//! Machine-readable event stream for non-interactive front-ends.
//!
//! When enabled with `--output json`, the host and viewer emit one JSON
//! object per line on **stdout**. The human banner and `tracing` logs stay
//! on **stderr**, so the two streams never interleave and a parser reading
//! stdout sees only events. Each line is flushed immediately so a front-end
//! reading the pipe gets events live rather than in block-buffered chunks.
//!
//! This is the shell-out counterpart to an in-process event channel: the
//! `--gui` front-end re-execs this binary as `pixelpass --host --output json`
//! and parses these lines to drive its window.

use std::io::Write;
use std::sync::atomic::{AtomicBool, Ordering};

use serde::Serialize;

static JSON_ENABLED: AtomicBool = AtomicBool::new(false);

/// Turn JSON event output on. Called once at startup from `--output json`.
pub fn set_json(enabled: bool) {
    JSON_ENABLED.store(enabled, Ordering::Relaxed);
}

/// Whether the JSON event stream is on — i.e. we're being driven by a
/// machine front-end (the `--gui` shell-out) rather than a human terminal.
/// Gates features that only make sense under that front-end, like the
/// stdin command channel the host reads `kick` requests from.
pub fn json_enabled() -> bool {
    JSON_ENABLED.load(Ordering::Relaxed)
}

/// One event in the stdout stream. Serialized as `{"event":"<tag>", ...}`.
#[derive(Serialize)]
#[serde(tag = "event", rename_all = "snake_case")]
pub enum Event<'a> {
    /// The relay-only ticket the viewer needs. Emitted once at host startup.
    Ticket { value: &'a str },
    /// One-shot host configuration summary, mirroring the banner fields.
    HostInfo {
        display_server: &'a str,
        capture: &'a str,
        quality: &'a str,
        dimensions: &'a str,
        hw_encode: bool,
        max_viewers: u32,
        max_viewers_source: &'a str,
    },
    /// A viewer joined. `id` is the viewer's endpoint id; `active` is the new
    /// total after the join.
    ViewerJoined { id: &'a str, active: u32, max: u32 },
    /// A viewer left — disconnected on their own or kicked by the host. `id`
    /// is the viewer's endpoint id; `active` is the new total after.
    ViewerLeft { id: &'a str, active: u32, max: u32 },
    /// Capture pipeline lifecycle (spawned on first viewer, torn down on last).
    Capture { state: CaptureState },
    /// A viewer was turned away (host full, or capture spawn failed).
    ViewerRefused { reason: &'a str },
    /// Viewer-side: the local player URL is ready to open.
    Connected { url: &'a str },
    /// Per-app audio routing state (only emitted when `--app` is set). `routed`
    /// = the chosen app's audio is now reaching viewers; `lost` = its last
    /// stream went away. Under `--strict-audio`, `lost` means viewers currently
    /// hear silence; without it, viewers fall back to whole-desktop audio.
    AppAudio { state: AppAudioState },
}

#[derive(Serialize)]
#[serde(rename_all = "snake_case")]
pub enum CaptureState {
    Started,
    Stopped,
}

#[derive(Serialize, Clone, Copy, Debug, PartialEq, Eq)]
#[serde(rename_all = "snake_case")]
pub enum AppAudioState {
    Routed,
    Lost,
}

/// Emit one event as a JSON line on stdout, flushed. No-op unless JSON
/// output was enabled with [`set_json`], so call sites can sprinkle these
/// unconditionally without branching.
pub fn emit(event: Event) {
    if !json_enabled() {
        return;
    }
    match serde_json::to_string(&event) {
        Ok(line) => {
            let mut out = std::io::stdout().lock();
            // Best-effort: a closed pipe (front-end gone) shouldn't crash the
            // host — it keeps streaming to any viewers already connected.
            let _ = writeln!(out, "{line}");
            let _ = out.flush();
        }
        Err(e) => tracing::warn!("failed to serialize event: {e}"),
    }
}

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

    // The app_audio event is the wire contract peerspeak parses to drive its
    // echo warning; pin the exact shape so a rename here is caught here.
    #[test]
    fn app_audio_event_wire_shape() {
        let routed = serde_json::to_string(&Event::AppAudio {
            state: AppAudioState::Routed,
        })
        .unwrap();
        assert_eq!(routed, r#"{"event":"app_audio","state":"routed"}"#);

        let lost = serde_json::to_string(&Event::AppAudio {
            state: AppAudioState::Lost,
        })
        .unwrap();
        assert_eq!(lost, r#"{"event":"app_audio","state":"lost"}"#);
    }
}