//! Forwards bytes between `infer.sock` and the llama-server router. It logs nothing.

use std::io::{self, copy};
use std::net::{Shutdown, TcpStream};
use std::os::unix::net::{UnixListener, UnixStream};
use std::sync::Arc;
use std::sync::atomic::{AtomicUsize, Ordering};
use std::thread;
use std::time::Instant;

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct Limits {
    pub max_connections: usize,
    pub burst: u32,
    pub per_second: u32,
}

impl Default for Limits {
    fn default() -> Self {
        Limits {
            max_connections: 8,
            burst: 10,
            per_second: 2,
        }
    }
}

/// Holds at most `burst` tokens and gains `per_second` tokens each second.
pub struct TokenBucket {
    tokens: u64,
    burst: u64,
    per_second: u32,
    last: Instant,
}

impl TokenBucket {
    pub fn new(burst: u32, per_second: u32, now: Instant) -> Self {
        let burst = u64::from(burst) * 1000;
        TokenBucket {
            tokens: burst,
            burst,
            per_second,
            last: now,
        }
    }

    pub fn take(&mut self, now: Instant) -> bool {
        let elapsed = now.saturating_duration_since(self.last);
        let earned = refill_thousandths(self.per_second, elapsed);
        let total = u128::from(self.tokens).saturating_add(earned);
        let capped = total.min(u128::from(self.burst));
        self.tokens = u64::try_from(capped).unwrap_or(self.burst);
        self.last = now;
        if self.tokens >= 1000 {
            self.tokens -= 1000;
            true
        } else {
            false
        }
    }
}

/// Thousandths of a token earned over `elapsed` at `per_second` tokens per second.
/// per_second * elapsed_nanoseconds / 1_000_000 keeps partial refills additive without
/// floating point, so checks 100 ms apart at 2/s accumulate correctly.
fn refill_thousandths(per_second: u32, elapsed: std::time::Duration) -> u128 {
    let nanos = u128::from(per_second).saturating_mul(elapsed.as_nanos());
    nanos.div_euclid(1_000_000)
}

struct OpenGuard {
    open: Arc<AtomicUsize>,
}

impl OpenGuard {
    fn new(open: Arc<AtomicUsize>) -> Self {
        open.fetch_add(1, Ordering::SeqCst);
        OpenGuard { open }
    }
}

impl Drop for OpenGuard {
    fn drop(&mut self) {
        self.open.fetch_sub(1, Ordering::SeqCst);
    }
}

/// Accepts connections forever. Returns only if `accept` itself fails.
pub fn serve(listener: UnixListener, upstream: String, limits: Limits) -> io::Result<()> {
    let open = Arc::new(AtomicUsize::new(0));
    let mut bucket = TokenBucket::new(limits.burst, limits.per_second, Instant::now());
    loop {
        let (client, _) = listener.accept()?;
        if !bucket.take(Instant::now()) {
            eprintln!("inferproxy: refused (rate limit)");
            drop(client);
            continue;
        }
        if open.load(Ordering::SeqCst) >= limits.max_connections {
            eprintln!("inferproxy: refused (connection limit)");
            drop(client);
            continue;
        }
        let open = Arc::clone(&open);
        let up = upstream.clone();
        thread::spawn(move || handle(client, up, open));
    }
}

fn handle(client: UnixStream, upstream: String, open: Arc<AtomicUsize>) {
    let server = match TcpStream::connect(&upstream) {
        Ok(s) => s,
        Err(_) => return,
    };
    let c2s = {
        let _guard = OpenGuard::new(open);
        match forward(&client, &server) {
            Ok(c2s) => c2s,
            Err(_) => return,
        }
    };
    let _ = client.shutdown(Shutdown::Both);
    let _ = server.shutdown(Shutdown::Both);
    let _ = c2s.join();
}

fn forward(client: &UnixStream, server: &TcpStream) -> io::Result<thread::JoinHandle<()>> {
    let mut c2s_in = client.try_clone()?;
    let mut s2c_out = client.try_clone()?;
    let mut s2c_in = server.try_clone()?;
    let mut c2s_out = server.try_clone()?;
    let c2s = thread::spawn(move || {
        let _ = copy(&mut c2s_in, &mut c2s_out);
        let _ = c2s_out.shutdown(Shutdown::Write);
    });
    let _ = thread::spawn(move || {
        let _ = copy(&mut s2c_in, &mut s2c_out);
    })
    .join();
    Ok(c2s)
}