//! Video library management: folder scanning, progress tracking, playlists,
//! subtitle integration, and background duration scanning.

use serde::{Deserialize, Serialize};
use serde_json::{json, Value};
use std::collections::HashMap;
use std::fs;
use std::path::{Path, PathBuf};
use std::sync::atomic::{AtomicBool, Ordering};
use std::sync::Arc;
use std::time::SystemTime;

use crate::ffmpeg::{self, VideoMetadata};
use crate::state::{atomic_write_json, load_json_with_fallbacks, BACKUP_COUNT};
use crate::subtitles;
use crate::utils::{
    clamp, compute_library_id, file_fingerprint, is_within_root, natural_key,
    pretty_title_from_filename,
};

// ---------------------------------------------------------------------------
// Constants
// ---------------------------------------------------------------------------

/// Supported video file extensions (lower-case, with leading dot).
pub const VIDEO_EXTS: &[&str] = &[
    ".mp4", ".m4v", ".mov", ".webm", ".mkv", ".avi", ".mpg", ".mpeg", ".m2ts", ".mts", ".ogv",
];

/// Current state file format version.
const STATE_VERSION: u32 = 1;

// ---------------------------------------------------------------------------
// Structs
// ---------------------------------------------------------------------------

/// Reference to a stored subtitle file for a video.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SubtitleRef {
    pub vtt: String,
    pub label: String,
}

/// Per-video metadata: position, progress, notes, subtitle reference.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct VideoMeta {
    pub pos: f64,
    pub watched: f64,
    pub duration: Option<f64>,
    pub finished: bool,
    pub note: String,
    pub last_open: u64,
    pub subtitle: Option<SubtitleRef>,
}

impl Default for VideoMeta {
    fn default() -> Self {
        Self {
            pos: 0.0,
            watched: 0.0,
            duration: None,
            finished: false,
            note: String::new(),
            last_open: 0,
            subtitle: None,
        }
    }
}

/// Serializable library state persisted to disk.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct LibraryState {
    pub version: u32,
    pub library_id: String,
    pub last_path: String,
    pub updated_at: u64,
    pub current_fid: Option<String>,
    pub current_time: f64,
    pub volume: f64,
    pub autoplay: bool,
    pub playback_rate: f64,
    pub order_fids: Vec<String>,
    pub videos: HashMap<String, VideoMeta>,
}

impl Default for LibraryState {
    fn default() -> Self {
        Self {
            version: STATE_VERSION,
            library_id: String::new(),
            last_path: String::new(),
            updated_at: 0,
            current_fid: None,
            current_time: 0.0,
            volume: 1.0,
            autoplay: false,
            playback_rate: 1.0,
            order_fids: Vec::new(),
            videos: HashMap::new(),
        }
    }
}

/// In-memory representation of the video library.
pub struct Library {
    pub root: Option<PathBuf>,
    pub files: Vec<PathBuf>,
    pub fids: Vec<String>,
    pub relpaths: Vec<String>,
    pub rel_to_fid: HashMap<String, String>,
    pub fid_to_rel: HashMap<String, String>,
    pub state: LibraryState,
    pub state_path: Option<PathBuf>,
    // ffmpeg paths
    pub ffprobe: Option<PathBuf>,
    pub ffmpeg: Option<PathBuf>,
    // metadata cache
    pub meta_cache: HashMap<String, VideoMetadata>,
    // background scan control
    scan_stop: Arc<AtomicBool>,
}

// ---------------------------------------------------------------------------
// Helper functions (private)
// ---------------------------------------------------------------------------

/// Return the current unix timestamp in seconds.
fn now_secs() -> u64 {
    SystemTime::now()
        .duration_since(SystemTime::UNIX_EPOCH)
        .map(|d| d.as_secs())
        .unwrap_or(0)
}

/// Natural-sort relative paths and return the corresponding fid list.
fn _sorted_default(relpaths: &[String], rel_to_fid: &HashMap<String, String>) -> Vec<String> {
    let mut sorted_rels: Vec<String> = relpaths.to_vec();
    sorted_rels.sort_by(|a, b| natural_key(a).cmp(&natural_key(b)));
    sorted_rels
        .iter()
        .filter_map(|r| rel_to_fid.get(r).cloned())
        .collect()
}

/// Apply a saved ordering to the full set of file IDs.
///
/// Files present in `order_fids` keep their saved order. Files not in the
/// saved order are appended at the end, naturally sorted by their relative
/// path.
fn _apply_saved_order(
    all_fids: &[String],
    fid_to_rel: &HashMap<String, String>,
    order_fids: &[String],
) -> Vec<String> {
    if order_fids.is_empty() {
        // Fall back to natural sort.
        let mut fids = all_fids.to_vec();
        fids.sort_by(|a, b| {
            let ra = fid_to_rel.get(a).cloned().unwrap_or_default();
            let rb = fid_to_rel.get(b).cloned().unwrap_or_default();
            natural_key(&ra).cmp(&natural_key(&rb))
        });
        return fids;
    }

    let all_set: std::collections::HashSet<&String> = all_fids.iter().collect();

    // Saved order, keeping only fids that still exist.
    let mut result: Vec<String> = order_fids
        .iter()
        .filter(|f| all_set.contains(f))
        .cloned()
        .collect();

    let ordered_set: std::collections::HashSet<&String> = result.iter().collect();

    // New files not in the saved order, naturally sorted.
    let mut new_fids: Vec<String> = all_fids
        .iter()
        .filter(|f| !ordered_set.contains(f))
        .cloned()
        .collect();
    new_fids.sort_by(|a, b| {
        let ra = fid_to_rel.get(a).cloned().unwrap_or_default();
        let rb = fid_to_rel.get(b).cloned().unwrap_or_default();
        natural_key(&ra).cmp(&natural_key(&rb))
    });

    result.extend(new_fids);
    result
}

/// Compute per-file tree display flags: depth, pipe characters, is_last, etc.
///
/// Depth is limited to 1 level (immediate parent folder only).
fn _tree_flags(rels: &[String]) -> HashMap<String, Value> {
    let mut result: HashMap<String, Value> = HashMap::new();

    // Group files by their immediate parent folder.
    // For a relpath like "subfolder/file.mp4", parent = "subfolder".
    // For "file.mp4", parent = "".
    let mut groups: HashMap<String, Vec<usize>> = HashMap::new();
    for (i, rel) in rels.iter().enumerate() {
        let parent = if let Some(pos) = rel.rfind('/') {
            rel[..pos].to_string()
        } else {
            String::new()
        };
        groups.entry(parent).or_default().push(i);
    }

    // Track which parents we have seen so far for has_prev_in_parent.
    let mut parent_seen_count: HashMap<String, usize> = HashMap::new();

    for (_i, rel) in rels.iter().enumerate() {
        let (parent, depth) = if let Some(pos) = rel.rfind('/') {
            (rel[..pos].to_string(), 1)
        } else {
            (String::new(), 0)
        };

        let group = groups.get(&parent).unwrap();
        let position_in_group = parent_seen_count.entry(parent.clone()).or_insert(0);
        let is_last = *position_in_group + 1 == group.len();
        let has_prev_in_parent = *position_in_group > 0;
        *parent_seen_count.get_mut(&parent).unwrap() += 1;

        // Build pipe prefix for tree display.
        let pipes = if depth == 0 {
            String::new()
        } else {
            if is_last {
                "\u{2514}\u{2500} ".to_string() // "-- "
            } else {
                "\u{251C}\u{2500} ".to_string() // "|-- "
            }
        };

        result.insert(
            rel.clone(),
            json!({
                "depth": depth,
                "pipes": pipes,
                "is_last": is_last,
                "has_prev_in_parent": has_prev_in_parent,
            }),
        );
    }

    result
}

/// Recursively scan a directory for video files.
fn scan_video_files(dir: &Path) -> Vec<PathBuf> {
    let mut results = Vec::new();
    _scan_recursive(dir, &mut results);
    results
}

fn _scan_recursive(dir: &Path, out: &mut Vec<PathBuf>) {
    let entries = match fs::read_dir(dir) {
        Ok(e) => e,
        Err(_) => return,
    };

    let mut dirs: Vec<PathBuf> = Vec::new();
    let mut files: Vec<PathBuf> = Vec::new();

    for entry in entries.flatten() {
        let path = entry.path();
        if path.is_dir() {
            dirs.push(path);
        } else if path.is_file() {
            if let Some(ext) = path.extension() {
                let ext_lower = format!(".{}", ext.to_string_lossy().to_lowercase());
                if VIDEO_EXTS.contains(&ext_lower.as_str()) {
                    files.push(path);
                }
            }
        }
    }

    // Sort files naturally for deterministic ordering.
    files.sort_by(|a, b| {
        let na = a.file_name().unwrap_or_default().to_string_lossy();
        let nb = b.file_name().unwrap_or_default().to_string_lossy();
        natural_key(&na).cmp(&natural_key(&nb))
    });
    out.extend(files);

    // Recurse into subdirectories in sorted order.
    dirs.sort_by(|a, b| {
        let na = a.file_name().unwrap_or_default().to_string_lossy();
        let nb = b.file_name().unwrap_or_default().to_string_lossy();
        natural_key(&na).cmp(&natural_key(&nb))
    });
    for d in &dirs {
        _scan_recursive(d, out);
    }
}

// ---------------------------------------------------------------------------
// Library implementation
// ---------------------------------------------------------------------------

impl Library {
    // -----------------------------------------------------------------------
    // new
    // -----------------------------------------------------------------------

    /// Create a new empty library with default state.
    pub fn new() -> Library {
        Library {
            root: None,
            files: Vec::new(),
            fids: Vec::new(),
            relpaths: Vec::new(),
            rel_to_fid: HashMap::new(),
            fid_to_rel: HashMap::new(),
            state: LibraryState::default(),
            state_path: None,
            ffprobe: None,
            ffmpeg: None,
            meta_cache: HashMap::new(),
            scan_stop: Arc::new(AtomicBool::new(false)),
        }
    }

    // -----------------------------------------------------------------------
    // set_root
    // -----------------------------------------------------------------------

    /// Open a folder as the video library root.
    ///
    /// Scans for video files, computes fingerprints, loads or creates state,
    /// merges with existing progress, and returns `get_library_info()`.
    pub fn set_root(&mut self, folder: &str, state_dir: &Path) -> Result<Value, String> {
        // 1. Resolve and verify folder.
        let folder_path = PathBuf::from(folder);
        let folder_path = folder_path
            .canonicalize()
            .map_err(|e| format!("Cannot resolve folder '{}': {}", folder, e))?;

        if !folder_path.is_dir() {
            return Err(format!("'{}' is not a directory", folder));
        }

        // Signal any running background scan to stop.
        self.scan_stop.store(true, Ordering::SeqCst);

        // 2. Recursively scan for video files.
        let found_files = scan_video_files(&folder_path);

        if found_files.is_empty() {
            return Err(format!("No video files found in '{}'", folder));
        }

        // 3. Compute fingerprints and build mappings.
        let mut rel_to_fid: HashMap<String, String> = HashMap::new();
        let mut fid_to_rel: HashMap<String, String> = HashMap::new();
        let mut all_fids: Vec<String> = Vec::new();
        let mut all_relpaths: Vec<String> = Vec::new();

        for file in &found_files {
            let relpath = file
                .strip_prefix(&folder_path)
                .unwrap_or(file.as_path())
                .to_string_lossy()
                .replace('\\', "/");

            let fid = file_fingerprint(file);

            // Skip duplicates (same fingerprint from hardlinks, etc.).
            if fid_to_rel.contains_key(&fid) {
                continue;
            }

            rel_to_fid.insert(relpath.clone(), fid.clone());
            fid_to_rel.insert(fid.clone(), relpath.clone());
            all_fids.push(fid);
            all_relpaths.push(relpath);
        }

        // 4. Compute library ID.
        let library_id = compute_library_id(&all_fids);

        // 5. Load existing state or create baseline.
        let state_file = state_dir.join(format!("library_{}.json", library_id));
        let baseline = LibraryState {
            version: STATE_VERSION,
            library_id: library_id.clone(),
            last_path: folder_path.to_string_lossy().to_string(),
            updated_at: now_secs(),
            current_fid: None,
            current_time: 0.0,
            volume: 1.0,
            autoplay: false,
            playback_rate: 1.0,
            order_fids: Vec::new(),
            videos: HashMap::new(),
        };

        let mut state = if let Some(loaded_val) = load_json_with_fallbacks(&state_file, BACKUP_COUNT)
        {
            // Try to deserialize the loaded JSON into LibraryState.
            if let Ok(loaded_state) = serde_json::from_value::<LibraryState>(loaded_val.clone()) {
                // 6. Merge: only if same library_id.
                if loaded_state.library_id == library_id {
                    self._merge_state(loaded_state, &baseline)
                } else {
                    baseline
                }
            } else {
                baseline
            }
        } else {
            baseline
        };

        // 7. Update last_path to current folder.
        state.last_path = folder_path.to_string_lossy().to_string();

        // 8. Normalize video metadata for all found fids.
        let existing_videos = state.videos.clone();
        let mut normalized: HashMap<String, VideoMeta> = HashMap::new();
        for fid in &all_fids {
            let meta = existing_videos.get(fid).cloned().unwrap_or_default();
            normalized.insert(fid.clone(), meta);
        }
        state.videos = normalized;

        // 9. Normalize settings.
        state.volume = clamp(state.volume, 0.0, 1.0);
        state.playback_rate = clamp(state.playback_rate, 0.25, 3.0);

        // 10. Clean order_fids.
        let valid_set: std::collections::HashSet<&String> = all_fids.iter().collect();
        state.order_fids.retain(|f| valid_set.contains(f));

        // 11. Apply saved ordering.
        let ordered_fids =
            _apply_saved_order(&all_fids, &fid_to_rel, &state.order_fids);
        state.order_fids = ordered_fids.clone();

        // 12. Build ordered file lists.
        let ordered_relpaths: Vec<String> = ordered_fids
            .iter()
            .filter_map(|fid| fid_to_rel.get(fid).cloned())
            .collect();
        let ordered_files: Vec<PathBuf> = ordered_relpaths
            .iter()
            .map(|rel| folder_path.join(rel.replace('/', std::path::MAIN_SEPARATOR_STR)))
            .collect();

        // 13. Validate current_fid.
        if let Some(ref cfid) = state.current_fid {
            if !fid_to_rel.contains_key(cfid) {
                state.current_fid = ordered_fids.first().cloned();
                state.current_time = 0.0;
            }
        }

        // Assign everything to self.
        self.root = Some(folder_path);
        self.files = ordered_files;
        self.fids = ordered_fids;
        self.relpaths = ordered_relpaths;
        self.rel_to_fid = rel_to_fid;
        self.fid_to_rel = fid_to_rel;
        self.state = state;
        self.state_path = Some(state_file);
        self.meta_cache.clear();
        self.scan_stop = Arc::new(AtomicBool::new(false));

        // 14. Save state.
        self.save_state();

        // 15. Return library info.
        Ok(self.get_library_info())
    }

    /// Merge a loaded state with a baseline, preferring loaded values.
    fn _merge_state(&self, loaded: LibraryState, baseline: &LibraryState) -> LibraryState {
        LibraryState {
            version: STATE_VERSION,
            library_id: loaded.library_id,
            last_path: baseline.last_path.clone(),
            updated_at: loaded.updated_at,
            current_fid: loaded.current_fid,
            current_time: loaded.current_time,
            volume: loaded.volume,
            autoplay: loaded.autoplay,
            playback_rate: loaded.playback_rate,
            order_fids: loaded.order_fids,
            videos: loaded.videos,
        }
    }

    // -----------------------------------------------------------------------
    // save_state
    // -----------------------------------------------------------------------

    /// Persist the current state to disk with backup rotation.
    pub fn save_state(&mut self) {
        self.state.updated_at = now_secs();
        if let Some(ref path) = self.state_path {
            if let Ok(val) = serde_json::to_value(&self.state) {
                atomic_write_json(path, &val, BACKUP_COUNT);
            }
        }
    }

    // -----------------------------------------------------------------------
    // get_library_info
    // -----------------------------------------------------------------------

    /// Build a comprehensive JSON object describing the library state.
    pub fn get_library_info(&self) -> Value {
        let folder = self
            .root
            .as_ref()
            .map(|p| {
                let s = p.to_string_lossy().to_string();
                // Strip Windows extended-length path prefix.
                s.strip_prefix("\\\\?\\").unwrap_or(&s).to_string()
            })
            .unwrap_or_default();

        let count = self.fids.len();

        let current_index = self
            .state
            .current_fid
            .as_ref()
            .and_then(|cfid| self.fids.iter().position(|f| f == cfid));

        // Build tree flags.
        let tree_flags = _tree_flags(&self.relpaths);

        // Check if any subdirectories exist.
        let has_subdirs = self.relpaths.iter().any(|r| r.contains('/'));

        // Build items array.
        let mut items: Vec<Value> = Vec::new();
        for (i, fid) in self.fids.iter().enumerate() {
            let relpath = self.fid_to_rel.get(fid).cloned().unwrap_or_default();
            let filename = relpath.rsplit('/').next().unwrap_or(&relpath);
            let title = pretty_title_from_filename(filename);

            let flags = tree_flags.get(&relpath);
            let depth = flags
                .and_then(|f| f.get("depth"))
                .and_then(|v| v.as_u64())
                .unwrap_or(0);
            let pipes = flags
                .and_then(|f| f.get("pipes"))
                .and_then(|v| v.as_str())
                .unwrap_or("");
            let is_last = flags
                .and_then(|f| f.get("is_last"))
                .and_then(|v| v.as_bool())
                .unwrap_or(true);
            let has_prev_in_parent = flags
                .and_then(|f| f.get("has_prev_in_parent"))
                .and_then(|v| v.as_bool())
                .unwrap_or(false);

            let meta = self.state.videos.get(fid);
            let pos = meta.map(|m| m.pos).unwrap_or(0.0);
            let watched = meta.map(|m| m.watched).unwrap_or(0.0);
            let duration = meta.and_then(|m| m.duration);
            let finished = meta.map(|m| m.finished).unwrap_or(false);
            let note_len = meta.map(|m| m.note.len()).unwrap_or(0);
            let last_open = meta.map(|m| m.last_open).unwrap_or(0);
            let has_sub = meta
                .map(|m| m.subtitle.is_some())
                .unwrap_or(false);

            items.push(json!({
                "index": i,
                "fid": fid,
                "name": filename,
                "title": title,
                "relpath": relpath,
                "depth": depth,
                "pipes": pipes,
                "is_last": is_last,
                "has_prev_in_parent": has_prev_in_parent,
                "pos": pos,
                "watched": watched,
                "duration": duration,
                "finished": finished,
                "note_len": note_len,
                "last_open": last_open,
                "has_sub": has_sub,
            }));
        }

        // Folder stats.
        let stats = self._folder_stats();

        // Next up: first unfinished video after current.
        let next_up = self._compute_next_up(current_index);

        json!({
            "ok": true,
            "folder": folder,
            "library_id": self.state.library_id,
            "count": count,
            "current_index": current_index,
            "current_fid": self.state.current_fid,
            "current_time": self.state.current_time,
            "folder_volume": self.state.volume,
            "folder_autoplay": self.state.autoplay,
            "folder_rate": self.state.playback_rate,
            "items": items,
            "has_subdirs": has_subdirs,
            "overall_progress": stats.get("overall_progress").cloned().unwrap_or(json!(0.0)),
            "durations_known": stats.get("durations_known").cloned().unwrap_or(json!(false)),
            "finished_count": stats.get("finished_count").cloned().unwrap_or(json!(0)),
            "remaining_count": stats.get("remaining_count").cloned().unwrap_or(json!(0)),
            "remaining_seconds_known": stats.get("remaining_seconds_known").cloned().unwrap_or(json!(0.0)),
            "top_folders": stats.get("top_folders").cloned().unwrap_or(json!([])),
            "next_up": next_up,
        })
    }

    /// Find the next unfinished video index after the current position.
    fn _compute_next_up(&self, current_index: Option<usize>) -> Value {
        let make_result = |i: usize| -> Value {
            let fid = &self.fids[i];
            let title = self.fid_to_rel.get(fid)
                .map(|r| pretty_title_from_filename(r))
                .unwrap_or_default();
            json!({"index": i, "title": title})
        };

        let start = current_index.map(|i| i + 1).unwrap_or(0);
        for i in start..self.fids.len() {
            let fid = &self.fids[i];
            let finished = self
                .state
                .videos
                .get(fid)
                .map(|m| m.finished)
                .unwrap_or(false);
            if !finished {
                return make_result(i);
            }
        }
        // Wrap around from beginning.
        let end = current_index.unwrap_or(self.fids.len());
        for i in 0..end.min(self.fids.len()) {
            let fid = &self.fids[i];
            let finished = self
                .state
                .videos
                .get(fid)
                .map(|m| m.finished)
                .unwrap_or(false);
            if !finished {
                return make_result(i);
            }
        }
        Value::Null
    }

    // -----------------------------------------------------------------------
    // update_progress
    // -----------------------------------------------------------------------

    /// Update playback progress for a video at the given index.
    ///
    /// The `finished` flag is sticky: once set, it remains true even if
    /// `current_time` decreases.
    pub fn update_progress(
        &mut self,
        index: usize,
        current_time: f64,
        duration: Option<f64>,
        playing: bool,
    ) -> Value {
        if self.fids.is_empty() {
            return json!({"ok": false, "error": "library is empty"});
        }

        let index = index.min(self.fids.len() - 1);
        let fid = self.fids[index].clone();

        let meta = self
            .state
            .videos
            .entry(fid.clone())
            .or_insert_with(VideoMeta::default);

        meta.pos = current_time;

        // High-water mark.
        if current_time > meta.watched {
            meta.watched = current_time;
        }

        // Update duration if provided and positive.
        if let Some(d) = duration {
            if d > 0.0 {
                meta.duration = Some(d);
            }
        }

        // Finished is sticky.
        let effective_duration = meta.duration.unwrap_or(0.0);
        let threshold = (effective_duration - 2.0).max(0.0);
        if effective_duration > 0.0 && current_time >= threshold {
            meta.finished = true;
        }
        // Note: if already finished, it stays finished.

        // Update last_open timestamp if actively playing.
        if playing {
            meta.last_open = now_secs();
        }

        // Update current tracking.
        self.state.current_fid = Some(fid);
        self.state.current_time = current_time;

        // Snapshot values before releasing the borrow on self.state.videos.
        let pos = meta.pos;
        let watched = meta.watched;
        let duration_val = meta.duration;
        let finished = meta.finished;

        self.save_state();

        json!({
            "ok": true,
            "index": index,
            "pos": pos,
            "watched": watched,
            "duration": duration_val,
            "finished": finished,
        })
    }

    // -----------------------------------------------------------------------
    // set_current
    // -----------------------------------------------------------------------

    /// Set the currently active video by index and timecode.
    pub fn set_current(&mut self, index: usize, timecode: f64) -> Value {
        if self.fids.is_empty() {
            return json!({"ok": false, "error": "library is empty"});
        }

        let index = index.min(self.fids.len() - 1);
        let fid = self.fids[index].clone();

        self.state.current_fid = Some(fid.clone());
        self.state.current_time = timecode;

        self.save_state();

        json!({
            "ok": true,
            "index": index,
            "fid": fid,
            "current_time": timecode,
        })
    }

    // -----------------------------------------------------------------------
    // set_folder_volume
    // -----------------------------------------------------------------------

    /// Set the folder-level volume (clamped 0.0 to 1.0).
    pub fn set_folder_volume(&mut self, volume: f64) -> Value {
        self.state.volume = clamp(volume, 0.0, 1.0);
        self.save_state();
        json!({"ok": true, "folder_volume": self.state.volume})
    }

    // -----------------------------------------------------------------------
    // set_folder_autoplay
    // -----------------------------------------------------------------------

    /// Set the folder-level autoplay preference.
    pub fn set_folder_autoplay(&mut self, enabled: bool) -> Value {
        self.state.autoplay = enabled;
        self.save_state();
        json!({"ok": true, "folder_autoplay": self.state.autoplay})
    }

    // -----------------------------------------------------------------------
    // set_folder_rate
    // -----------------------------------------------------------------------

    /// Set the folder-level playback rate (clamped 0.25 to 3.0).
    pub fn set_folder_rate(&mut self, rate: f64) -> Value {
        self.state.playback_rate = clamp(rate, 0.25, 3.0);
        self.save_state();
        json!({"ok": true, "folder_rate": self.state.playback_rate})
    }

    // -----------------------------------------------------------------------
    // set_order
    // -----------------------------------------------------------------------

    /// Reorder the playlist. `fids` contains the desired ordering; any fids
    /// not in the list are appended at the end (naturally sorted).
    pub fn set_order(&mut self, fids: Vec<String>) -> Value {
        let valid_set: std::collections::HashSet<&String> = self.fids.iter().collect();

        // Keep only fids that actually exist.
        let mut ordered: Vec<String> = fids.into_iter().filter(|f| valid_set.contains(f)).collect();

        // Append remaining fids not in the new order.
        let ordered_set: std::collections::HashSet<&String> = ordered.iter().collect();
        let mut remaining: Vec<String> = self
            .fids
            .iter()
            .filter(|f| !ordered_set.contains(f))
            .cloned()
            .collect();
        remaining.sort_by(|a, b| {
            let ra = self.fid_to_rel.get(a).cloned().unwrap_or_default();
            let rb = self.fid_to_rel.get(b).cloned().unwrap_or_default();
            natural_key(&ra).cmp(&natural_key(&rb))
        });
        ordered.extend(remaining);

        // Rebuild file lists.
        self.fids = ordered.clone();
        self.relpaths = ordered
            .iter()
            .filter_map(|fid| self.fid_to_rel.get(fid).cloned())
            .collect();
        if let Some(ref root) = self.root {
            self.files = self
                .relpaths
                .iter()
                .map(|rel| root.join(rel.replace('/', std::path::MAIN_SEPARATOR_STR)))
                .collect();
        }

        self.state.order_fids = ordered;
        self.save_state();

        self.get_library_info()
    }

    // -----------------------------------------------------------------------
    // get_note / set_note
    // -----------------------------------------------------------------------

    /// Return the note for the given fid, or an empty string if not found.
    pub fn get_note(&self, fid: &str) -> String {
        self.state
            .videos
            .get(fid)
            .map(|m| m.note.clone())
            .unwrap_or_default()
    }

    /// Set a note for the given fid and save state.
    pub fn set_note(&mut self, fid: &str, note: &str) -> Value {
        if let Some(meta) = self.state.videos.get_mut(fid) {
            meta.note = note.to_string();
        } else {
            let mut m = VideoMeta::default();
            m.note = note.to_string();
            self.state.videos.insert(fid.to_string(), m);
        }
        self.save_state();
        json!({"ok": true, "fid": fid, "note_len": note.len()})
    }

    // -----------------------------------------------------------------------
    // get_video_path
    // -----------------------------------------------------------------------

    /// Get the full file path for a video by index.
    ///
    /// Performs bounds checking and path-traversal protection.
    pub fn get_video_path(&self, index: usize) -> Result<PathBuf, String> {
        if self.files.is_empty() {
            return Err("No files in library".to_string());
        }
        if index >= self.files.len() {
            return Err(format!(
                "Index {} out of range (0..{})",
                index,
                self.files.len()
            ));
        }

        let path = &self.files[index];

        // Path-traversal protection.
        if let Some(ref root) = self.root {
            if !is_within_root(root, path) {
                return Err("Path traversal detected".to_string());
            }
        }

        if !path.exists() {
            return Err(format!("File not found: {}", path.display()));
        }

        Ok(path.clone())
    }

    // -----------------------------------------------------------------------
    // get_current_video_metadata
    // -----------------------------------------------------------------------

    /// Return basic file metadata plus cached ffprobe metadata for the
    /// current video.
    pub fn get_current_video_metadata(&mut self) -> Value {
        let fid = match &self.state.current_fid {
            Some(f) => f.clone(),
            None => return json!({"ok": false, "error": "no current video"}),
        };

        let index = match self.fids.iter().position(|f| f == &fid) {
            Some(i) => i,
            None => return json!({"ok": false, "error": "current fid not in library"}),
        };

        let basic = self._basic_file_meta(&fid);

        // Probe if not cached.
        if !self.meta_cache.contains_key(&fid) {
            if let Some(ref ffprobe) = self.ffprobe.clone() {
                if index < self.files.len() {
                    if let Some(meta) =
                        ffmpeg::ffprobe_video_metadata(&self.files[index], ffprobe)
                    {
                        self.meta_cache.insert(fid.clone(), meta);
                    }
                }
            }
        }

        let mut result = json!({
            "ok": true,
            "basic": basic,
            "ffprobe_found": self.ffprobe.is_some(),
        });

        if let Some(cached) = self.meta_cache.get(&fid) {
            if let Ok(meta_val) = serde_json::to_value(cached) {
                result
                    .as_object_mut()
                    .unwrap()
                    .insert("probe".to_string(), meta_val);
            }
        }

        result
    }

    // -----------------------------------------------------------------------
    // Subtitle methods
    // -----------------------------------------------------------------------

    /// Build a protocol URL for a stored subtitle VTT path.
    /// The `vtt` field is like `"subtitles/{fid}_{name}.vtt"` — extract the filename.
    fn _sub_url(vtt: &str) -> String {
        let filename = vtt.rsplit('/').next().unwrap_or(vtt);
        format!("http://tutdock.localhost/sub/{}", filename)
    }

    /// Build a successful subtitle JSON response with `has`, `url`, and `label`.
    fn _sub_response(vtt: &str, label: &str) -> Value {
        json!({
            "ok": true,
            "has": true,
            "url": Self::_sub_url(vtt),
            "label": label,
        })
    }

    /// Get subtitle for the current video.
    ///
    /// Priority: stored subtitle -> sidecar -> embedded.
    pub fn get_subtitle_for_current(&mut self, state_dir: &Path) -> Value {
        let fid = match &self.state.current_fid {
            Some(f) => f.clone(),
            None => return json!({"ok": false, "error": "no current video"}),
        };

        let index = match self.fids.iter().position(|f| f == &fid) {
            Some(i) => i,
            None => return json!({"ok": false, "error": "current fid not in library"}),
        };

        // 1. Check stored subtitle.
        if let Some(meta) = self.state.videos.get(&fid) {
            if let Some(ref sub_ref) = meta.subtitle {
                let vtt_path = state_dir.join(&sub_ref.vtt);
                if vtt_path.exists() {
                    return Self::_sub_response(&sub_ref.vtt, &sub_ref.label);
                }
            }
        }

        // 2. Try sidecar.
        if index < self.files.len() {
            if let Some(sidecar_path) = subtitles::auto_subtitle_sidecar(&self.files[index]) {
                let subs_dir = state_dir.join("subtitles");
                if let Some(stored) =
                    subtitles::store_subtitle_for_fid(&fid, &sidecar_path, &subs_dir)
                {
                    // Save reference.
                    if let Some(meta) = self.state.videos.get_mut(&fid) {
                        meta.subtitle = Some(SubtitleRef {
                            vtt: stored.vtt.clone(),
                            label: stored.label.clone(),
                        });
                    }
                    self.save_state();
                    return Self::_sub_response(&stored.vtt, &stored.label);
                }
            }
        }

        // 3. Try embedded (first subtitle track).
        if let Some(ref ffmpeg_path) = self.ffmpeg.clone() {
            if let Some(ref ffprobe_path) = self.ffprobe.clone() {
                if index < self.files.len() {
                    if let Some(meta) =
                        ffmpeg::ffprobe_video_metadata(&self.files[index], ffprobe_path)
                    {
                        if let Some(track) = meta.subtitle_tracks.first() {
                            let subs_dir = state_dir.join("subtitles");
                            if let Ok(stored) = subtitles::extract_embedded_subtitle(
                                &self.files[index],
                                track.index,
                                ffmpeg_path,
                                &subs_dir,
                                &fid,
                            ) {
                                if let Some(vmeta) = self.state.videos.get_mut(&fid) {
                                    vmeta.subtitle = Some(SubtitleRef {
                                        vtt: stored.vtt.clone(),
                                        label: stored.label.clone(),
                                    });
                                }
                                self.save_state();
                                return Self::_sub_response(&stored.vtt, &stored.label);
                            }
                        }
                    }
                }
            }
        }

        json!({"ok": true, "has": false})
    }

    /// Store a user-selected subtitle file for the current video.
    pub fn set_subtitle_for_current(&mut self, file_path: &str, state_dir: &Path) -> Value {
        let fid = match &self.state.current_fid {
            Some(f) => f.clone(),
            None => return json!({"ok": false, "error": "no current video"}),
        };

        let src = PathBuf::from(file_path);
        if !src.is_file() {
            return json!({"ok": false, "error": "subtitle file not found"});
        }

        let subs_dir = state_dir.join("subtitles");
        match subtitles::store_subtitle_for_fid(&fid, &src, &subs_dir) {
            Some(stored) => {
                if let Some(meta) = self.state.videos.get_mut(&fid) {
                    meta.subtitle = Some(SubtitleRef {
                        vtt: stored.vtt.clone(),
                        label: stored.label.clone(),
                    });
                }
                self.save_state();
                Self::_sub_response(&stored.vtt, &stored.label)
            }
            None => {
                json!({"ok": false, "error": "unsupported subtitle format"})
            }
        }
    }

    /// Get list of embedded subtitle tracks using ffprobe.
    pub fn get_embedded_subtitles(&self) -> Value {
        let fid = match &self.state.current_fid {
            Some(f) => f.clone(),
            None => return json!({"ok": false, "error": "no current video"}),
        };

        let index = match self.fids.iter().position(|f| f == &fid) {
            Some(i) => i,
            None => return json!({"ok": false, "error": "current fid not in library"}),
        };

        if let Some(ref ffprobe_path) = self.ffprobe {
            if index < self.files.len() {
                if let Some(meta) =
                    ffmpeg::ffprobe_video_metadata(&self.files[index], ffprobe_path)
                {
                    let tracks: Vec<Value> = meta
                        .subtitle_tracks
                        .iter()
                        .map(|t| {
                            json!({
                                "index": t.index,
                                "codec": t.codec,
                                "language": t.language,
                                "title": t.title,
                            })
                        })
                        .collect();
                    return json!({"ok": true, "tracks": tracks});
                }
            }
        }

        json!({"ok": true, "tracks": []})
    }

    /// Extract an embedded subtitle track by index.
    pub fn extract_embedded_subtitle(
        &mut self,
        track_index: u32,
        state_dir: &Path,
    ) -> Value {
        let fid = match &self.state.current_fid {
            Some(f) => f.clone(),
            None => return json!({"ok": false, "error": "no current video"}),
        };

        let index = match self.fids.iter().position(|f| f == &fid) {
            Some(i) => i,
            None => return json!({"ok": false, "error": "current fid not in library"}),
        };

        let ffmpeg_path = match &self.ffmpeg {
            Some(p) => p.clone(),
            None => return json!({"ok": false, "error": "ffmpeg not available"}),
        };

        if index >= self.files.len() {
            return json!({"ok": false, "error": "index out of range"});
        }

        let subs_dir = state_dir.join("subtitles");
        match subtitles::extract_embedded_subtitle(
            &self.files[index],
            track_index,
            &ffmpeg_path,
            &subs_dir,
            &fid,
        ) {
            Ok(stored) => {
                if let Some(meta) = self.state.videos.get_mut(&fid) {
                    meta.subtitle = Some(SubtitleRef {
                        vtt: stored.vtt.clone(),
                        label: stored.label.clone(),
                    });
                }
                self.save_state();
                Self::_sub_response(&stored.vtt, &stored.label)
            }
            Err(e) => {
                json!({"ok": false, "error": e})
            }
        }
    }

    /// Get all available sidecar and embedded subtitles for the current video.
    ///
    /// Sidecar discovery uses normalized matching:
    /// - Check all .srt/.vtt files in video's parent directory
    /// - Match by: exact stem, normalized stem, stem with language suffix
    /// - Extract language labels from filename suffixes
    /// - Sort: English first, then alphabetical
    /// - Deduplicate by label
    pub fn get_available_subtitles(&self) -> Value {
        let fid = match &self.state.current_fid {
            Some(f) => f.clone(),
            None => return json!({"ok": false, "error": "no current video"}),
        };

        let index = match self.fids.iter().position(|f| f == &fid) {
            Some(i) => i,
            None => return json!({"ok": false, "error": "current fid not in library"}),
        };

        let mut sidecar_subs: Vec<Value> = Vec::new();

        if index < self.files.len() {
            let video_path = &self.files[index];
            if let Some(parent) = video_path.parent() {
                let video_stem = video_path
                    .file_stem()
                    .map(|s| s.to_string_lossy().to_string())
                    .unwrap_or_default();
                let video_stem_lower = video_stem.to_lowercase();
                let video_stem_norm = _normalize_stem(&video_stem);

                if let Ok(entries) = fs::read_dir(parent) {
                    let mut found: Vec<(String, String, bool)> = Vec::new(); // (label, path, is_english)
                    let mut seen_labels: std::collections::HashSet<String> =
                        std::collections::HashSet::new();

                    for entry in entries.flatten() {
                        let path = entry.path();
                        if !path.is_file() {
                            continue;
                        }

                        let fname = match path.file_name() {
                            Some(n) => n.to_string_lossy().to_string(),
                            None => continue,
                        };
                        let fname_lower = fname.to_lowercase();

                        let is_sub = subtitles::SUB_EXTS
                            .iter()
                            .any(|ext| fname_lower.ends_with(ext));
                        if !is_sub {
                            continue;
                        }

                        let sub_stem = match path.file_stem() {
                            Some(s) => s.to_string_lossy().to_string(),
                            None => continue,
                        };
                        let sub_stem_lower = sub_stem.to_lowercase();
                        let sub_stem_norm = _normalize_stem(&sub_stem);

                        // Check if this subtitle matches the video.
                        let mut matched = false;
                        let mut lang_label = String::new();
                        let mut is_english = false;

                        // Exact stem match.
                        if sub_stem_lower == video_stem_lower {
                            matched = true;
                            lang_label = "Default".to_string();
                        }

                        // Normalized match.
                        if !matched && sub_stem_norm == video_stem_norm {
                            matched = true;
                            lang_label = "Default".to_string();
                        }

                        // Language suffix match.
                        if !matched {
                            if let Some(dot_pos) = sub_stem.rfind('.') {
                                let base = &sub_stem[..dot_pos];
                                let suffix = &sub_stem[dot_pos + 1..];
                                let base_lower = base.to_lowercase();
                                let base_norm = _normalize_stem(base);
                                let suffix_lower = suffix.to_lowercase();

                                if base_lower == video_stem_lower
                                    || base_norm == video_stem_norm
                                {
                                    matched = true;
                                    lang_label = _language_label(&suffix_lower);
                                    is_english = ["en", "eng", "english"]
                                        .contains(&suffix_lower.as_str());
                                }
                            }
                        }

                        if matched {
                            if seen_labels.insert(lang_label.clone()) {
                                found.push((
                                    lang_label,
                                    path.to_string_lossy().to_string(),
                                    is_english,
                                ));
                            }
                        }
                    }

                    // Sort: English first, then alphabetical.
                    found.sort_by(|a, b| {
                        if a.2 && !b.2 {
                            std::cmp::Ordering::Less
                        } else if !a.2 && b.2 {
                            std::cmp::Ordering::Greater
                        } else {
                            a.0.cmp(&b.0)
                        }
                    });

                    for (label, path, _) in &found {
                        let ext = std::path::Path::new(path)
                            .extension()
                            .map(|e| e.to_string_lossy().to_uppercase())
                            .unwrap_or_default();
                        sidecar_subs.push(json!({
                            "type": "sidecar",
                            "label": label,
                            "path": path,
                            "format": ext,
                        }));
                    }
                }
            }
        }

        // Embedded subtitles.
        let mut embedded_subs: Vec<Value> = Vec::new();
        if let Some(ref ffprobe_path) = self.ffprobe {
            if index < self.files.len() {
                if let Some(meta) =
                    ffmpeg::ffprobe_video_metadata(&self.files[index], ffprobe_path)
                {
                    for track in &meta.subtitle_tracks {
                        let label = if !track.title.is_empty() {
                            track.title.clone()
                        } else if !track.language.is_empty() {
                            _language_label(&track.language)
                        } else {
                            format!("Track {}", track.index)
                        };
                        embedded_subs.push(json!({
                            "type": "embedded",
                            "label": label,
                            "index": track.index,
                            "codec": track.codec,
                            "language": track.language,
                        }));
                    }
                }
            }
        }

        json!({
            "ok": true,
            "sidecar": sidecar_subs,
            "embedded": embedded_subs,
        })
    }

    /// Load a specific sidecar subtitle file for the current video.
    pub fn load_sidecar_subtitle(
        &mut self,
        file_path: &str,
        state_dir: &Path,
    ) -> Value {
        let fid = match &self.state.current_fid {
            Some(f) => f.clone(),
            None => return json!({"ok": false, "error": "no current video"}),
        };

        let src = PathBuf::from(file_path);
        if !src.is_file() {
            return json!({"ok": false, "error": "subtitle file not found"});
        }

        let subs_dir = state_dir.join("subtitles");
        match subtitles::store_subtitle_for_fid(&fid, &src, &subs_dir) {
            Some(stored) => {
                if let Some(meta) = self.state.videos.get_mut(&fid) {
                    meta.subtitle = Some(SubtitleRef {
                        vtt: stored.vtt.clone(),
                        label: stored.label.clone(),
                    });
                }
                self.save_state();
                Self::_sub_response(&stored.vtt, &stored.label)
            }
            None => {
                json!({"ok": false, "error": "unsupported subtitle format or read error"})
            }
        }
    }

    // -----------------------------------------------------------------------
    // reset_watch_progress
    // -----------------------------------------------------------------------

    /// Reset pos, watched, and finished for ALL videos.
    ///
    /// Preserves notes, durations, and subtitle references.
    pub fn reset_watch_progress(&mut self) -> Value {
        for meta in self.state.videos.values_mut() {
            meta.pos = 0.0;
            meta.watched = 0.0;
            meta.finished = false;
        }
        self.state.current_time = 0.0;
        self.save_state();
        json!({"ok": true})
    }

    // -----------------------------------------------------------------------
    // Duration scanning
    // -----------------------------------------------------------------------

    /// Return a list of (fid, file_path) pairs where duration is unknown.
    pub fn get_pending_scans(&self) -> Vec<(String, PathBuf)> {
        let mut pending = Vec::new();
        for (i, fid) in self.fids.iter().enumerate() {
            let has_duration = self
                .state
                .videos
                .get(fid)
                .and_then(|m| m.duration)
                .is_some();
            if !has_duration && i < self.files.len() {
                pending.push((fid.clone(), self.files[i].clone()));
            }
        }
        pending
    }

    /// Apply a scanned duration to a video's metadata.
    ///
    /// Also re-checks the finished status: if watched >= duration - 2,
    /// mark finished.
    pub fn apply_scanned_duration(&mut self, fid: &str, duration: f64) {
        if duration <= 0.0 {
            return;
        }
        let meta = self
            .state
            .videos
            .entry(fid.to_string())
            .or_insert_with(VideoMeta::default);
        meta.duration = Some(duration);

        // Re-check finished status with the new duration.
        let threshold = (duration - 2.0).max(0.0);
        if meta.watched >= threshold {
            meta.finished = true;
        }

        self.save_state();
    }

    /// Start a background duration scan.
    ///
    /// This method signals any previous scan to stop, then prepares the
    /// pending list. The actual scanning is driven externally via
    /// `get_pending_scans()` and `apply_scanned_duration()`.
    pub fn start_duration_scan(&mut self) {
        // Signal any running scan to stop.
        self.scan_stop.store(true, Ordering::SeqCst);
        // Create a fresh stop flag for any new scan cycle.
        self.scan_stop = Arc::new(AtomicBool::new(false));
    }

    /// Check whether the background scan has been signalled to stop.
    pub fn is_scan_stopped(&self) -> bool {
        self.scan_stop.load(Ordering::SeqCst)
    }

    /// Get a reference to the scan stop flag for external use.
    pub fn scan_stop_flag(&self) -> Arc<AtomicBool> {
        Arc::clone(&self.scan_stop)
    }

    // -----------------------------------------------------------------------
    // Private helpers
    // -----------------------------------------------------------------------

    /// Compute folder statistics: finished/remaining counts, progress, etc.
    fn _folder_stats(&self) -> Value {
        let total = self.fids.len();
        let mut finished_count: usize = 0;
        let mut total_duration: f64 = 0.0;
        let mut total_watched: f64 = 0.0;
        let mut all_durations_known = true;
        let mut remaining_seconds: f64 = 0.0;

        // Track top-level folder progress.
        let mut folder_totals: HashMap<String, (usize, usize)> = HashMap::new(); // (total, finished)

        for fid in &self.fids {
            let meta = self.state.videos.get(fid);
            let finished = meta.map(|m| m.finished).unwrap_or(false);
            let duration = meta.and_then(|m| m.duration);
            let watched = meta.map(|m| m.watched).unwrap_or(0.0);

            if finished {
                finished_count += 1;
            }

            if let Some(d) = duration {
                total_duration += d;
                total_watched += watched.min(d);
                if !finished {
                    remaining_seconds += (d - watched).max(0.0);
                }
            } else {
                all_durations_known = false;
            }

            // Top-level folder grouping.
            let relpath = self.fid_to_rel.get(fid).cloned().unwrap_or_default();
            let top_folder = if let Some(pos) = relpath.find('/') {
                relpath[..pos].to_string()
            } else {
                String::new()
            };
            let entry = folder_totals.entry(top_folder).or_insert((0, 0));
            entry.0 += 1;
            if finished {
                entry.1 += 1;
            }
        }

        let overall_progress = if total > 0 {
            if total_duration > 0.0 {
                (total_watched / total_duration * 100.0).min(100.0)
            } else {
                (finished_count as f64 / total as f64) * 100.0
            }
        } else {
            0.0
        };

        let remaining_count = total - finished_count;

        // Build top_folders list (only if there are subdirs).
        let mut top_folders: Vec<Value> = Vec::new();
        let mut folder_names: Vec<String> = folder_totals.keys().cloned().collect();
        folder_names.sort_by(|a, b| natural_key(a).cmp(&natural_key(b)));
        for name in &folder_names {
            if name.is_empty() {
                continue; // Skip root-level files.
            }
            let (ft, ff) = folder_totals[name];
            top_folders.push(json!({
                "name": name,
                "total": ft,
                "finished": ff,
            }));
        }

        json!({
            "finished_count": finished_count,
            "remaining_count": remaining_count,
            "remaining_seconds_known": remaining_seconds,
            "overall_progress": overall_progress,
            "durations_known": all_durations_known,
            "top_folders": top_folders,
        })
    }

    /// Get basic file metadata (extension, size, mtime, folder).
    fn _basic_file_meta(&self, fid: &str) -> Value {
        let relpath = self.fid_to_rel.get(fid).cloned().unwrap_or_default();
        let index = self.fids.iter().position(|f| f == fid);

        if let Some(i) = index {
            if i < self.files.len() {
                let path = &self.files[i];
                let ext = path
                    .extension()
                    .map(|e| e.to_string_lossy().to_string())
                    .unwrap_or_default();

                let (size, mtime) = match fs::metadata(path) {
                    Ok(m) => {
                        let size = m.len();
                        let mtime = m
                            .modified()
                            .ok()
                            .and_then(|t| t.duration_since(SystemTime::UNIX_EPOCH).ok())
                            .map(|d| d.as_secs())
                            .unwrap_or(0);
                        (size, mtime)
                    }
                    Err(_) => (0, 0),
                };

                let folder = path
                    .parent()
                    .map(|p| {
                        let s = p.to_string_lossy().to_string();
                        s.strip_prefix("\\\\?\\").unwrap_or(&s).to_string()
                    })
                    .unwrap_or_default();

                return json!({
                    "fid": fid,
                    "relpath": relpath,
                    "ext": ext,
                    "size": size,
                    "mtime": mtime,
                    "folder": folder,
                });
            }
        }

        json!({
            "fid": fid,
            "relpath": relpath,
            "ext": "",
            "size": 0,
            "mtime": 0,
            "folder": "",
        })
    }
}

// ---------------------------------------------------------------------------
// Module-level helpers
// ---------------------------------------------------------------------------

/// Normalize a stem for fuzzy matching (lowercase, collapse separators).
fn _normalize_stem(s: &str) -> String {
    let lower = s.to_lowercase();
    let re = regex::Regex::new(r"[-_\s]+").unwrap();
    let replaced = re.replace_all(&lower, " ");
    replaced.trim().to_string()
}

/// Convert a language suffix to a human-readable label.
fn _language_label(lang: &str) -> String {
    match lang.to_lowercase().as_str() {
        "en" | "eng" | "english" => "English".to_string(),
        "fr" | "fre" | "fra" | "french" => "French".to_string(),
        "de" | "deu" | "ger" | "german" => "German".to_string(),
        "es" | "spa" | "spanish" => "Spanish".to_string(),
        "it" | "ita" | "italian" => "Italian".to_string(),
        "pt" | "por" | "portuguese" => "Portuguese".to_string(),
        "ru" | "rus" | "russian" => "Russian".to_string(),
        "ja" | "jpn" | "japanese" => "Japanese".to_string(),
        "ko" | "kor" | "korean" => "Korean".to_string(),
        "zh" | "zho" | "chi" | "chinese" => "Chinese".to_string(),
        "ar" | "ara" | "arabic" => "Arabic".to_string(),
        "hi" | "hin" | "hindi" => "Hindi".to_string(),
        "nl" | "dut" | "nld" | "dutch" => "Dutch".to_string(),
        "sv" | "swe" | "swedish" => "Swedish".to_string(),
        "pl" | "pol" | "polish" => "Polish".to_string(),
        other => {
            // Capitalize first letter.
            let mut c = other.chars();
            match c.next() {
                Some(first) => {
                    let upper: String = first.to_uppercase().collect();
                    format!("{}{}", upper, c.as_str())
                }
                None => String::new(),
            }
        }
    }
}

// ===========================================================================
// Tests
// ===========================================================================

#[cfg(test)]
mod tests {
    use super::*;
    use std::fs;
    use tempfile::TempDir;

    // -----------------------------------------------------------------------
    // 1. test_new_library_is_empty
    // -----------------------------------------------------------------------

    #[test]
    fn test_new_library_is_empty() {
        let lib = Library::new();
        assert!(lib.root.is_none());
        assert!(lib.files.is_empty());
        assert!(lib.fids.is_empty());
        assert!(lib.relpaths.is_empty());
        assert!(lib.rel_to_fid.is_empty());
        assert!(lib.fid_to_rel.is_empty());
        assert!(lib.state_path.is_none());
        assert!(lib.state.current_fid.is_none());
        assert_eq!(lib.state.library_id, "");
        assert_eq!(lib.state.volume, 1.0);
        assert_eq!(lib.state.playback_rate, 1.0);
        assert!(!lib.state.autoplay);
    }

    // -----------------------------------------------------------------------
    // 2. test_sorted_default
    // -----------------------------------------------------------------------

    #[test]
    fn test_sorted_default() {
        let relpaths = vec![
            "video10.mp4".to_string(),
            "video2.mp4".to_string(),
            "video1.mp4".to_string(),
            "video20.mp4".to_string(),
        ];
        let mut rel_to_fid = HashMap::new();
        for (i, rel) in relpaths.iter().enumerate() {
            rel_to_fid.insert(rel.clone(), format!("fid{}", i));
        }

        let sorted = _sorted_default(&relpaths, &rel_to_fid);
        let sorted_rels: Vec<String> = sorted
            .iter()
            .map(|fid| {
                rel_to_fid
                    .iter()
                    .find(|(_, v)| *v == fid)
                    .map(|(k, _)| k.clone())
                    .unwrap()
            })
            .collect();

        assert_eq!(
            sorted_rels,
            vec![
                "video1.mp4".to_string(),
                "video2.mp4".to_string(),
                "video10.mp4".to_string(),
                "video20.mp4".to_string(),
            ]
        );
    }

    // -----------------------------------------------------------------------
    // 3. test_apply_saved_order_with_valid_order
    // -----------------------------------------------------------------------

    #[test]
    fn test_apply_saved_order_with_valid_order() {
        let all_fids = vec!["a".to_string(), "b".to_string(), "c".to_string()];
        let mut fid_to_rel = HashMap::new();
        fid_to_rel.insert("a".to_string(), "01.mp4".to_string());
        fid_to_rel.insert("b".to_string(), "02.mp4".to_string());
        fid_to_rel.insert("c".to_string(), "03.mp4".to_string());

        let order = vec!["c".to_string(), "a".to_string(), "b".to_string()];
        let result = _apply_saved_order(&all_fids, &fid_to_rel, &order);
        assert_eq!(result, vec!["c", "a", "b"]);
    }

    // -----------------------------------------------------------------------
    // 4. test_apply_saved_order_with_new_files
    // -----------------------------------------------------------------------

    #[test]
    fn test_apply_saved_order_with_new_files() {
        let all_fids = vec![
            "a".to_string(),
            "b".to_string(),
            "c".to_string(),
            "d".to_string(),
        ];
        let mut fid_to_rel = HashMap::new();
        fid_to_rel.insert("a".to_string(), "01.mp4".to_string());
        fid_to_rel.insert("b".to_string(), "02.mp4".to_string());
        fid_to_rel.insert("c".to_string(), "03.mp4".to_string());
        fid_to_rel.insert("d".to_string(), "04.mp4".to_string());

        // Saved order only has a and c; b and d are "new".
        let order = vec!["c".to_string(), "a".to_string()];
        let result = _apply_saved_order(&all_fids, &fid_to_rel, &order);

        // c, a come first (saved order), then b, d (naturally sorted).
        assert_eq!(result, vec!["c", "a", "b", "d"]);
    }

    // -----------------------------------------------------------------------
    // 5. test_apply_saved_order_empty
    // -----------------------------------------------------------------------

    #[test]
    fn test_apply_saved_order_empty() {
        let all_fids = vec!["b".to_string(), "a".to_string(), "c".to_string()];
        let mut fid_to_rel = HashMap::new();
        fid_to_rel.insert("a".to_string(), "01_alpha.mp4".to_string());
        fid_to_rel.insert("b".to_string(), "02_beta.mp4".to_string());
        fid_to_rel.insert("c".to_string(), "03_gamma.mp4".to_string());

        let result = _apply_saved_order(&all_fids, &fid_to_rel, &[]);

        // Falls back to natural sort by relpath.
        assert_eq!(result, vec!["a", "b", "c"]);
    }

    // -----------------------------------------------------------------------
    // 6. test_tree_flags_flat
    // -----------------------------------------------------------------------

    #[test]
    fn test_tree_flags_flat() {
        let rels = vec![
            "video1.mp4".to_string(),
            "video2.mp4".to_string(),
            "video3.mp4".to_string(),
        ];
        let flags = _tree_flags(&rels);

        for rel in &rels {
            let f = flags.get(rel).unwrap();
            assert_eq!(f["depth"], 0);
            assert_eq!(f["pipes"], "");
        }

        // Last file should have is_last = true.
        assert_eq!(flags["video3.mp4"]["is_last"], true);
        // First file should not have has_prev_in_parent.
        assert_eq!(flags["video1.mp4"]["has_prev_in_parent"], false);
        // Second file should have has_prev_in_parent.
        assert_eq!(flags["video2.mp4"]["has_prev_in_parent"], true);
    }

    // -----------------------------------------------------------------------
    // 7. test_tree_flags_nested
    // -----------------------------------------------------------------------

    #[test]
    fn test_tree_flags_nested() {
        let rels = vec![
            "intro.mp4".to_string(),
            "chapter1/lesson1.mp4".to_string(),
            "chapter1/lesson2.mp4".to_string(),
            "chapter2/lesson1.mp4".to_string(),
        ];
        let flags = _tree_flags(&rels);

        // Root-level file.
        assert_eq!(flags["intro.mp4"]["depth"], 0);

        // Nested files.
        assert_eq!(flags["chapter1/lesson1.mp4"]["depth"], 1);
        assert_eq!(flags["chapter1/lesson2.mp4"]["depth"], 1);
        assert_eq!(flags["chapter2/lesson1.mp4"]["depth"], 1);

        // chapter1/lesson1 should not have prev in parent.
        assert_eq!(flags["chapter1/lesson1.mp4"]["has_prev_in_parent"], false);
        // chapter1/lesson2 should have prev in parent.
        assert_eq!(flags["chapter1/lesson2.mp4"]["has_prev_in_parent"], true);
        // chapter1/lesson2 is the last in chapter1.
        assert_eq!(flags["chapter1/lesson2.mp4"]["is_last"], true);
        // chapter2/lesson1 is the only (and last) in chapter2.
        assert_eq!(flags["chapter2/lesson1.mp4"]["is_last"], true);
    }

    // -----------------------------------------------------------------------
    // 8. test_set_root_with_video_files
    // -----------------------------------------------------------------------

    #[test]
    fn test_set_root_with_video_files() {
        let dir = TempDir::new().unwrap();
        let state_dir = TempDir::new().unwrap();

        // Create dummy video files.
        fs::write(dir.path().join("video_a.mp4"), b"fake video a content").unwrap();
        fs::write(dir.path().join("video_b.mp4"), b"fake video b content").unwrap();
        fs::write(dir.path().join("video_c.mp4"), b"fake video c content").unwrap();

        // Also create a non-video file (should be ignored).
        fs::write(dir.path().join("readme.txt"), b"not a video").unwrap();

        let mut lib = Library::new();
        let folder_str = dir.path().to_string_lossy().to_string();
        let result = lib.set_root(&folder_str, state_dir.path());

        assert!(result.is_ok(), "set_root failed: {:?}", result.err());

        let info = result.unwrap();
        assert_eq!(info["ok"], true);
        assert_eq!(info["count"], 3);
        assert_eq!(lib.fids.len(), 3);
        assert_eq!(lib.relpaths.len(), 3);
        assert_eq!(lib.files.len(), 3);

        // All relpaths should be video files.
        for rel in &lib.relpaths {
            assert!(rel.ends_with(".mp4"), "unexpected relpath: {}", rel);
        }

        // State file should exist.
        assert!(lib.state_path.is_some());
        assert!(lib.state_path.as_ref().unwrap().exists());

        // Library ID should be non-empty.
        assert!(!lib.state.library_id.is_empty());
    }

    // -----------------------------------------------------------------------
    // 9. test_update_progress_finished_sticky
    // -----------------------------------------------------------------------

    #[test]
    fn test_update_progress_finished_sticky() {
        let dir = TempDir::new().unwrap();
        let state_dir = TempDir::new().unwrap();

        fs::write(dir.path().join("video.mp4"), b"content").unwrap();

        let mut lib = Library::new();
        let folder_str = dir.path().to_string_lossy().to_string();
        lib.set_root(&folder_str, state_dir.path()).unwrap();

        // Set duration and advance to near the end to trigger finished.
        let result = lib.update_progress(0, 98.5, Some(100.0), true);
        assert_eq!(result["finished"], true);

        // Now go back to an earlier time -- finished should remain true.
        let result = lib.update_progress(0, 10.0, Some(100.0), true);
        assert_eq!(result["finished"], true);

        // Verify pos updated but finished stayed sticky.
        assert_eq!(result["pos"], 10.0);

        // Watched should still be the high-water mark.
        assert_eq!(result["watched"], 98.5);
    }

    // -----------------------------------------------------------------------
    // 10. test_reset_watch_progress
    // -----------------------------------------------------------------------

    #[test]
    fn test_reset_watch_progress() {
        let dir = TempDir::new().unwrap();
        let state_dir = TempDir::new().unwrap();

        fs::write(dir.path().join("vid1.mp4"), b"video 1").unwrap();
        fs::write(dir.path().join("vid2.mp4"), b"video 2").unwrap();

        let mut lib = Library::new();
        let folder_str = dir.path().to_string_lossy().to_string();
        lib.set_root(&folder_str, state_dir.path()).unwrap();

        // Add some progress and notes.
        lib.update_progress(0, 50.0, Some(100.0), true);
        lib.update_progress(1, 99.0, Some(100.0), true);
        lib.set_note(&lib.fids[0].clone(), "Important lecture");

        // Verify pre-conditions.
        let fid0 = lib.fids[0].clone();
        let fid1 = lib.fids[1].clone();
        assert!(lib.state.videos[&fid0].watched > 0.0);
        assert!(lib.state.videos[&fid1].finished);
        assert!(!lib.state.videos[&fid0].note.is_empty());
        assert!(lib.state.videos[&fid0].duration.is_some());

        // Reset.
        let result = lib.reset_watch_progress();
        assert_eq!(result["ok"], true);

        // Progress should be reset.
        assert_eq!(lib.state.videos[&fid0].pos, 0.0);
        assert_eq!(lib.state.videos[&fid0].watched, 0.0);
        assert!(!lib.state.videos[&fid0].finished);
        assert_eq!(lib.state.videos[&fid1].pos, 0.0);
        assert_eq!(lib.state.videos[&fid1].watched, 0.0);
        assert!(!lib.state.videos[&fid1].finished);

        // Notes and durations should be preserved.
        assert_eq!(lib.state.videos[&fid0].note, "Important lecture");
        assert!(lib.state.videos[&fid0].duration.is_some());
        assert!(lib.state.videos[&fid1].duration.is_some());
    }
}