//! TCP accept loop, per-connection handling, and the background purge loop.

use std::net::{TcpListener, TcpStream};
use std::sync::{Arc, Mutex};
use std::time::Duration;

use crate::bind::TransportBindValues;
use crate::channel::{handshake, Channel};
use crate::config::DomainConfig;
use crate::crypto::{b32, derive_public};
use crate::proto::{FETCH_BUDGET, HEADER_TIMEOUT_SECS, KEY_LEN, MALFORMED, PURGE_INTERVAL_SECS};
use crate::ratelimit::RateLimiter;
use crate::session::{ServerConfig, Session};
use crate::store::Store;

pub struct ServeArgs {
    pub key_path: String,
    pub db_path: String,
    pub host: String,
    pub port: u16,
    pub max_envelope: usize,
    pub quota: i64,
    pub requests_quota: i64,
    pub retention_days: i64,
    pub requests_retention_days: i64,
    pub max_tokens: u16,
    pub invite_token: Option<String>,
    pub rate_connections: u32,
    pub rate_sends: u32,
    pub rate_tokens: u32,
    pub max_connections: usize,
}

/// The single-domain CLI path: one anonymous domain.
pub fn run(args: ServeArgs) -> anyhow::Result<()> {
    serve_domains(vec![DomainConfig {
        name: "default".to_string(),
        key_path: args.key_path,
        db_path: args.db_path,
        host: args.host,
        port: args.port,
        max_envelope: args.max_envelope,
        quota: args.quota,
        requests_quota: args.requests_quota,
        retention_days: args.retention_days,
        requests_retention_days: args.requests_retention_days,
        max_tokens: args.max_tokens,
        invite_token: args.invite_token.map(String::into_bytes),
        rate_connections: args.rate_connections,
        rate_sends: args.rate_sends,
        rate_tokens: args.rate_tokens,
        max_connections: args.max_connections,
    }])
}

/// Serves every domain: one listener, key, mailbox database, config and
/// purge loop per domain, nothing shared between them.
pub fn serve_domains(domains: Vec<DomainConfig>) -> anyhow::Result<()> {
    // Read every key and bind every listener first, so a missing key or a
    // taken port fails the whole process before any domain serves.
    let mut bound = Vec::with_capacity(domains.len());
    for domain in &domains {
        let static_key = read_static_key(&domain.key_path)?;
        let listener = TcpListener::bind((domain.host.as_str(), domain.port))?;
        let config = Arc::new(ServerConfig {
            max_envelope: domain.max_envelope,
            fetch_budget: FETCH_BUDGET,
            main_quota: domain.quota,
            requests_quota: domain.requests_quota,
            max_tokens: domain.max_tokens,
            invite_token: domain.invite_token.clone(),
            conn_limiter: RateLimiter::new(domain.rate_connections),
            send_limiter: RateLimiter::new(domain.rate_sends),
            token_limiter: RateLimiter::new(domain.rate_tokens),
        });
        bound.push((domain, static_key, listener, config));
    }

    for (domain, static_key, listener, config) in bound {
        log::info!(
            "domain {}: listening on {}:{}",
            domain.name,
            domain.host,
            domain.port
        );
        log::info!(
            "domain {}: server public key: {}",
            domain.name,
            b32(&derive_public(&static_key))
        );

        let purge_db_path = domain.db_path.clone();
        let main_retention_secs = domain.retention_days * 86400;
        let requests_retention_secs = domain.requests_retention_days * 86400;
        std::thread::spawn(move || {
            purge_loop(purge_db_path, main_retention_secs, requests_retention_secs)
        });

        let gate = Arc::new(ConnGate::new(domain.max_connections));
        let db_path = domain.db_path.clone();
        std::thread::spawn(move || accept_loop(listener, config, static_key, db_path, gate));
    }

    // Each domain's accept loop runs in its own thread; nothing fails here.
    loop {
        std::thread::sleep(Duration::from_secs(3600));
    }
}

fn read_static_key(path: &str) -> anyhow::Result<[u8; KEY_LEN]> {
    let key =
        std::fs::read(path).map_err(|e| anyhow::anyhow!("cannot read server key {path}: {e}"))?;
    anyhow::ensure!(
        key.len() == KEY_LEN,
        "server key {path} must be {KEY_LEN} raw bytes, got {}",
        key.len()
    );
    Ok(key.try_into().unwrap())
}

fn accept_loop(
    listener: TcpListener,
    config: Arc<ServerConfig>,
    static_key: [u8; KEY_LEN],
    db_path: String,
    gate: Arc<ConnGate>,
) {
    for incoming in listener.incoming() {
        let stream = match incoming {
            Ok(s) => s,
            Err(e) => {
                log::warn!("accept error: {e}");
                continue;
            }
        };
        let peer_ip = stream
            .peer_addr()
            .map(|a| a.ip().to_string())
            .unwrap_or_else(|_| "unknown".to_string());

        if !config.conn_limiter.allow(&peer_ip) {
            log::warn!("rate limited {peer_ip}");
            continue;
        }

        let permit = match gate.try_acquire() {
            Some(p) => p,
            None => {
                log::warn!("connection limit reached, refusing {peer_ip}");
                continue;
            }
        };

        let config = Arc::clone(&config);
        let db_path = db_path.clone();
        std::thread::spawn(move || {
            // The permit is dropped with the connection, freeing its slot.
            let _permit = permit;
            if let Err(e) = handle_connection(stream, &config, &static_key, &db_path, &peer_ip) {
                log::info!("connection error from {peer_ip}: {e}");
            }
        });
    }
}

/// Cap on concurrently served TCP connections, the mirror of the RNS
/// carrier's link cap (SPEC.md sec 13.8): without one, a connection flood
/// would exhaust threads one unbounded spawn at a time. `max == 0` disables
/// the cap; past it, connections are refused, never queued.
struct ConnGate {
    max: usize,
    active: Mutex<usize>,
}

impl ConnGate {
    fn new(max: usize) -> Self {
        ConnGate {
            max,
            active: Mutex::new(0),
        }
    }

    fn try_acquire(self: &Arc<Self>) -> Option<ConnPermit> {
        if self.max == 0 {
            return Some(ConnPermit {
                gate: Arc::clone(self),
            });
        }
        let mut active = self.active.lock().unwrap();
        if *active >= self.max {
            return None;
        }
        *active += 1;
        Some(ConnPermit {
            gate: Arc::clone(self),
        })
    }
}

/// Dropping releases the slot, so the count stays accurate whatever return
/// path or panic closes the connection.
struct ConnPermit {
    gate: Arc<ConnGate>,
}

impl Drop for ConnPermit {
    fn drop(&mut self) {
        if self.gate.max == 0 {
            return;
        }
        *self.gate.active.lock().unwrap() -= 1;
    }
}

/// Also the hostile harness's entry point: it drives real connections
/// through the same accept/handshake/session path the TCP carrier serves.
pub(crate) fn handle_connection(
    mut stream: TcpStream,
    config: &ServerConfig,
    static_key: &[u8; KEY_LEN],
    db_path: &str,
    peer_ip: &str,
) -> anyhow::Result<()> {
    // The handshake runs under the header budget; once it completes,
    // Channel::read_noise picks between the header and idle deadlines per
    // frame state.
    stream.set_read_timeout(Some(Duration::from_secs(HEADER_TIMEOUT_SECS)))?;

    let handshake_started = std::time::Instant::now();
    let (transport, handshake_hash) = match handshake(&mut stream, static_key) {
        Ok(v) => v,
        Err(e) => {
            log::info!("handshake failed from {peer_ip}: {e}");
            return Ok(());
        }
    };
    log::debug!(
        "handshake from {peer_ip} in {:?}",
        handshake_started.elapsed()
    );

    // Drop logs the summary once, whatever return path closes the session.
    let summary = SessionSummary {
        peer_ip,
        started: std::time::Instant::now(),
        ops: std::cell::Cell::new(0),
    };

    let store = Store::open(db_path)?;
    let server_static = derive_public(static_key);
    let bind = TransportBindValues::tcp(&handshake_hash, &server_static)?;
    let mut session = Session::new(config, store, peer_ip.to_string(), bind);
    let mut channel = Channel::new(stream, transport);

    loop {
        let (op, body) = match channel.read_frame() {
            Ok(v) => v,
            Err(e) => {
                if is_eof_like(&e) {
                    return Ok(());
                }
                log::info!("bad frame from {peer_ip}: {e}");
                let _ = channel.write_frame(0, &[MALFORMED]);
                return Ok(());
            }
        };
        summary.ops.set(summary.ops.get() + 1);

        let (status, payload) = session.dispatch(op, &body);
        let mut response = Vec::with_capacity(1 + payload.len());
        response.push(status);
        response.extend_from_slice(&payload);
        if channel.write_frame(op, &response).is_err() {
            return Ok(());
        }
    }
}

struct SessionSummary<'a> {
    peer_ip: &'a str,
    started: std::time::Instant,
    ops: std::cell::Cell<u64>,
}

impl Drop for SessionSummary<'_> {
    fn drop(&mut self) {
        log::debug!(
            "session {} closed: {} op(s) in {:?}",
            self.peer_ip,
            self.ops.get(),
            self.started.elapsed()
        );
    }
}

fn is_eof_like(e: &anyhow::Error) -> bool {
    if let Some(io_err) = e.downcast_ref::<std::io::Error>() {
        return matches!(
            io_err.kind(),
            std::io::ErrorKind::UnexpectedEof
                | std::io::ErrorKind::ConnectionReset
                | std::io::ErrorKind::BrokenPipe
                | std::io::ErrorKind::TimedOut
                | std::io::ErrorKind::WouldBlock
        );
    }
    false
}

fn purge_loop(db_path: String, main_retention_secs: i64, requests_retention_secs: i64) {
    let store = match Store::open(&db_path) {
        Ok(s) => s,
        Err(e) => {
            log::error!("purge thread failed to open store: {e}");
            return;
        }
    };
    loop {
        std::thread::sleep(Duration::from_secs(PURGE_INTERVAL_SECS));
        let now = std::time::SystemTime::now()
            .duration_since(std::time::UNIX_EPOCH)
            .unwrap()
            .as_secs() as i64;
        match store.purge(now - main_retention_secs, now - requests_retention_secs) {
            Ok(0) => {}
            Ok(n) => log::info!("expired {n} message(s)"),
            Err(e) => log::error!("purge failed: {e}"),
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn gate_refuses_past_cap_and_frees_on_drop() {
        let gate = Arc::new(ConnGate::new(2));
        let a = gate.try_acquire().unwrap();
        let b = gate.try_acquire().unwrap();
        assert!(gate.try_acquire().is_none());
        drop(b);
        assert!(gate.try_acquire().is_some());
        drop(a);
    }

    #[test]
    fn zero_cap_means_unlimited() {
        let gate = Arc::new(ConnGate::new(0));
        for _ in 0..100 {
            assert!(gate.try_acquire().is_some());
        }
    }
}