//! Server event draining: applies incoming [`UIEvent`]s to the TUI state.

use tokio::sync::mpsc;
use tracing::debug;

use super::{AppState, DisplayMessage};
use crate::core::types::{Role, StampedEvent, UIEvent};
use crate::tui::tool_display;

/// Drain all pending [`UIEvent`]s from `event_rx` and apply them to `state`.
///
/// This is non-blocking: it processes all currently-available events and returns
/// immediately when the channel is empty.
///
/// Tool events use in-place replacement: when a `ToolExecuting` or `ToolResult`
/// arrives, the handler searches `state.messages` for an existing entry with the
/// same `tool_use_id` and replaces its content rather than appending a new row.
///
/// | Event             | Effect                                                     |
/// |-------------------|------------------------------------------------------------|
/// | `StreamDelta(s)`  | Append `s` to last message if it's `Assistant`; else push  |
/// | `ToolExecuting`   | Push new executing message (or replace in-place)           |
/// | `ToolResult`      | Replace executing message in-place (or push new)           |
/// | `TurnComplete`    | No structural change; logged at debug level                |
/// | `Error(msg)`      | Push `(Assistant, "[error] {msg}")`                        |
pub(super) fn drain_ui_events(event_rx: &mut mpsc::Receiver<StampedEvent>, state: &mut AppState) {
    while let Ok(stamped) = event_rx.try_recv() {
        // Discard events from before the most recent :clear.
        if stamped.epoch < state.epoch {
            debug!(
                event_epoch = stamped.epoch,
                state_epoch = state.epoch,
                "dropping stale event"
            );
            continue;
        }
        match stamped.event {
            UIEvent::StreamDelta(chunk) => {
                if let Some(msg) = state.messages.last_mut() {
                    if msg.role == Role::Assistant && msg.tool_use_id.is_none() {
                        msg.content.push_str(&chunk);
                    } else {
                        state.messages.push(DisplayMessage {
                            role: Role::Assistant,
                            content: chunk,
                            tool_use_id: None,
                        });
                    }
                } else {
                    state.messages.push(DisplayMessage {
                        role: Role::Assistant,
                        content: chunk,
                        tool_use_id: None,
                    });
                }
                state.content_changed = true;
            }
            UIEvent::ToolExecuting {
                tool_use_id,
                tool_name,
                display,
            } => {
                let content = tool_display::format_executing(&tool_name, &display);
                replace_or_push(state, &tool_use_id, content);
                state.content_changed = true;
            }
            UIEvent::ToolResult {
                tool_use_id,
                tool_name,
                display,
                is_error,
            } => {
                let content = tool_display::format_result(&tool_name, &display, is_error);
                replace_or_push(state, &tool_use_id, content);
                state.content_changed = true;
            }
            UIEvent::TurnComplete => {
                debug!("turn complete");
            }
            UIEvent::NetworkPolicyChanged(allowed) => {
                state.network_allowed = allowed;
            }
            UIEvent::Error(msg) => {
                state.messages.push(DisplayMessage {
                    role: Role::Assistant,
                    content: format!("[error] {msg}"),
                    tool_use_id: None,
                });
                state.content_changed = true;
            }
        }
    }
}

/// Find the message with the given `tool_use_id` and replace its content,
/// or push a new message if not found.
fn replace_or_push(state: &mut AppState, tool_use_id: &str, content: String) {
    if let Some(msg) = state
        .messages
        .iter_mut()
        .rev()
        .find(|m| m.tool_use_id.as_deref() == Some(tool_use_id))
    {
        msg.content = content;
    } else {
        state.messages.push(DisplayMessage {
            role: Role::Assistant,
            content,
            tool_use_id: Some(tool_use_id.to_string()),
        });
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::core::types::ToolDisplay;

    /// Wrap a [`UIEvent`] in a [`StampedEvent`] at epoch 0 for tests.
    fn stamp(event: UIEvent) -> StampedEvent {
        StampedEvent { epoch: 0, event }
    }

    #[tokio::test]
    async fn drain_appends_to_existing_assistant() {
        let (tx, mut rx) = tokio::sync::mpsc::channel(8);
        let mut state = AppState::new();
        state.messages.push(DisplayMessage {
            role: Role::Assistant,
            content: "hello".to_string(),
            tool_use_id: None,
        });
        tx.send(stamp(UIEvent::StreamDelta(" world".to_string())))
            .await
            .unwrap();
        drop(tx);
        drain_ui_events(&mut rx, &mut state);
        assert_eq!(state.messages.last().unwrap().content, "hello world");
    }

    #[tokio::test]
    async fn drain_creates_assistant_on_user_last() {
        let (tx, mut rx) = tokio::sync::mpsc::channel(8);
        let mut state = AppState::new();
        state.messages.push(DisplayMessage {
            role: Role::User,
            content: "hi".to_string(),
            tool_use_id: None,
        });
        tx.send(stamp(UIEvent::StreamDelta("hello".to_string())))
            .await
            .unwrap();
        drop(tx);
        drain_ui_events(&mut rx, &mut state);
        assert_eq!(state.messages.len(), 2);
        assert_eq!(state.messages[1].role, Role::Assistant);
        assert_eq!(state.messages[1].content, "hello");
    }

    #[tokio::test]
    async fn drain_tool_result_replaces_existing_message() {
        let (tx, mut rx) = tokio::sync::mpsc::channel(8);
        let mut state = AppState::new();
        // Simulate an existing executing message.
        state.messages.push(DisplayMessage {
            role: Role::Assistant,
            content: "$ cargo test".to_string(),
            tool_use_id: Some("t1".to_string()),
        });
        tx.send(stamp(UIEvent::ToolResult {
            tool_use_id: "t1".to_string(),
            tool_name: "shell_exec".to_string(),
            display: ToolDisplay::ShellExec {
                command: "cargo test\nok".to_string(),
            },
            is_error: false,
        }))
        .await
        .unwrap();
        drop(tx);
        drain_ui_events(&mut rx, &mut state);
        // Should replace, not append.
        assert_eq!(state.messages.len(), 1);
        assert!(state.messages[0].content.contains("cargo test"));
    }

    #[tokio::test]
    async fn drain_discards_stale_epoch_events() {
        let (tx, mut rx) = tokio::sync::mpsc::channel(8);
        let mut state = AppState::new();
        state.epoch = 2;
        tx.send(StampedEvent {
            epoch: 1,
            event: UIEvent::StreamDelta("ghost".to_string()),
        })
        .await
        .unwrap();
        tx.send(StampedEvent {
            epoch: 2,
            event: UIEvent::StreamDelta("real".to_string()),
        })
        .await
        .unwrap();
        drop(tx);
        drain_ui_events(&mut rx, &mut state);
        assert_eq!(state.messages.len(), 1);
        assert_eq!(state.messages[0].content, "real");
    }

    #[tokio::test]
    async fn drain_sets_content_changed() {
        let (tx, mut rx) = tokio::sync::mpsc::channel(8);
        let mut state = AppState::new();
        assert!(!state.content_changed);
        tx.send(stamp(UIEvent::StreamDelta("hi".to_string())))
            .await
            .unwrap();
        drop(tx);
        drain_ui_events(&mut rx, &mut state);
        assert!(state.content_changed);
    }
}