//! Filesystem watcher for `.story_kit/work/`.
//!
//! Watches the work pipeline directories for file changes, infers the lifecycle
//! stage from the target directory name, auto-commits with a deterministic message,
//! and broadcasts a [`WatcherEvent`] to all connected WebSocket clients.
//!
//! # Debouncing
//! Events are buffered for 300 ms after the last activity. All changes within the
//! window are batched into a single `git add + commit`. This avoids double-commits
//! when `fs::rename` fires both a remove and a create event.
//!
//! # Race conditions
//! If a mutation handler (e.g. `move_story_to_current`) already committed the
//! change, `git commit` will return "nothing to commit". The watcher detects this
//! via exit-code inspection and silently skips the commit while still broadcasting
//! the event so connected clients stay in sync.

use crate::slog;
use notify::{EventKind, RecommendedWatcher, RecursiveMode, Watcher, recommended_watcher};
use serde::Serialize;
use std::collections::HashMap;
use std::path::{Path, PathBuf};
use std::sync::mpsc;
use std::time::{Duration, Instant};
use tokio::sync::broadcast;

/// A lifecycle event emitted by the filesystem watcher after auto-committing.
#[derive(Clone, Debug, Serialize)]
pub struct WatcherEvent {
    /// Pipeline stage directory (e.g. `"2_current"`, `"5_archived"`).
    pub stage: String,
    /// Work item ID (filename stem without extension, e.g. `"42_story_my_feature"`).
    pub item_id: String,
    /// Semantic action inferred from the stage (e.g. `"start"`, `"accept"`).
    pub action: String,
    /// The deterministic git commit message used (or that would have been used).
    pub commit_msg: String,
}

/// Map a pipeline directory name to a (action, commit-message-prefix) pair.
fn stage_metadata(stage: &str, item_id: &str) -> Option<(&'static str, String)> {
    let (action, prefix) = match stage {
        "1_upcoming" => ("create", format!("story-kit: create {item_id}")),
        "2_current"  => ("start",  format!("story-kit: start {item_id}")),
        "3_qa"       => ("qa",     format!("story-kit: queue {item_id} for QA")),
        "4_merge"    => ("merge",  format!("story-kit: queue {item_id} for merge")),
        "5_archived" => ("accept", format!("story-kit: accept {item_id}")),
        _ => return None,
    };
    Some((action, prefix))
}

/// Return the pipeline stage name for a path if it is a `.md` file living
/// directly inside one of the known work subdirectories, otherwise `None`.
///
/// Explicitly returns `None` for any path under `.story_kit/worktrees/` so
/// that code changes made by agents in their isolated worktrees are never
/// auto-committed to master by the watcher.
fn stage_for_path(path: &Path) -> Option<String> {
    // Reject any path that passes through the worktrees directory.
    if path
        .components()
        .any(|c| c.as_os_str() == "worktrees")
    {
        return None;
    }

    if path.extension().is_none_or(|e| e != "md") {
        return None;
    }
    let stage = path
        .parent()
        .and_then(|p| p.file_name())
        .and_then(|n| n.to_str())?;
    matches!(stage, "1_upcoming" | "2_current" | "3_qa" | "4_merge" | "5_archived")
        .then(|| stage.to_string())
}

/// Stage all changes in the work directory and commit with the given message.
///
/// Uses `git add -A .story_kit/work/` to catch both additions and deletions in
/// a single commit. Returns `Ok(true)` if a commit was made, `Ok(false)` if
/// there was nothing to commit, and `Err` for unexpected failures.
fn git_add_work_and_commit(git_root: &Path, message: &str) -> Result<bool, String> {
    let work_rel = PathBuf::from(".story_kit").join("work");

    let add_out = std::process::Command::new("git")
        .args(["add", "-A"])
        .arg(&work_rel)
        .current_dir(git_root)
        .output()
        .map_err(|e| format!("git add: {e}"))?;
    if !add_out.status.success() {
        return Err(format!(
            "git add failed: {}",
            String::from_utf8_lossy(&add_out.stderr)
        ));
    }

    let commit_out = std::process::Command::new("git")
        .args(["commit", "-m", message])
        .current_dir(git_root)
        .output()
        .map_err(|e| format!("git commit: {e}"))?;

    if commit_out.status.success() {
        return Ok(true);
    }

    let stderr = String::from_utf8_lossy(&commit_out.stderr);
    let stdout = String::from_utf8_lossy(&commit_out.stdout);
    if stdout.contains("nothing to commit") || stderr.contains("nothing to commit") {
        return Ok(false);
    }

    Err(format!("git commit failed: {stderr}"))
}

/// Process a batch of pending (path → stage) entries: commit and broadcast.
///
/// Only files that still exist on disk are used to derive the commit message
/// (they represent the destination of a move or a new file). Deletions are
/// captured by `git add -A .story_kit/work/` automatically.
fn flush_pending(
    pending: &HashMap<PathBuf, String>,
    git_root: &Path,
    event_tx: &broadcast::Sender<WatcherEvent>,
) {
    // Separate into files that exist (additions) vs gone (deletions).
    let mut additions: Vec<(&PathBuf, &str)> = Vec::new();
    for (path, stage) in pending {
        if path.exists() {
            additions.push((path, stage.as_str()));
        }
    }

    // Pick the commit message from the first addition (the meaningful side of a move).
    // If there are only deletions, use a generic message.
    let (action, item_id, commit_msg) = if let Some((path, stage)) = additions.first() {
        let item = path.file_stem().and_then(|s| s.to_str()).unwrap_or("unknown");
        if let Some((act, msg)) = stage_metadata(stage, item) {
            (act, item.to_string(), msg)
        } else {
            return;
        }
    } else {
        // Only deletions — pick any pending path for the item name.
        let Some((path, _)) = pending.iter().next() else {
            return;
        };
        let item = path.file_stem().and_then(|s| s.to_str()).unwrap_or("unknown");
        ("remove", item.to_string(), format!("story-kit: remove {item}"))
    };

    slog!("[watcher] flush: {commit_msg}");
    match git_add_work_and_commit(git_root, &commit_msg) {
        Ok(committed) => {
            if committed {
                slog!("[watcher] committed: {commit_msg}");
            } else {
                slog!("[watcher] skipped (already committed): {commit_msg}");
            }
            let stage = additions.first().map_or("unknown", |(_, s)| s);
            let evt = WatcherEvent {
                stage: stage.to_string(),
                item_id,
                action: action.to_string(),
                commit_msg,
            };
            let _ = event_tx.send(evt);
        }
        Err(e) => {
            slog!("[watcher] git error: {e}");
        }
    }
}

/// Start the filesystem watcher on a dedicated OS thread.
///
/// `work_dir`  — absolute path to `.story_kit/work/` (watched recursively).
/// `git_root`  — project root (passed to `git` commands as cwd).
/// `event_tx`  — broadcast sender; each connected WebSocket client holds a receiver.
pub fn start_watcher(
    work_dir: PathBuf,
    git_root: PathBuf,
    event_tx: broadcast::Sender<WatcherEvent>,
) {
    std::thread::spawn(move || {
        let (notify_tx, notify_rx) = mpsc::channel::<notify::Result<notify::Event>>();

        let mut watcher: RecommendedWatcher = match recommended_watcher(move |res| {
            let _ = notify_tx.send(res);
        }) {
            Ok(w) => w,
            Err(e) => {
                slog!("[watcher] failed to create watcher: {e}");
                return;
            }
        };

        if let Err(e) = watcher.watch(&work_dir, RecursiveMode::Recursive) {
            slog!("[watcher] failed to watch {}: {e}", work_dir.display());
            return;
        }

        slog!("[watcher] watching {}", work_dir.display());

        const DEBOUNCE: Duration = Duration::from_millis(300);

        // Map path → stage for pending (uncommitted) changes.
        let mut pending: HashMap<PathBuf, String> = HashMap::new();
        let mut deadline: Option<Instant> = None;

        loop {
            // How long until the debounce window closes (or wait for next event).
            let timeout = deadline.map_or(Duration::from_secs(60), |d| {
                d.saturating_duration_since(Instant::now())
            });

            let flush = match notify_rx.recv_timeout(timeout) {
                Ok(Ok(event)) => {
                    // Track creates, modifies, AND removes. Removes are needed so
                    // that standalone deletions trigger a flush, and so that moves
                    // (which fire Remove + Create) land in the same debounce window.
                    let is_relevant_kind = matches!(
                        event.kind,
                        EventKind::Create(_) | EventKind::Modify(_) | EventKind::Remove(_)
                    );

                    if is_relevant_kind {
                        for path in event.paths {
                            if let Some(stage) = stage_for_path(&path) {
                                pending.insert(path, stage);
                                deadline = Some(Instant::now() + DEBOUNCE);
                            }
                        }
                    }
                    false
                }
                Ok(Err(e)) => {
                    slog!("[watcher] notify error: {e}");
                    false
                }
                // Debounce window expired — time to flush.
                Err(mpsc::RecvTimeoutError::Timeout) => true,
                Err(mpsc::RecvTimeoutError::Disconnected) => {
                    slog!("[watcher] channel disconnected, shutting down");
                    break;
                }
            };

            if flush && !pending.is_empty() {
                flush_pending(&pending, &git_root, &event_tx);
                pending.clear();
                deadline = None;
            }
        }
    });
}

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

    #[test]
    fn stage_for_path_recognises_pipeline_dirs() {
        let base = PathBuf::from("/proj/.story_kit/work");
        assert_eq!(
            stage_for_path(&base.join("2_current/42_story_foo.md")),
            Some("2_current".to_string())
        );
        assert_eq!(
            stage_for_path(&base.join("5_archived/10_bug_bar.md")),
            Some("5_archived".to_string())
        );
        assert_eq!(stage_for_path(&base.join("other/file.md")), None);
        assert_eq!(
            stage_for_path(&base.join("2_current/42_story_foo.txt")),
            None
        );
    }

    #[test]
    fn stage_for_path_ignores_worktree_paths() {
        let worktrees = PathBuf::from("/proj/.story_kit/worktrees");

        // Code changes inside a worktree must be ignored.
        assert_eq!(
            stage_for_path(&worktrees.join("42_story_foo/server/src/main.rs")),
            None,
        );

        // Even if a worktree happens to contain a path component that looks
        // like a pipeline stage, it must still be ignored.
        assert_eq!(
            stage_for_path(&worktrees.join("42_story_foo/.story_kit/work/2_current/42_story_foo.md")),
            None,
        );

        // A path that only contains the word "worktrees" as part of a longer
        // segment (not an exact component) must NOT be filtered out.
        assert_eq!(
            stage_for_path(&PathBuf::from("/proj/.story_kit/work/2_current/not_worktrees_story.md")),
            Some("2_current".to_string()),
        );
    }

    #[test]
    fn stage_metadata_returns_correct_actions() {
        let (action, msg) = stage_metadata("2_current", "42_story_foo").unwrap();
        assert_eq!(action, "start");
        assert_eq!(msg, "story-kit: start 42_story_foo");

        let (action, msg) = stage_metadata("5_archived", "42_story_foo").unwrap();
        assert_eq!(action, "accept");
        assert_eq!(msg, "story-kit: accept 42_story_foo");

        assert!(stage_metadata("unknown", "id").is_none());
    }
}