if let Some(mut c) = cached {
                if futures::future::poll_fn(|cx| c.poll_ready(cx))
                    .await
                    .is_ok()
                {
                    return Ok(c);
                }
                debug!("Client physical connection dead, preparing to recover...");
            }

            let mut lock = self.current_client.lock().await;
            if lock.is_some()
                && futures::future::poll_fn(|cx| lock.as_mut().unwrap().poll_ready(cx))
                    .await
                    .is_ok()
            {
                continue;
            }

            if let Some(config) = &self.config {
                let mut retry_delay = Duration::from_secs(1);
                let mut sid = self.session_id.load(Ordering::Relaxed);

                let new_client = loop {
                    match do_client_reconnect(config, &mut sid).await {
                        Ok(TunnelEndpointRaw::Client(c)) => {
                            self.session_id.store(sid, Ordering::Relaxed);
                            break c;
                        }
                        Ok(_) => return Err(anyhow::anyhow!("Identity mismatch on reconnect")),
                        Err(e) => {
                            error!("Reconnect failed: {}. Retrying in {:?}...", e, retry_delay);
                            drop(e);
                            sleep(retry_delay).await;
                            retry_delay = std::cmp::min(retry_delay * 2, Duration::from_secs(30));
                        }
                    }
                };
                *lock = Some(*new_client.clone());
                return Ok(*new_client);
            } else {
                *lock = None;
                drop(lock);
                match timeout(Duration::from_secs(15), self.notify.notified()).await {
                    Ok(_) => continue,
                    Err(_) => {
                        anyhow::bail!("Session did not reconnect within 15s. Throwing error!")
                    }
                }
            }
        }
    }
}

pub struct ServerManager {
    pub session_id: AtomicU64,
    pub current_server: Mutex<Option<server::Connection<TcpStream, Bytes>>>,
    pub config: Option<ClientTunnelConfig>,
    pub notify: Arc<Notify>,
}

impl ServerManager {
    pub async fn accept(
        &self,
    ) -> Option<Result<(Request<RecvStream>, server::SendResponse<Bytes>), h2::Error>> {
        loop {
            let mut conn_guard = self.current_server.lock().await;
            if let Some(conn) = conn_guard.as_mut() {
                if let Some(res) = conn.accept().await {
                    return Some(res);
                }
                *conn_guard = None;
                log::warn!("Server physical connection dropped, waiting for recovery...");
            }
            drop(conn_guard);

            if let Some(config) = &self.config {
                let mut retry_delay = Duration::from_secs(1);
                let mut sid = self.session_id.load(Ordering::Relaxed);

                let new_server = loop {
                    match do_client_reconnect(config, &mut sid).await {
                        Ok(TunnelEndpointRaw::Server(s)) => {
                            self.session_id.store(sid, Ordering::Relaxed);
                            break s;
                        }
                        Ok(_) => {
                            error!("Identity mismatch on reconnect");
                            return None;
                        }
                        Err(e) => {
                            error!("Reconnect failed: {}. Retrying in {:?}...", e, retry_delay);
                            drop(e);
                            sleep(retry_delay).await;
                            retry_delay = std::cmp::min(retry_delay * 2, Duration::from_secs(30));
                        }
                    }
                };
                *self.current_server.lock().await = Some(*new_server);
                continue;
            } else {
                match timeout(Duration::from_secs(15), self.notify.notified()).await {
                    Ok(_) => continue,
                    Err(_) => {
                        error!(
                            "Session did not reconnect within 15s. Throwing error (returning None)!"
                        );
                        return None;
                    }
                }
            }
        }
    }
}

enum ResumeResult {
    NotResume(TcpStream),
    NewSession(TunnelEndpointRaw, u64),
    ResumedExisting,
    Invalid,
}

pub struct TunnelListener {
    listener: TcpListener,
    config: ServerTunnelConfig,
    pending_plain_sessions: Mutex<HashSet<u64>>,
    next_session_id: AtomicU64,
    active_sessions: Mutex<HashMap<u64, TunnelEndpoint>>,
}

impl TunnelListener {
    pub async fn bind(config: ServerTunnelConfig) -> anyhow::Result<Self> {
        let listener = TcpListener::bind(&config.url).await?;
        info!("TCP tunnel listener bound to {}", &config.url);
        Ok(Self {
            listener,
            config,
            pending_plain_sessions: Mutex::new(HashSet::new()),
            next_session_id: AtomicU64::new(1),
            active_sessions: Mutex::new(HashMap::new()),
        })
    }

    pub async fn accept(&self) -> anyhow::Result<TunnelEndpoint> {
        loop {
            let (stream, peer_addr) = self.listener.accept().await?;
            debug!("[{}] Connected on tcp", peer_addr);

            if is_tls_client_hello(&stream).await {
                if let Err(e) = self.handle_tls_bootstrap(stream, peer_addr).await {
                    error!("[{}] TLS bootstrap failed: {}", peer_addr, e);
                }
                continue;
            }

            let mut stream = match self.try_resume_plain_session(stream, peer_addr).await? {
                ResumeResult::NewSession(ep_raw, sid) => {
                    let ep = wrap_raw_endpoint(sid, ep_raw, None);
                    self.active_sessions.lock().await.insert(sid, ep.clone());
                    return Ok(ep);
                }
                ResumeResult::ResumedExisting => continue,
                ResumeResult::Invalid => continue,
                ResumeResult::NotResume(s) => s,
            };

            if perform_server_handshake(&mut stream, &self.config.token, self.config.identity)
                .await
                .is_err()
            {
                debug!("[{}] Plain handshake failed", peer_addr);
                continue;
            }

            debug!(
                "[{}] Plain handshake completed, upgrading to H2...",
                peer_addr
            );
            let ep_raw = upgrade_to_h2_raw(stream, self.config.identity)
                .await
                .map_err(|e| anyhow::anyhow!("{}", e))?;

            let sid = self.next_session_id.fetch_add(1, Ordering::Relaxed);
            let ep = wrap_raw_endpoint(sid, ep_raw, None);
            self.active_sessions.lock().await.insert(sid, ep.clone());
            return Ok(ep);
        }
    }

    async fn handle_tls_bootstrap(
        &self,
        stream: TcpStream,
        peer_addr: std::net::SocketAddr,
    ) -> Result<(), Box<dyn Error>> {
        let cert_cfg = self
            .config
            .cert
            .as_ref()
            .ok_or("TLS is not enabled on server")?;

        let acceptor = TlsAcceptor::from(build_server_tls_config(cert_cfg)?);
        let mut tls_stream = acceptor.accept(stream).await?;

        perform_server_handshake(&mut tls_stream, &self.config.token, self.config.identity).await?;

        let mut magic = vec![0u8; TLS_BOOTSTRAP_MAGIC.len()];
        tls_stream.read_exact(&mut magic).await?;
        if magic != TLS_BOOTSTRAP_MAGIC {
            return Err("TLS bootstrap marker mismatch".into());
        }

        let session_id = self.next_session_id.fetch_add(1, Ordering::Relaxed);
        {
            let mut pending = self.pending_plain_sessions.lock().await;
            pending.insert(session_id);
        }

        tls_stream.write_all(TLS_BOOTSTRAP_MAGIC).await?;
        tls_stream.write_all(&session_id.to_be_bytes()).await?;
        debug!(
            "[{}] TLS bootstrap done, issued plain-H2 session {}",
            peer_addr, session_id
        );
        Ok(())
    }

    async fn try_resume_plain_session(
        &self,
        mut stream: TcpStream,
        peer_addr: std::net::SocketAddr,
    ) -> anyhow::Result<ResumeResult> {
        let mut peek_buf = vec![0u8; RESUME_MAGIC.len()];
        let n = stream.peek(&mut peek_buf).await?;
        if n < RESUME_MAGIC.len() || peek_buf != RESUME_MAGIC {
            return Ok(ResumeResult::NotResume(stream));
        }

        let mut magic = vec![0u8; RESUME_MAGIC.len()];
        stream.read_exact(&mut magic).await?;
        let mut sid_buf = [0u8; 8];
        stream.read_exact(&mut sid_buf).await?;
        let session_id = u64::from_be_bytes(sid_buf);

        let is_pending = {
            let mut pending = self.pending_plain_sessions.lock().await;
            pending.remove(&session_id)
        };

        if is_pending {
            let ep_raw = upgrade_to_h2_raw(stream, self.config.identity)
                .await
                .map_err(|e| anyhow::anyhow!("{}", e))?;
            return Ok(ResumeResult::NewSession(ep_raw, session_id));
        }

        let active = self.active_sessions.lock().await.get(&session_id).cloned();
        if let Some(ep) = active {
            let ep_raw = upgrade_to_h2_raw(stream, self.config.identity)
                .await
                .map_err(|e| anyhow::anyhow!("{}", e))?;
            update_endpoint(&ep, ep_raw).await;
            info!(
                "[{}] Successfully resumed existing session {}",
                peer_addr, session_id
            );
            return Ok(ResumeResult::ResumedExisting);
        }

        error!("[{}] Invalid plain-H2 session {}", peer_addr, session_id);
        Ok(ResumeResult::Invalid)
    }
}

async fn is_tls_client_hello(stream: &TcpStream) -> bool {
    let mut header = [0u8; 3];
    match stream.peek(&mut header).await {
        Ok(n) if n >= 3 => header[0] == 0x16 && header[1] == 0x03 && (1..=4).contains(&header[2]),
        _ => false,
    }
}

async fn perform_server_handshake<S>(
    stream: &mut S,
    token: &str,
    identity: Identity,
) -> Result<(), Box<dyn Error>>
where
    S: tokio::io::AsyncRead + tokio::io::AsyncWrite + Unpin,
{
    let mut client_hello_buf = vec![0u8; CLIENT_HELLO.len()];
    stream.read_exact(&mut client_hello_buf).await?;
    if client_hello_buf != CLIENT_HELLO {
        return Err("Client Hello mismatch".into());
    }

    let mut token_buf = vec![0u8; token.len()];
    stream.read_exact(&mut token_buf).await?;
    if String::from_utf8_lossy(&token_buf) != token {
        return Err("Wrong token".into());
    }

    stream.write_all(SERVER_HELLO).await?;

    let mut peer_id_buf = [0u8; 1];
    stream.read_exact(&mut peer_id_buf).await?;
    stream.write_all(&[identity.as_u8()]).await?;

    if peer_id_buf[0] == identity.as_u8() {
        return Err("Identity collision with peer".into());
    }

    Ok(())
}

async fn perform_client_handshake<S>(
    stream: &mut S,
    token: &str,
    identity: Identity,
) -> anyhow::Result<()>
where
    S: tokio::io::AsyncRead + tokio::io::AsyncWrite + Unpin,
{
    stream.write_all(CLIENT_HELLO).await?;
    stream.write_all(token.as_bytes()).await?;

    let mut server_hello_buf = vec![0u8; SERVER_HELLO.len()];
    stream.read_exact(&mut server_hello_buf).await?;
    if server_hello_buf != SERVER_HELLO {
        return Err(anyhow!("Server Hello mismatch"));
    }

    stream.write_all(&[identity.as_u8()]).await?;
    let mut peer_id_buf = [0u8; 1];
    stream.read_exact(&mut peer_id_buf).await?;

    if peer_id_buf[0] == identity.as_u8() {
        return Err(anyhow!("Identity collision with server"));
    }

    Ok(())
}

fn build_server_tls_config(
    cert_cfg: &SslConfig,
) -> Result<Arc<rustls::ServerConfig>, Box<dyn Error>> {
    let cert_file = File::open(&cert_cfg.cert)?;
    let mut cert_reader = BufReader::new(cert_file);
    let cert_chain: Vec<CertificateDer<'static>> =
        rustls_pemfile::certs(&mut cert_reader).collect::<Result<Vec<_>, _>>()?;
    if cert_chain.is_empty() {
        return Err("No certificate found in PEM".into());
    }

    let key_file = File::open(&cert_cfg.key)?;
    let mut key_reader = BufReader::new(key_file);
    let private_key =
        rustls_pemfile::private_key(&mut key_reader)?.ok_or("No private key found in PEM")?;

    let server_config = rustls::ServerConfig::builder()
        .with_no_client_auth()
        .with_single_cert(cert_chain, private_key)?;
    Ok(Arc::new(server_config))
}

fn build_client_tls_connector() -> TlsConnector {
    let root_store =
        rustls::RootCertStore::from_iter(webpki_roots::TLS_SERVER_ROOTS.iter().cloned());
    let config = rustls::ClientConfig::builder()
        .with_root_certificates(root_store)
        .with_no_client_auth();
    TlsConnector::from(Arc::new(config))
}

async fn upgrade_to_h2_raw(
    stream: TcpStream,
    identity: Identity,
) -> anyhow::Result<TunnelEndpointRaw> {
    match identity {
        Identity::Client => {
            let (h2_client, h2_conn) = client::handshake(stream).await?;
            tokio::spawn(async move {
                if let Err(e) = h2_conn.await {
                    debug!("H2 connection driver finished/error: {:?}", e);
                }
            });
            Ok(TunnelEndpointRaw::Client(Box::new(h2_client)))
        }
        Identity::Server => {
            let h2_conn = server::handshake(stream).await?;
            Ok(TunnelEndpointRaw::Server(Box::new(h2_conn)))
        }
    }
}

fn wrap_raw_endpoint(
    sid: u64,
    raw: TunnelEndpointRaw,
    config: Option<ClientTunnelConfig>,
) -> TunnelEndpoint {
    match raw {
        TunnelEndpointRaw::Client(c) => TunnelEndpoint::Client(Arc::new(ClientManager {
            session_id: AtomicU64::new(sid),
            current_client: Mutex::new(Some(*c)),
            config,
            notify: Arc::new(Notify::new()),
        })),
        TunnelEndpointRaw::Server(s) => TunnelEndpoint::Server(Arc::new(ServerManager {
            session_id: AtomicU64::new(sid),
            current_server: Mutex::new(Some(*s)),
            config,
            notify: Arc::new(Notify::new()),
        })),
    }
}

async fn update_endpoint(ep: &TunnelEndpoint, ep_raw: TunnelEndpointRaw) {
    match (ep, ep_raw) {
        (TunnelEndpoint::Client(mgr), TunnelEndpointRaw::Client(c)) => {
            *mgr.current_client.lock().await = Some(*c);
            mgr.notify.notify_waiters();
        }
        (TunnelEndpoint::Server(mgr), TunnelEndpointRaw::Server(s)) => {
            *mgr.current_server.lock().await = Some(*s);
            mgr.notify.notify_waiters();
        }
        _ => error!("Identity mismatch during session resume!"),
    }
}

pub async fn connect_tunnel(config: ClientTunnelConfig) -> Result<TunnelEndpoint, Box<dyn Error>> {
    info!("Connecting to tunnel at {}", &config.url);
    let mut sid = 0;
    let ep_raw = do_client_reconnect(&config, &mut sid).await?;
    Ok(wrap_raw_endpoint(sid, ep_raw, Some(config)))
}

async fn do_client_reconnect(
    config: &ClientTunnelConfig,
    current_sid: &mut u64,
) -> anyhow::Result<TunnelEndpointRaw> {
    if *current_sid != 0 {
        match resume_tunnel_client(config, *current_sid).await {
            Ok(ep) => {
                info!("Resumed existing session {}", current_sid);
                return Ok(ep);
            }
            Err(e) => {
                log::warn!("Resume failed: {}. Falling back to full bootstrap.", e);
            }
        }
    }

    if let Some(host) = config.host.clone() {
        let (sid, _) = bootstrap_tls_and_get_sid(config, &host).await?;
        *current_sid = sid;
        resume_tunnel_client(config, sid).await
    } else {
        let mut stream = TcpStream::connect(&config.url).await?;
        perform_client_handshake(&mut stream, &config.token, config.identity).await?;
        let raw = upgrade_to_h2_raw(stream, config.identity).await?;
        *current_sid = 0;
        Ok(raw)
    }
}

async fn bootstrap_tls_and_get_sid(
    config: &ClientTunnelConfig,
    host: &str,
) -> anyhow::Result<(u64, ())> {
    let connector = build_client_tls_connector();
    let tcp = TcpStream::connect(&config.url).await?;
    let server_name = ServerName::try_from(host.to_string())
        .map_err(|_| anyhow!("Invalid TLS host: {}", host))?
        .to_owned();

    let mut tls_stream = connector
        .connect(server_name, tcp)
        .await
        .map_err(|e| anyhow!("TLS handshake failed (server may not support TLS): {}", e))?;

    perform_client_handshake(&mut tls_stream, &config.token, config.identity).await?;

    tls_stream.write_all(TLS_BOOTSTRAP_MAGIC).await?;
    let mut magic = vec![0u8; TLS_BOOTSTRAP_MAGIC.len()];
    tls_stream.read_exact(&mut magic).await?;
    if magic != TLS_BOOTSTRAP_MAGIC {
        return Err(anyhow!("TLS bootstrap response mismatch"));
    }

    let mut sid_buf = [0u8; 8];
    tls_stream.read_exact(&mut sid_buf).await?;
    Ok((u64::from_be_bytes(sid_buf), ()))
}

async fn resume_tunnel_client(
    config: &ClientTunnelConfig,
    session_id: u64,
) -> anyhow::Result<TunnelEndpointRaw> {
    let mut plain_stream = TcpStream::connect(&config.url).await?;
    plain_stream.write_all(RESUME_MAGIC).await?;
    plain_stream.write_all(&session_id.to_be_bytes()).await?;
    upgrade_to_h2_raw(plain_stream, config.identity).await
}