//! SSE endpoint for watching a chat conversation in real-time via NATS.
//!
//! Unlike the primary SSE stream (which triggers AI execution), this endpoint
//! passively subscribes to NATS Core subjects and forwards chat messages and
//! stream chunks to connected clients. This enables multiple viewers to watch
//! the same AI conversation in real-time.

use actix_web::{web, HttpResponse, Result};
use futures::StreamExt;
use service::AppService;
use std::pin::Pin;
use uuid::Uuid;

use crate::error::ApiError;

/// SSE endpoint for watching a chat conversation.
///
/// `GET /api/ai/conversations/{conversation_id}/watch`
///
/// Subscribes to NATS Core subjects (`chat.chunk.{id}` and `chat.message.{id}`)
/// and forwards received events as SSE to the connected client.
///
/// SSE events:
/// - `chunk` — a stream chunk (thinking, token, tool_call, tool_result, done, error)
/// - `message` — a complete chat message
/// - `error` — an error event
pub fn create_watch_sse_stream(
    service: AppService,
    conversation_id: Uuid,
) -> Pin<Box<dyn futures::Stream<Item = Result<actix_web::web::Bytes, actix_web::Error>> + Send>> {
    let (tx, rx) = tokio::sync::mpsc::channel::<String>(200);

    tokio::spawn(async move {
        let nats = match &service.queue_producer.nats {
            Some(n) => n.clone(),
            None => {
                let _ = tx.send(format!(
                    "data: {{\"event\":\"error\",\"data\":{}}}\n\n",
                    serde_json::to_string("NATS not available").unwrap_or_default()
                )).await;
                return;
            }
        };

        // Subscribe to chat chunks
        let chunk_subject = format!("chat.chunk.{}", conversation_id);
        let mut chunk_sub = match nats.subscribe(&chunk_subject).await {
            Ok(s) => s,
            Err(e) => {
                let _ = tx.send(format!(
                    "data: {{\"event\":\"error\",\"data\":{}}}\n\n",
                    serde_json::to_string(&e.to_string()).unwrap_or_default()
                )).await;
                return;
            }
        };

        // Subscribe to chat messages
        let msg_subject = format!("chat.message.{}", conversation_id);
        let mut msg_sub = match nats.subscribe(&msg_subject).await {
            Ok(s) => s,
            Err(e) => {
                let _ = tx.send(format!(
                    "data: {{\"event\":\"error\",\"data\":{}}}\n\n",
                    serde_json::to_string(&e.to_string()).unwrap_or_default()
                )).await;
                return;
            }
        };

        let _ = tx.send(":ok\n\n".to_string()).await;

        loop {
            tokio::select! {
                chunk_msg = chunk_sub.next() => {
                    match chunk_msg {
                        Some(msg) => {
                            let payload = String::from_utf8_lossy(&msg.payload);
                            // Parse to get chunk_type for the event field
                            let event_type = if let Ok(parsed) = serde_json::from_str::<serde_json::Value>(&payload) {
                                parsed.get("chunk_type")
                                    .and_then(|v| v.as_str())
                                    .unwrap_or("chunk")
                                    .to_string()
                            } else {
                                "chunk".to_string()
                            };
                            let sse = format!(
                                "data: {{\"event\":\"{}\",\"data\":{}}}\n\n",
                                event_type, payload
                            );
                            if tx.send(sse).await.is_err() {
                                break;
                            }
                        }
                        None => break,
                    }
                }
                msg = msg_sub.next() => {
                    match msg {
                        Some(msg) => {
                            let payload = String::from_utf8_lossy(&msg.payload);
                            let sse = format!(
                                "data: {{\"event\":\"message\",\"data\":{}}}\n\n",
                                payload
                            );
                            if tx.send(sse).await.is_err() {
                                break;
                            }
                        }
                        None => break,
                    }
                }
            }
        }
    });

    Box::pin(tokio_stream::wrappers::ReceiverStream::new(rx).map(|s| {
        Ok(actix_web::web::Bytes::from(s))
    }))
}

#[utoipa::path(
    get,
    path = "/api/ai/conversations/{conversation_id}/watch",
    params(
        ("conversation_id" = Uuid, Path, description = "Conversation ID"),
    ),
    responses(
        (status = 200, description = "SSE stream of conversation events"),
        (status = 404, description = "Not found"),
    ),
    tag = "AI Chat"
)]
pub async fn conversation_watch(
    service: web::Data<AppService>,
    session: session::Session,
    path: web::Path<Uuid>,
) -> Result<HttpResponse, ApiError> {
    let user_id = session.user().ok_or_else(|| ApiError::from(service::error::AppError::Unauthorized))?;
    let conversation_id = path.into_inner();

    // Verify access (view-only is sufficient)
    let _conv = service
        .find_conversation_owned(conversation_id, user_id)
        .await?;

    let response = HttpResponse::Ok()
        .content_type("text/event-stream")
        .insert_header(("Cache-Control", "no-cache"))
        .insert_header(("X-Accel-Buffering", "no"))
        .streaming(create_watch_sse_stream(
            service.get_ref().clone(),
            conversation_id,
        ));

    Ok(response.into())
}