//! Generic **structured reply ↔ Tauri Channel** bridge infrastructure.
//!
//! Twin of [`crate::pty::PtyBridge`] (ARCHITECTURE §17.7). Where `PtyBridge`
//! routes raw PTY byte chunks to a per-session [`tauri::ipc::Channel`], this
//! bridge routes typed [`ReplyChunk`]s — the serialised
//! [`domain::ports::ReplyEvent`]s of an [`domain::ports::AgentSession`] turn — to
//! the chat cell that owns the session.
//!
//! Design (mirrors the PTY path so the lifecycle guarantees are identical):
//! - The frontend opens (or re-attaches) a chat cell and passes a
//!   [`tauri::ipc::Channel`] for that session. The backend registers it here keyed
//!   by `SessionId`, bumping a monotonic **generation** so a superseded pump can't
//!   tear down the channel of the attach that replaced it (see [`unregister_if`]).
//! - The `agent_send` pump drains the session's [`domain::ports::ReplyStream`],
//!   maps each event to a [`ReplyChunk`], and forwards it via [`send_output`].
//! - Unlike a PTY, an [`domain::ports::AgentSession`] keeps **no** scrollback (the
//!   port is a per-turn stream, not a persistent byte ring). So the bridge itself
//!   retains the rendered chunks per session — the **conversation scrollback** —
//!   so a re-attach can repaint the turns already streamed, exactly as the PTY
//!   adapter's ring buffer lets `reattach_terminal` repaint xterm. This lives on
//!   the transport side (D4 owns transport), keeping the domain port pure.
//!
//! [`unregister_if`]: ChatBridge::unregister_if
//! [`send_output`]: ChatBridge::send_output

use std::collections::HashMap;
use std::sync::Mutex;

use tauri::ipc::Channel;

use domain::ids::SessionId;
use domain::ports::ReplyEvent;

use crate::dto::ReplyChunk;

/// Per-session transport state: the current attach generation, its output
/// channel, and the conversation scrollback recorded so far.
struct ChatEntry {
    /// Monotonic generation of the current (re-)attach (anti double-pump).
    generation: u64,
    /// The output channel of the current attach, if one is registered. `None`
    /// when the session has been recorded (scrollback kept) but no view is
    /// currently attached — the pump then only appends to the scrollback.
    channel: Option<Channel<ReplyChunk>>,
    /// Every chunk routed for this session, in order — the conversation
    /// scrollback replayed on re-attach. Bounded only by the conversation length
    /// (a turn count), like the PTY ring buffer is bounded by its byte capacity.
    scrollback: Vec<ReplyChunk>,
}

/// Registry mapping live structured (chat) sessions to their reply [`Channel`]
/// plus a retained conversation scrollback.
///
/// Thread-safe; a cloned `Arc<ChatBridge>` is held in [`crate::state::AppState`],
/// the twin of [`crate::pty::PtyBridge`].
#[derive(Default)]
pub struct ChatBridge {
    entries: Mutex<HashMap<SessionId, ChatEntry>>,
}

impl ChatBridge {
    /// Creates an empty bridge.
    #[must_use]
    pub fn new() -> Self {
        Self {
            entries: Mutex::new(HashMap::new()),
        }
    }

    /// Registers (or replaces) the reply channel for a session and returns the
    /// **generation** of this registration. Each call for a session bumps the
    /// generation, so the caller's pump can later tear down *only its own*
    /// registration via [`unregister_if`](Self::unregister_if). The retained
    /// conversation scrollback is **preserved** across re-attaches (only the
    /// channel and generation change), mirroring how the PTY ring buffer survives
    /// a `reattach_terminal`.
    pub fn register(&self, session: SessionId, channel: Channel<ReplyChunk>) -> u64 {
        if let Ok(mut map) = self.entries.lock() {
            match map.get_mut(&session) {
                Some(entry) => {
                    entry.generation = entry.generation.wrapping_add(1);
                    entry.channel = Some(channel);
                    entry.generation
                }
                None => {
                    map.insert(
                        session,
                        ChatEntry {
                            generation: 0,
                            channel: Some(channel),
                            scrollback: Vec::new(),
                        },
                    );
                    0
                }
            }
        } else {
            0
        }
    }

    /// Returns the conversation scrollback retained for a session (the chunks
    /// already streamed), or an empty vector if the session is unknown.
    ///
    /// Called by `reattach_agent_chat` to repaint the prior turns into the
    /// re-mounting chat view before the new stream is wired — the typed twin of
    /// `PtyPort::scrollback`.
    #[must_use]
    pub fn scrollback(&self, session: &SessionId) -> Vec<ReplyChunk> {
        self.entries
            .lock()
            .ok()
            .and_then(|m| m.get(session).map(|e| e.scrollback.clone()))
            .unwrap_or_default()
    }

    /// Removes a session's transport state **and** its retained scrollback
    /// unconditionally (chat cell explicitly closed). Twin of
    /// [`PtyBridge::unregister`](crate::pty::PtyBridge::unregister).
    pub fn unregister(&self, session: &SessionId) {
        if let Ok(mut map) = self.entries.lock() {
            map.remove(session);
        }
    }

    /// Detaches a session's channel **only if** `gen` is still the current
    /// generation, leaving the scrollback intact. A pump calls this when its turn
    /// stream ends: if the session was re-attached meanwhile (newer generation),
    /// this is a no-op so the dying pump never detaches the live channel that
    /// superseded it. Twin of
    /// [`PtyBridge::unregister_if`](crate::pty::PtyBridge::unregister_if), but it
    /// keeps the conversation scrollback (the conversation outlives a single
    /// turn's pump — closing the cell is what discards it, via [`unregister`]).
    ///
    /// [`unregister`]: ChatBridge::unregister
    pub fn detach_if(&self, session: &SessionId, gen: u64) {
        if let Ok(mut map) = self.entries.lock() {
            if let Some(entry) = map.get_mut(session) {
                if entry.generation == gen {
                    entry.channel = None;
                }
            }
        }
    }

    /// Records a chunk in the session's scrollback and, if a view is attached at
    /// the current generation, forwards it to that channel.
    ///
    /// Returns `true` if the chunk was delivered to a live channel, `false` if no
    /// channel is currently attached (e.g. the view navigated away — the chunk is
    /// still retained in scrollback for the next re-attach) or the send failed.
    /// The pump keeps draining either way so the turn still completes and the
    /// scrollback stays whole.
    pub fn send_output(&self, session: &SessionId, chunk: ReplyChunk) -> bool {
        let Ok(mut map) = self.entries.lock() else {
            return false;
        };
        let Some(entry) = map.get_mut(session) else {
            return false;
        };
        entry.scrollback.push(chunk.clone());
        match &entry.channel {
            Some(channel) => channel.send(chunk).is_ok(),
            None => false,
        }
    }

    /// Number of currently-tracked sessions (handy for tests/diagnostics).
    #[must_use]
    pub fn active_sessions(&self) -> usize {
        self.entries.lock().map(|m| m.len()).unwrap_or(0)
    }
}

/// Maps a domain [`ReplyEvent`] to its wire [`ReplyChunk`]. Pure translation, no
/// I/O — the single point where the typed turn event becomes a serialisable chunk
/// (kept here so the pump and any test share one mapping).
///
/// Returns `None` for [`ReplyEvent::Heartbeat`]: a heartbeat is a non-terminal
/// liveness proof (readiness/heartbeat lot 1) with **no chat content**, so it maps
/// to no wire chunk — the pump simply skips it. Likewise [`ReplyEvent::RateLimited`]
/// is non-terminal and content-free (ports §21.2-T4): the UI badge comes from the
/// `DomainEvent::AgentRateLimited` bus, not the chat stream, so it maps to `None`
/// too. Every content-bearing event still maps to exactly one chunk.
#[must_use]
pub fn chunk_from_event(event: ReplyEvent) -> Option<ReplyChunk> {
    match event {
        ReplyEvent::TextDelta { text } => Some(ReplyChunk::TextDelta { text }),
        ReplyEvent::ToolActivity { label } => Some(ReplyChunk::ToolActivity { label }),
        ReplyEvent::Final { content } => Some(ReplyChunk::Final { content }),
        ReplyEvent::Announcement { .. } => None,
        ReplyEvent::Heartbeat => None,
        ReplyEvent::RateLimited { .. } => None,
    }
}