//! Bitcask-style file storage adapter.
//!
//! Each collection is stored as a directory:
//! ```text
//! {base_path}/{db}/{coll}/
//!   data.rdb       — append-only binary record log
//!   wal.rdb        — binary write-ahead log
//!   keydir.hint    — persisted KeyDir for fast restart
//!   indexes.json   — index metadata
//! ```
//!
//! Point operations (insert, update, delete, find_by_id) are O(1).
//! Full scans (find_all) are O(n) in live document count.

use std::collections::HashSet;
use std::io::{Seek, SeekFrom, Write};
use std::path::PathBuf;
use std::sync::atomic::{AtomicU64, Ordering};
use std::sync::Arc;

use async_trait::async_trait;
use bson::{doc, oid::ObjectId, Document};
use dashmap::DashMap;
use tracing::debug;

use crate::adapter::StorageAdapter;
use crate::binary_wal::{BinaryWal, WalOpType};
use crate::error::{StorageError, StorageResult};
use crate::keydir::{KeyDir, KeyDirEntry};
use crate::record::{now_ms, DataRecord, FileHeader, FileType, FILE_HEADER_SIZE};

// ---------------------------------------------------------------------------
// Per-collection state
// ---------------------------------------------------------------------------

struct CollectionState {
    /// In-memory document location index.
    keydir: KeyDir,
    /// Path to the collection directory.
    coll_dir: PathBuf,
    /// Write-ahead log.
    wal: std::sync::Mutex<BinaryWal>,
    /// Serializes append writes to the data file.
    write_lock: std::sync::Mutex<()>,
    /// Live document count.
    doc_count: AtomicU64,
    /// Bytes occupied by dead (superseded/tombstone) records. For compaction.
    dead_bytes: AtomicU64,
    /// Total size of the data file.
    data_file_size: AtomicU64,
}

impl CollectionState {
    fn data_path(&self) -> PathBuf {
        self.coll_dir.join("data.rdb")
    }

    fn hint_path(&self) -> PathBuf {
        self.coll_dir.join("keydir.hint")
    }

    fn index_path(&self) -> PathBuf {
        self.coll_dir.join("indexes.json")
    }

    /// Read a single document from the data file at the given KeyDir entry.
    fn read_document(&self, entry: &KeyDirEntry) -> StorageResult<Document> {
        let data_path = self.data_path();
        let mut file = std::fs::File::open(&data_path)?;
        file.seek(SeekFrom::Start(entry.offset))?;

        let (record, _) = DataRecord::decode_from(&mut file)?
            .ok_or_else(|| StorageError::CorruptRecord("unexpected EOF reading document".into()))?;

        if record.is_tombstone() {
            return Err(StorageError::CorruptRecord(
                "KeyDir pointed to a tombstone record".into(),
            ));
        }

        // Parse BSON from raw bytes
        bson::from_slice(&record.value)
            .map_err(|e| StorageError::SerializationError(format!("BSON decode: {e}")))
    }

    /// Append a data record and update the KeyDir. Must be called under write_lock.
    fn append_record(
        &self,
        key: &str,
        bson_bytes: &[u8],
        timestamp: u64,
    ) -> StorageResult<KeyDirEntry> {
        let record = DataRecord {
            timestamp,
            key: key.as_bytes().to_vec(),
            value: bson_bytes.to_vec(),
        };
        let encoded = record.encode();
        let disk_size = encoded.len() as u32;

        let data_path = self.data_path();
        let mut file = std::fs::OpenOptions::new()
            .create(true)
            .append(true)
            .open(&data_path)?;

        let offset = file.seek(SeekFrom::End(0))?;
        file.write_all(&encoded)?;
        file.sync_all()?;

        let entry = KeyDirEntry {
            offset,
            record_len: disk_size,
            value_len: bson_bytes.len() as u32,
            timestamp,
        };

        self.data_file_size
            .fetch_add(disk_size as u64, Ordering::Relaxed);

        Ok(entry)
    }

    /// Append a tombstone record. Must be called under write_lock.
    fn append_tombstone(&self, key: &str, timestamp: u64) -> StorageResult<u32> {
        let record = DataRecord {
            timestamp,
            key: key.as_bytes().to_vec(),
            value: vec![],
        };
        let encoded = record.encode();
        let disk_size = encoded.len() as u32;

        let data_path = self.data_path();
        let mut file = std::fs::OpenOptions::new()
            .create(true)
            .append(true)
            .open(&data_path)?;

        file.write_all(&encoded)?;
        file.sync_all()?;

        self.data_file_size
            .fetch_add(disk_size as u64, Ordering::Relaxed);

        Ok(disk_size)
    }

    /// Try compaction if dead bytes exceed the threshold.
    /// Must be called while holding the write_lock.
    fn try_compact(&self) {
        let dead = self.dead_bytes.load(Ordering::Relaxed);
        let total = self.data_file_size.load(Ordering::Relaxed);
        if crate::compaction::should_compact(dead, total) {
            debug!("triggering compaction for {:?} (dead={dead}, total={total})", self.coll_dir);
            if let Err(e) = crate::compaction::compact_data_file(
                &self.data_path(),
                &self.keydir,
                &self.dead_bytes,
                &self.data_file_size,
            ) {
                tracing::warn!("compaction failed for {:?}: {e}", self.coll_dir);
            }
        }
    }
}

// ---------------------------------------------------------------------------
// Collection cache key: "db\0coll"
// ---------------------------------------------------------------------------

fn coll_cache_key(db: &str, coll: &str) -> String {
    format!("{db}\0{coll}")
}

// ---------------------------------------------------------------------------
// FileStorageAdapter
// ---------------------------------------------------------------------------

/// Bitcask-style file storage adapter.
pub struct FileStorageAdapter {
    base_path: PathBuf,
    /// Cache of initialized collection states, keyed by "db\0coll".
    collections: DashMap<String, Arc<CollectionState>>,
    /// Handle to the periodic compaction task. Dropped on close.
    compaction_handle: std::sync::Mutex<Option<tokio::task::JoinHandle<()>>>,
}

impl FileStorageAdapter {
    pub fn new(base_path: impl Into<PathBuf>) -> Self {
        Self {
            base_path: base_path.into(),
            collections: DashMap::new(),
            compaction_handle: std::sync::Mutex::new(None),
        }
    }

    fn db_dir(&self, db: &str) -> PathBuf {
        self.base_path.join(db)
    }

    fn coll_dir(&self, db: &str, coll: &str) -> PathBuf {
        self.db_dir(db).join(coll)
    }

    /// Get or initialize the CollectionState for a given db/coll.
    fn get_or_init_collection(
        &self,
        db: &str,
        coll: &str,
    ) -> StorageResult<Arc<CollectionState>> {
        let key = coll_cache_key(db, coll);
        if let Some(state) = self.collections.get(&key) {
            return Ok(Arc::clone(state.value()));
        }

        let coll_dir = self.coll_dir(db, coll);
        if !coll_dir.exists() {
            return Err(StorageError::NotFound(format!(
                "collection '{db}.{coll}'"
            )));
        }

        let state = self.init_collection_state(&coll_dir)?;
        let state = Arc::new(state);
        self.collections.insert(key, Arc::clone(&state));
        Ok(state)
    }

    /// Initialize a CollectionState from disk.
    fn init_collection_state(&self, coll_dir: &PathBuf) -> StorageResult<CollectionState> {
        let data_path = coll_dir.join("data.rdb");
        let wal_path = coll_dir.join("wal.rdb");
        let hint_path = coll_dir.join("keydir.hint");

        // Try loading from hint file first, fall back to data file scan
        let (keydir, dead_bytes) = if hint_path.exists() && data_path.exists() {
            match KeyDir::load_from_hint_file(&hint_path) {
                Ok(Some(kd)) => {
                    debug!("loaded KeyDir from hint file: {:?}", hint_path);
                    // We don't know dead_bytes from the hint file; estimate from file size
                    let file_size = std::fs::metadata(&data_path)
                        .map(|m| m.len())
                        .unwrap_or(FILE_HEADER_SIZE as u64);
                    let live_bytes: u64 = {
                        let mut total = 0u64;
                        kd.for_each(|_, e| total += e.record_len as u64);
                        total
                    };
                    let dead = file_size.saturating_sub(FILE_HEADER_SIZE as u64).saturating_sub(live_bytes);
                    (kd, dead)
                }
                _ => {
                    debug!("hint file invalid, rebuilding KeyDir from data file");
                    KeyDir::build_from_data_file(&data_path)?
                }
            }
        } else if data_path.exists() {
            KeyDir::build_from_data_file(&data_path)?
        } else {
            (KeyDir::new(), 0)
        };

        let doc_count = keydir.len();
        let data_file_size = if data_path.exists() {
            std::fs::metadata(&data_path)?.len()
        } else {
            FILE_HEADER_SIZE as u64
        };

        // Initialize WAL and recover
        let wal = BinaryWal::new(wal_path);
        wal.initialize()?;

        // Recover uncommitted WAL entries
        let uncommitted = wal.recover()?;
        if !uncommitted.is_empty() {
            debug!(
                "recovering {} uncommitted WAL entries for {:?}",
                uncommitted.len(),
                coll_dir
            );
        }

        let state = CollectionState {
            keydir,
            coll_dir: coll_dir.clone(),
            wal: std::sync::Mutex::new(wal),
            write_lock: std::sync::Mutex::new(()),
            doc_count: AtomicU64::new(doc_count),
            dead_bytes: AtomicU64::new(dead_bytes),
            data_file_size: AtomicU64::new(data_file_size),
        };

        // Replay uncommitted entries
        for entry in uncommitted {
            let key_str = String::from_utf8(entry.key.clone())
                .map_err(|e| StorageError::CorruptRecord(format!("invalid UTF-8 key in WAL: {e}")))?;

            match entry.op {
                WalOpType::Insert | WalOpType::Update => {
                    if !entry.value.is_empty() {
                        let ts = now_ms();
                        let new_entry = state.append_record(&key_str, &entry.value, ts)?;

                        if entry.op == WalOpType::Insert {
                            if state.keydir.insert(key_str, new_entry).is_none() {
                                state.doc_count.fetch_add(1, Ordering::Relaxed);
                            }
                        } else {
                            if let Some(prev) = state.keydir.insert(key_str, new_entry) {
                                state.dead_bytes.fetch_add(prev.record_len as u64, Ordering::Relaxed);
                            }
                        }
                    }
                }
                WalOpType::Delete => {
                    let ts = now_ms();
                    let tombstone_size = state.append_tombstone(&key_str, ts)?;
                    if let Some(prev) = state.keydir.remove(&key_str) {
                        state.dead_bytes.fetch_add(prev.record_len as u64 + tombstone_size as u64, Ordering::Relaxed);
                        state.doc_count.fetch_sub(1, Ordering::Relaxed);
                    }
                }
            }
        }

        // Truncate WAL after successful recovery
        if let Ok(wal) = state.wal.lock() {
            wal.truncate()?;
        }

        Ok(state)
    }

    /// Create a fresh, empty collection on disk. Returns the new CollectionState.
    fn create_empty_collection(&self, db: &str, coll: &str) -> StorageResult<Arc<CollectionState>> {
        let coll_dir = self.coll_dir(db, coll);
        std::fs::create_dir_all(&coll_dir)?;

        // Write empty data file with header
        let data_path = coll_dir.join("data.rdb");
        {
            let mut f = std::fs::File::create(&data_path)?;
            let hdr = FileHeader::new(FileType::Data);
            f.write_all(&hdr.encode())?;
            f.sync_all()?;
        }

        // Write default _id index
        let idx_spec = vec![doc! { "name": "_id_", "key": { "_id": 1 } }];
        let idx_json = serde_json::to_string_pretty(&idx_spec)?;
        let idx_path = coll_dir.join("indexes.json");
        std::fs::write(&idx_path, idx_json)?;

        // Initialize WAL
        let wal_path = coll_dir.join("wal.rdb");
        let wal = BinaryWal::new(wal_path);
        wal.initialize()?;

        let state = Arc::new(CollectionState {
            keydir: KeyDir::new(),
            coll_dir,
            wal: std::sync::Mutex::new(wal),
            write_lock: std::sync::Mutex::new(()),
            doc_count: AtomicU64::new(0),
            dead_bytes: AtomicU64::new(0),
            data_file_size: AtomicU64::new(FILE_HEADER_SIZE as u64),
        });

        let key = coll_cache_key(db, coll);
        self.collections.insert(key, Arc::clone(&state));

        Ok(state)
    }

    fn extract_id_hex(doc: &Document) -> StorageResult<String> {
        match doc.get("_id") {
            Some(bson::Bson::ObjectId(oid)) => Ok(oid.to_hex()),
            Some(bson::Bson::String(s)) => Ok(s.clone()),
            Some(other) => Ok(format!("{other}")),
            None => Err(StorageError::NotFound("document missing _id".into())),
        }
    }

    /// Scan database directory for collection subdirectories.
    fn list_collection_dirs(&self, db: &str) -> StorageResult<Vec<String>> {
        let db_dir = self.db_dir(db);
        if !db_dir.exists() {
            return Err(StorageError::NotFound(format!("database '{db}'")));
        }
        let mut colls = Vec::new();
        let entries = std::fs::read_dir(&db_dir)?;
        for entry in entries {
            let entry = entry?;
            if entry.file_type()?.is_dir() {
                if let Some(name) = entry.file_name().to_str() {
                    // Only consider directories that contain data.rdb
                    let data_path = entry.path().join("data.rdb");
                    if data_path.exists() {
                        colls.push(name.to_string());
                    }
                }
            }
        }
        Ok(colls)
    }
}

// ---------------------------------------------------------------------------
// StorageAdapter implementation
// ---------------------------------------------------------------------------

#[async_trait]
impl StorageAdapter for FileStorageAdapter {
    async fn initialize(&self) -> StorageResult<()> {
        std::fs::create_dir_all(&self.base_path)?;
        debug!("FileStorageAdapter initialized at {:?}", self.base_path);

        // Pre-load all existing collections
        if let Ok(entries) = std::fs::read_dir(&self.base_path) {
            for entry in entries.flatten() {
                if entry.file_type().map(|ft| ft.is_dir()).unwrap_or(false) {
                    if let Some(db_name) = entry.file_name().to_str() {
                        let db_name = db_name.to_string();
                        if let Ok(colls) = self.list_collection_dirs(&db_name) {
                            for coll_name in colls {
                                let _ = self.get_or_init_collection(&db_name, &coll_name);
                            }
                        }
                    }
                }
            }
        }

        // Start periodic compaction task (runs every 24 hours)
        {
            let collections = self.collections.clone();
            let handle = tokio::spawn(async move {
                let mut interval = tokio::time::interval(std::time::Duration::from_secs(24 * 60 * 60));
                interval.tick().await; // first tick is immediate — skip it
                loop {
                    interval.tick().await;
                    debug!("running periodic compaction sweep");
                    for entry in collections.iter() {
                        let state = entry.value();
                        let _guard = state.write_lock.lock().unwrap();
                        state.try_compact();
                    }
                }
            });
            *self.compaction_handle.lock().unwrap() = Some(handle);
        }

        Ok(())
    }

    async fn close(&self) -> StorageResult<()> {
        // Stop the periodic compaction task
        if let Some(handle) = self.compaction_handle.lock().unwrap().take() {
            handle.abort();
        }

        // Persist all KeyDir hint files
        for entry in self.collections.iter() {
            let state = entry.value();
            let _ = state.keydir.persist_to_hint_file(&state.hint_path());
        }
        debug!("FileStorageAdapter closed");
        Ok(())
    }

    // ---- database ----

    async fn list_databases(&self) -> StorageResult<Vec<String>> {
        let mut dbs = Vec::new();
        let entries = std::fs::read_dir(&self.base_path)?;
        for entry in entries {
            let entry = entry?;
            if entry.file_type()?.is_dir() {
                if let Some(name) = entry.file_name().to_str() {
                    dbs.push(name.to_string());
                }
            }
        }
        Ok(dbs)
    }

    async fn create_database(&self, db: &str) -> StorageResult<()> {
        let dir = self.db_dir(db);
        if dir.exists() {
            return Err(StorageError::AlreadyExists(format!("database '{db}'")));
        }
        std::fs::create_dir_all(&dir)?;
        Ok(())
    }

    async fn drop_database(&self, db: &str) -> StorageResult<()> {
        // Remove cached states for all collections in this db
        let prefix = format!("{db}\0");
        self.collections.retain(|k, _| !k.starts_with(&prefix));

        let dir = self.db_dir(db);
        if dir.exists() {
            std::fs::remove_dir_all(&dir)?;
        }
        Ok(())
    }

    async fn database_exists(&self, db: &str) -> StorageResult<bool> {
        Ok(self.db_dir(db).exists())
    }

    // ---- collection ----

    async fn list_collections(&self, db: &str) -> StorageResult<Vec<String>> {
        self.list_collection_dirs(db)
    }

    async fn create_collection(&self, db: &str, coll: &str) -> StorageResult<()> {
        let coll_dir = self.coll_dir(db, coll);
        if coll_dir.exists() {
            return Err(StorageError::AlreadyExists(format!(
                "collection '{db}.{coll}'"
            )));
        }
        std::fs::create_dir_all(self.db_dir(db))?;
        self.create_empty_collection(db, coll)?;
        Ok(())
    }

    async fn drop_collection(&self, db: &str, coll: &str) -> StorageResult<()> {
        let key = coll_cache_key(db, coll);
        self.collections.remove(&key);

        let coll_dir = self.coll_dir(db, coll);
        if coll_dir.exists() {
            std::fs::remove_dir_all(&coll_dir)?;
        }
        Ok(())
    }

    async fn collection_exists(&self, db: &str, coll: &str) -> StorageResult<bool> {
        let coll_dir = self.coll_dir(db, coll);
        Ok(coll_dir.exists() && coll_dir.join("data.rdb").exists())
    }

    async fn rename_collection(
        &self,
        db: &str,
        old_name: &str,
        new_name: &str,
    ) -> StorageResult<()> {
        let old_dir = self.coll_dir(db, old_name);
        let new_dir = self.coll_dir(db, new_name);
        if !old_dir.exists() {
            return Err(StorageError::NotFound(format!(
                "collection '{db}.{old_name}'"
            )));
        }
        if new_dir.exists() {
            return Err(StorageError::AlreadyExists(format!(
                "collection '{db}.{new_name}'"
            )));
        }

        // Remove old cache entry
        let old_key = coll_cache_key(db, old_name);
        self.collections.remove(&old_key);

        // Rename directory
        std::fs::rename(&old_dir, &new_dir)?;

        // Re-init under new name
        let _ = self.get_or_init_collection(db, new_name);

        Ok(())
    }

    // ---- document writes ----

    async fn insert_one(
        &self,
        db: &str,
        coll: &str,
        mut doc: Document,
    ) -> StorageResult<String> {
        if !doc.contains_key("_id") {
            doc.insert("_id", ObjectId::new());
        }
        let id = Self::extract_id_hex(&doc)?;

        let state = self.get_or_init_collection(db, coll)?;

        // Check for duplicate
        if state.keydir.contains(&id) {
            return Err(StorageError::AlreadyExists(format!("document '{id}'")));
        }

        let bson_bytes = bson::to_vec(&doc)
            .map_err(|e| StorageError::SerializationError(e.to_string()))?;
        let ts = now_ms();

        let _guard = state.write_lock.lock().unwrap();

        // WAL → data → KeyDir → WAL commit
        {
            let wal = state.wal.lock().unwrap();
            let seq = wal.append(WalOpType::Insert, id.as_bytes(), &bson_bytes)?;
            let entry = state.append_record(&id, &bson_bytes, ts)?;
            state.keydir.insert(id.clone(), entry);
            state.doc_count.fetch_add(1, Ordering::Relaxed);
            wal.append_commit(seq)?;
        }

        // Insert doesn't create dead bytes, but check anyway for consistency
        state.try_compact();

        Ok(id)
    }

    async fn insert_many(
        &self,
        db: &str,
        coll: &str,
        mut docs: Vec<Document>,
    ) -> StorageResult<Vec<String>> {
        let state = self.get_or_init_collection(db, coll)?;
        let mut ids = Vec::with_capacity(docs.len());

        let _guard = state.write_lock.lock().unwrap();

        for doc in &mut docs {
            if !doc.contains_key("_id") {
                doc.insert("_id", ObjectId::new());
            }
            let id = Self::extract_id_hex(doc)?;

            let bson_bytes = bson::to_vec(doc)
                .map_err(|e| StorageError::SerializationError(e.to_string()))?;
            let ts = now_ms();

            let wal = state.wal.lock().unwrap();
            let seq = wal.append(WalOpType::Insert, id.as_bytes(), &bson_bytes)?;
            let entry = state.append_record(&id, &bson_bytes, ts)?;
            state.keydir.insert(id.clone(), entry);
            state.doc_count.fetch_add(1, Ordering::Relaxed);
            wal.append_commit(seq)?;

            ids.push(id);
        }

        state.try_compact();

        Ok(ids)
    }

    async fn update_by_id(
        &self,
        db: &str,
        coll: &str,
        id: &str,
        doc: Document,
    ) -> StorageResult<()> {
        let state = self.get_or_init_collection(db, coll)?;

        if !state.keydir.contains(id) {
            return Err(StorageError::NotFound(format!("document '{id}'")));
        }

        let bson_bytes = bson::to_vec(&doc)
            .map_err(|e| StorageError::SerializationError(e.to_string()))?;
        let ts = now_ms();

        let _guard = state.write_lock.lock().unwrap();

        let wal = state.wal.lock().unwrap();
        let seq = wal.append(WalOpType::Update, id.as_bytes(), &bson_bytes)?;
        let new_entry = state.append_record(id, &bson_bytes, ts)?;

        if let Some(prev) = state.keydir.insert(id.to_string(), new_entry) {
            state
                .dead_bytes
                .fetch_add(prev.record_len as u64, Ordering::Relaxed);
        }
        wal.append_commit(seq)?;

        state.try_compact();

        Ok(())
    }

    async fn delete_by_id(
        &self,
        db: &str,
        coll: &str,
        id: &str,
    ) -> StorageResult<()> {
        let state = self.get_or_init_collection(db, coll)?;

        if !state.keydir.contains(id) {
            return Err(StorageError::NotFound(format!("document '{id}'")));
        }

        let ts = now_ms();
        let _guard = state.write_lock.lock().unwrap();

        let wal = state.wal.lock().unwrap();
        let seq = wal.append(WalOpType::Delete, id.as_bytes(), &[])?;
        let tombstone_size = state.append_tombstone(id, ts)?;

        if let Some(prev) = state.keydir.remove(id) {
            state.dead_bytes.fetch_add(
                prev.record_len as u64 + tombstone_size as u64,
                Ordering::Relaxed,
            );
            state.doc_count.fetch_sub(1, Ordering::Relaxed);
        }
        wal.append_commit(seq)?;

        state.try_compact();

        Ok(())
    }

    async fn delete_by_ids(
        &self,
        db: &str,
        coll: &str,
        ids: &[String],
    ) -> StorageResult<()> {
        let state = self.get_or_init_collection(db, coll)?;
        let _guard = state.write_lock.lock().unwrap();

        for id in ids {
            if !state.keydir.contains(id) {
                continue; // skip non-existent
            }

            let ts = now_ms();
            let wal = state.wal.lock().unwrap();
            let seq = wal.append(WalOpType::Delete, id.as_bytes(), &[])?;
            let tombstone_size = state.append_tombstone(id, ts)?;

            if let Some(prev) = state.keydir.remove(id) {
                state.dead_bytes.fetch_add(
                    prev.record_len as u64 + tombstone_size as u64,
                    Ordering::Relaxed,
                );
                state.doc_count.fetch_sub(1, Ordering::Relaxed);
            }
            wal.append_commit(seq)?;
        }

        state.try_compact();

        Ok(())
    }

    // ---- document reads ----

    async fn find_all(&self, db: &str, coll: &str) -> StorageResult<Vec<Document>> {
        let state = self.get_or_init_collection(db, coll)?;
        let mut docs = Vec::with_capacity(state.doc_count.load(Ordering::Relaxed) as usize);

        state.keydir.for_each(|_key, entry| {
            // We collect entries first, then read. This avoids holding DashMap refs during I/O.
            docs.push(*entry);
        });

        let mut result = Vec::with_capacity(docs.len());
        for entry in &docs {
            match state.read_document(entry) {
                Ok(doc) => result.push(doc),
                Err(e) => {
                    tracing::warn!("skipping unreadable document: {e}");
                }
            }
        }

        Ok(result)
    }

    async fn find_by_ids(
        &self,
        db: &str,
        coll: &str,
        ids: HashSet<String>,
    ) -> StorageResult<Vec<Document>> {
        let state = self.get_or_init_collection(db, coll)?;
        let mut docs = Vec::with_capacity(ids.len());

        for id in &ids {
            if let Some(entry) = state.keydir.get(id) {
                match state.read_document(&entry) {
                    Ok(doc) => docs.push(doc),
                    Err(e) => {
                        tracing::warn!("skipping unreadable document {id}: {e}");
                    }
                }
            }
        }

        Ok(docs)
    }

    async fn find_by_id(
        &self,
        db: &str,
        coll: &str,
        id: &str,
    ) -> StorageResult<Option<Document>> {
        let state = self.get_or_init_collection(db, coll)?;

        match state.keydir.get(id) {
            Some(entry) => {
                let doc = state.read_document(&entry)?;
                Ok(Some(doc))
            }
            None => Ok(None),
        }
    }

    async fn count(&self, db: &str, coll: &str) -> StorageResult<u64> {
        let state = self.get_or_init_collection(db, coll)?;
        Ok(state.doc_count.load(Ordering::Relaxed))
    }

    // ---- indexes ----

    async fn save_index(
        &self,
        db: &str,
        coll: &str,
        name: &str,
        spec: Document,
    ) -> StorageResult<()> {
        let state = self.get_or_init_collection(db, coll)?;
        let path = state.index_path();

        let mut indexes = self.read_index_file(&path)?;
        indexes.retain(|s| {
            s.get_str("name").unwrap_or("") != name
        });
        let mut full_spec = spec;
        full_spec.insert("name", name);
        indexes.push(full_spec);
        self.write_index_file(&path, &indexes)?;
        Ok(())
    }

    async fn get_indexes(
        &self,
        db: &str,
        coll: &str,
    ) -> StorageResult<Vec<Document>> {
        let state = self.get_or_init_collection(db, coll)?;
        self.read_index_file(&state.index_path())
    }

    async fn drop_index(
        &self,
        db: &str,
        coll: &str,
        name: &str,
    ) -> StorageResult<()> {
        let state = self.get_or_init_collection(db, coll)?;
        let path = state.index_path();

        let mut indexes = self.read_index_file(&path)?;
        let before = indexes.len();
        indexes.retain(|s| {
            s.get_str("name").unwrap_or("") != name
        });
        if indexes.len() == before {
            return Err(StorageError::NotFound(format!("index '{name}'")));
        }
        self.write_index_file(&path, &indexes)?;
        Ok(())
    }

    // ---- snapshot / conflict detection ----

    async fn create_snapshot(&self, _db: &str, _coll: &str) -> StorageResult<i64> {
        Ok(now_ms() as i64)
    }

    async fn has_conflicts(
        &self,
        db: &str,
        coll: &str,
        ids: &HashSet<String>,
        snapshot_time: i64,
    ) -> StorageResult<bool> {
        let state = self.get_or_init_collection(db, coll)?;
        let snapshot_ts = snapshot_time as u64;

        for id in ids {
            if let Some(entry) = state.keydir.get(id) {
                if entry.timestamp > snapshot_ts {
                    return Ok(true);
                }
            }
        }
        Ok(false)
    }
}

// ---------------------------------------------------------------------------
// Index file helpers (JSON — kept simple, rarely written)
// ---------------------------------------------------------------------------

impl FileStorageAdapter {
    fn read_index_file(&self, path: &PathBuf) -> StorageResult<Vec<Document>> {
        if !path.exists() {
            return Ok(vec![]);
        }
        let data = std::fs::read_to_string(path)?;
        let json_docs: Vec<serde_json::Value> = serde_json::from_str(&data)?;
        let mut docs = Vec::with_capacity(json_docs.len());
        for jv in json_docs {
            let bson_val: bson::Bson = serde_json::from_value(jv)
                .map_err(|e| StorageError::SerializationError(e.to_string()))?;
            let doc = bson_val
                .as_document()
                .ok_or_else(|| StorageError::SerializationError("expected document".into()))?
                .clone();
            docs.push(doc);
        }
        Ok(docs)
    }

    fn write_index_file(&self, path: &PathBuf, specs: &[Document]) -> StorageResult<()> {
        let json_vals: Vec<serde_json::Value> = specs
            .iter()
            .map(|d| {
                let b = bson::to_bson(d)
                    .map_err(|e| StorageError::SerializationError(e.to_string()))?;
                serde_json::to_value(&b)
                    .map_err(|e| StorageError::SerializationError(e.to_string()))
            })
            .collect::<StorageResult<Vec<_>>>()?;
        let json = serde_json::to_string_pretty(&json_vals)?;
        std::fs::write(path, json)?;
        Ok(())
    }
}