//! Loopback JSON-lines control server: one request per line, one reply per line. Agents and
//! tests drive the app through it; the UI thread answers each request between frames
//! (`fieldcraft_ui_egui::control`).
//!
//! Adapted from SolidCraft's `apps/solidcraft/src/control_server.rs` (MIT OR Apache-2.0), see
//! ATTRIBUTION.md.

use std::io::{BufRead, BufReader, Write};
use std::net::{TcpListener, TcpStream};
use std::sync::atomic::{AtomicUsize, Ordering};
use std::sync::mpsc::{Receiver, Sender, channel};
use std::time::Duration;

use fieldcraft_ui_egui::ControlRequest;
use serde_json::{Value, json};

/// Longest request line (bytes).
const MAX_LINE: usize = 16 * 1024 * 1024;
/// Connections served at once. The channel has no authentication beyond being bound to
/// loopback, so a flood must not be able to spawn threads without end.
const MAX_CONNECTIONS: usize = 16;
/// Lines that are not JSON before the connection is closed, so garbage cannot loop forever.
const MAX_BAD_LINES: usize = 8;
/// How long a request waits for the UI thread.
const REPLY_TIMEOUT: Duration = Duration::from_secs(60);

static OPEN: AtomicUsize = AtomicUsize::new(0);

pub fn start(port: u16, ctx: egui::Context) -> Receiver<ControlRequest> {
    let (tx, rx) = channel::<ControlRequest>();
    let listener = match TcpListener::bind(("127.0.0.1", port)) {
        Ok(l) => l,
        Err(e) => {
            eprintln!("fieldcraft: control server failed to bind 127.0.0.1:{port}: {e}");
            return rx;
        }
    };
    eprintln!("fieldcraft: control server listening on 127.0.0.1:{port}");
    std::thread::spawn(move || {
        for stream in listener.incoming().flatten() {
            if OPEN.fetch_add(1, Ordering::Relaxed) >= MAX_CONNECTIONS {
                OPEN.fetch_sub(1, Ordering::Relaxed);
                let mut refused = stream;
                let _ = writeln!(refused, "{}", json!({"ok": false, "error": "too many connections"}));
                continue;
            }
            let tx = tx.clone();
            let ctx = ctx.clone();
            std::thread::spawn(move || {
                serve(stream, tx, ctx);
                OPEN.fetch_sub(1, Ordering::Relaxed);
            });
        }
    });
    rx
}

/// One line without its newline, or `None` at the end of the stream. A client that sends
/// gigabytes with no newline would otherwise make this thread allocate until the process is
/// killed, so the line is refused as soon as it passes `max`.
fn read_line(r: &mut impl BufRead, max: usize) -> std::io::Result<Option<Vec<u8>>> {
    let mut line = Vec::new();
    loop {
        let (done, used) = {
            let available = r.fill_buf()?;
            if available.is_empty() {
                return Ok((!line.is_empty()).then_some(line));
            }
            match available.iter().position(|b| *b == b'\n') {
                Some(i) => {
                    line.extend_from_slice(available.get(..i).unwrap_or(&[]));
                    (true, i + 1)
                }
                None => {
                    line.extend_from_slice(available);
                    (false, available.len())
                }
            }
        };
        r.consume(used);
        if line.len() > max {
            return Err(std::io::Error::new(std::io::ErrorKind::InvalidData, "request too large"));
        }
        if done {
            return Ok(Some(line));
        }
    }
}

fn serve(stream: TcpStream, tx: Sender<ControlRequest>, ctx: egui::Context) {
    let Ok(read) = stream.try_clone() else { return };
    let mut out = stream;
    let mut reader = BufReader::new(read);
    let mut bad = 0usize;
    loop {
        let line = match read_line(&mut reader, MAX_LINE) {
            Ok(Some(l)) => String::from_utf8_lossy(&l).into_owned(),
            Ok(None) => break,
            Err(e) => {
                let _ = writeln!(out, "{}", json!({"ok": false, "error": e.to_string()}));
                break;
            }
        };
        if line.trim().is_empty() {
            continue;
        }
        let reply = match serde_json::from_str::<Value>(&line) {
            Ok(msg) => {
                let id = msg.get("id").cloned().unwrap_or(Value::Null);
                let method = msg.get("method").and_then(Value::as_str).unwrap_or("").to_string();
                let params = msg.get("params").cloned().unwrap_or(json!({}));
                let (req, rrx) = ControlRequest::new(method, params);
                if tx.send(req).is_err() {
                    break;
                }
                ctx.request_repaint();
                let mut r = rrx.recv_timeout(REPLY_TIMEOUT).unwrap_or_else(|_| json!({"ok": false, "error": "timeout"}));
                // A request waiting for a job (a solve, a mesher): its reply comes when the job
                // ends, however long it runs; the job always answers, even when cancelled.
                if r.get(fieldcraft_ui_egui::control::PENDING).is_some() {
                    r = rrx.recv().unwrap_or_else(|_| json!({"ok": false, "error": "the job ended without a reply"}));
                }
                if let Some(o) = r.as_object_mut() {
                    o.insert("id".into(), id);
                }
                r
            }
            Err(e) => {
                bad += 1;
                json!({"ok": false, "error": format!("bad JSON: {e}")})
            }
        };
        if writeln!(out, "{reply}").is_err() || bad >= MAX_BAD_LINES {
            break;
        }
    }
}

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

    /// A request longer than the cap is refused instead of allocated.
    #[test]
    fn lines_past_the_cap_are_refused() {
        let flood = vec![b'A'; MAX_LINE + 2];
        let mut r = BufReader::new(flood.as_slice());
        let e = read_line(&mut r, MAX_LINE).unwrap_err();
        assert_eq!(e.kind(), std::io::ErrorKind::InvalidData);
    }

    /// Lines split on newlines, blank lines kept, no trailing newline needed; a line spanning
    /// several reads arrives whole.
    #[test]
    fn lines_split_on_newlines() {
        let text = b"{\"a\":1}\n\n{\"b\":2}\nlast".to_vec();
        let mut r = BufReader::with_capacity(3, text.as_slice());
        let mut got = Vec::new();
        while let Some(line) = read_line(&mut r, MAX_LINE).unwrap() {
            got.push(String::from_utf8(line).unwrap());
        }
        assert_eq!(got, vec!["{\"a\":1}", "", "{\"b\":2}", "last"]);
        let mut r = BufReader::with_capacity(4, b"0123456789\n".as_slice());
        assert!(read_line(&mut r, 5).is_err());
    }
}