armdb 0.2.0

use std::collections::HashMap;
use std::hash::Hash;
use std::mem::size_of;
use std::sync::Arc;
use std::sync::atomic::{AtomicUsize, Ordering as AtomicOrdering};

use crate::Key;
use crate::byte_view::ByteView;
use crate::cache::{BlockCache, BlockKey};
use crate::compaction::{CompactionIndex, compact_shard};
use crate::config::Config;
use crate::disk_loc::DiskLoc;
use crate::engine::Engine;
use crate::error::{DbError, DbResult};
use crate::hook::{NoHook, WriteHook};
use crate::io::aligned_buf::AlignedBuf;
use crate::recovery::recover_var_map;
use crate::shard::ShardInner;
use crate::sync::{self, Mutex, MutexGuard};

const MAX_STALE_RETRIES: usize = 3;

/// A map with fixed-size keys and variable-length values.
/// Uses per-shard HashMap for O(1) lookup. Values are stored on disk,
/// cached at 4096-byte block granularity via `BlockCache`.
/// No ordered iteration — use `VarTree` if you need prefix/range scans.
///
/// Each `VarMap` owns its storage engine — one map = one database directory.
pub struct VarMap<K: Key + Send + Sync + Hash + Eq, H: WriteHook<K> = NoHook> {
    indexes: Vec<Mutex<HashMap<K, DiskLoc>>>,
    engine: Engine,
    cache: BlockCache,
    compaction_threshold: f64,
    shard_prefix_bits: usize,
    hook: H,
    entry_count: AtomicUsize,
}

impl<K: Key + Send + Sync + Hash + Eq> VarMap<K> {
    /// Open or create a `VarMap` at the given path.
    /// Recovers the index from existing data files on disk.
    pub fn open(path: impl AsRef<std::path::Path>, config: Config) -> DbResult<Self> {
        Self::open_inner(path, config, NoHook)
    }
}

impl<K: Key + Send + Sync + Hash + Eq, H: WriteHook<K>> VarMap<K, H> {
    /// Open or create a `VarMap` with a write hook for secondary index maintenance.
    pub fn open_hooked(
        path: impl AsRef<std::path::Path>,
        config: Config,
        hook: H,
    ) -> DbResult<Self> {
        Self::open_inner(path, config, hook)
    }

    fn open_inner(path: impl AsRef<std::path::Path>, config: Config, hook: H) -> DbResult<Self> {
        let compaction_threshold = config.compaction_threshold;
        let shard_prefix_bits = config.shard_prefix_bits;
        let cache = BlockCache::new(&config.cache);
        let engine = Engine::open(path, config)?;

        let shard_count = engine.shards().len();
        let mut indexes = Vec::with_capacity(shard_count);
        for _ in 0..shard_count {
            indexes.push(Mutex::new(HashMap::new()));
        }

        let map = Self {
            indexes,
            engine,
            cache,
            compaction_threshold,
            shard_prefix_bits,
            hook,
            entry_count: AtomicUsize::new(0),
        };

        // Recover index from disk
        let shard_dirs = map.engine.shard_dirs();
        let shard_dir_refs = Engine::shard_dir_refs(&shard_dirs);
        let shard_ids = map.engine.shard_ids();

        let hints = map.engine.hints();
        let outcome = recover_var_map::<K>(
            &shard_dir_refs,
            &shard_ids,
            map.indexes(),
            hints,
            #[cfg(feature = "encryption")]
            map.engine.cipher(),
        )?;
        for tail in &outcome.active_tails {
            map.engine.shards()[tail.shard_idx].apply_recovery_tail(tail)?;
        }
        for (shard_idx, dead) in outcome.shard_dead_bytes {
            map.engine.shards()[shard_idx].install_dead_bytes(dead);
        }
        let max_gsn = outcome.max_gsn;

        let initial_len: usize = map.indexes.iter().map(|m| sync::lock(m).len()).sum();
        map.entry_count.store(initial_len, AtomicOrdering::Relaxed);

        map.engine
            .gsn()
            .fetch_max(max_gsn + 1, AtomicOrdering::Relaxed);
        if hints {
            for shard in map.engine.shards().iter() {
                shard.set_key_len(size_of::<K>());
            }
        }
        tracing::info!(
            key_size = size_of::<K>(),
            entries = map.len(),
            "var_map recovered"
        );

        Ok(map)
    }

    /// Graceful shutdown: write hint files (if enabled), flush write buffers + fsync.
    pub fn close(self) -> DbResult<()> {
        if self.engine.hints() {
            self.sync_hints()?;
        }
        self.engine.flush()
    }

    /// Flush all shard write buffers to disk (without fsync).
    pub fn flush_buffers(&self) -> DbResult<()> {
        self.engine.flush_buffers()
    }

    /// Get the database configuration.
    pub fn config(&self) -> &Config {
        self.engine.config()
    }
}

impl<K: Key + Send + Sync + Hash + Eq, H: WriteHook<K>> CompactionIndex<K> for VarMap<K, H> {
    fn update_if_match(&self, key: &K, old_loc: DiskLoc, new_loc: DiskLoc) -> bool {
        let mut index = sync::lock(&self.indexes[self.shard_for(key)]);
        if let Some(disk) = index.get_mut(key)
            && *disk == old_loc
        {
            *disk = new_loc;
            return true;
        }
        false
    }

    fn invalidate_blocks(&self, shard_id: u8, file_id: u32, total_bytes: u64) {
        self.cache.invalidate_file(shard_id, file_id, total_bytes);
    }

    fn contains_key(&self, key: &K) -> bool {
        self.contains(key)
    }
}

impl<K: Key + Send + Sync + Hash + Eq, H: WriteHook<K>> VarMap<K, H> {
    /// Trigger a compaction pass across all shards.
    pub fn compact(&self) -> DbResult<usize> {
        let mut total_compacted = 0;
        for shard in self.engine.shards().iter() {
            total_compacted += compact_shard(shard, self, self.compaction_threshold)?;
        }
        Ok(total_compacted)
    }

    /// Get a value by key. Checks block cache first, then reads from disk.
    pub fn get(&self, key: &K) -> Option<ByteView> {
        metrics::counter!("armdb.ops", "op" => "get", "tree" => "var_map").increment(1);
        #[cfg(feature = "hot-path-tracing")]
        tracing::trace!("var_map.get");
        let shard_id = self.shard_for(key);
        for _ in 0..MAX_STALE_RETRIES {
            let disk = {
                let index = sync::lock(&self.indexes[shard_id]);
                match index.get(key) {
                    Some(d) => *d,
                    None => return None,
                }
            };
            match self.read_value_cached_inner(&disk) {
                Ok(v) => return Some(v),
                Err(DbError::StaleDiskLoc) => {
                    metrics::counter!("armdb.read.stale_retry", "tree" => "var_map").increment(1);
                    continue;
                }
                Err(_e) => {
                    #[cfg(feature = "hot-path-tracing")]
                    tracing::error!("VarMap read_value_cached error: {:?}", _e);
                    return None;
                }
            }
        }
        None
    }

    /// Get a value by key, returning `Err(KeyNotFound)` if absent.
    pub fn get_or_err(&self, key: &K) -> DbResult<ByteView> {
        self.get(key).ok_or(DbError::KeyNotFound)
    }

    /// Insert or update a key-value pair.
    pub fn put(&self, key: &K, value: &[u8]) -> DbResult<()> {
        metrics::counter!("armdb.ops", "op" => "put", "tree" => "var_map").increment(1);
        #[cfg(feature = "hot-path-tracing")]
        tracing::trace!("var_map.put");
        let shard_id = self.shard_for(key);
        let mut inner = self.engine.shards()[shard_id].lock();
        let mut index = sync::lock(&self.indexes[shard_id]);
        let old_value = if H::NEEDS_OLD_VALUE {
            if let Some(disk) = index.get(key) {
                Some(self.read_value_locked_result(disk, &inner)?)
            } else {
                None
            }
        } else {
            None
        };
        self.put_locked(shard_id, &mut inner, &mut index, key, value)?;
        drop(index);
        drop(inner);
        self.hook.on_write(key, old_value.as_deref(), Some(value));
        Ok(())
    }

    /// Insert a key-value pair only if the key does not exist.
    /// Returns `Err(KeyExists)` if the key is already present.
    pub fn insert(&self, key: &K, value: &[u8]) -> DbResult<()> {
        metrics::counter!("armdb.ops", "op" => "insert", "tree" => "var_map").increment(1);
        #[cfg(feature = "hot-path-tracing")]
        tracing::trace!("var_map.insert");
        let shard_id = self.shard_for(key);
        let mut inner = self.engine.shards()[shard_id].lock();
        let mut index = sync::lock(&self.indexes[shard_id]);
        self.insert_locked(shard_id, &mut inner, &mut index, key, value)?;
        drop(index);
        drop(inner);
        self.hook.on_write(key, None, Some(value));
        Ok(())
    }

    /// Delete a key. Returns `true` if the key existed.
    pub fn delete(&self, key: &K) -> DbResult<bool> {
        metrics::counter!("armdb.ops", "op" => "delete", "tree" => "var_map").increment(1);
        #[cfg(feature = "hot-path-tracing")]
        tracing::trace!("var_map.delete");
        let shard_id = self.shard_for(key);
        let mut inner = self.engine.shards()[shard_id].lock();
        let mut index = sync::lock(&self.indexes[shard_id]);
        let old_value = if H::NEEDS_OLD_VALUE {
            if let Some(disk) = index.get(key) {
                Some(self.read_value_locked_result(disk, &inner)?)
            } else {
                None
            }
        } else {
            None
        };
        let existed = self.delete_locked(shard_id, &mut inner, &mut index, key)?;
        drop(index);
        drop(inner);
        if existed {
            self.hook.on_write(key, old_value.as_deref(), None);
        }
        Ok(existed)
    }

    /// Compare-and-swap: if current value == expected, replace with new_value.
    /// Returns `Ok(())` on success, `Err(CasMismatch)` if current != expected,
    /// `Err(KeyNotFound)` if key doesn't exist.
    ///
    /// **Caveat:** Holds the shard lock while reading the current value. On a
    /// block-cache miss this performs disk I/O under the lock, blocking all
    /// writes to the same shard. Pre-warm the cache to avoid latency spikes.
    pub fn cas(&self, key: &K, expected: &[u8], new_value: &[u8]) -> DbResult<()> {
        metrics::counter!("armdb.ops", "op" => "cas", "tree" => "var_map").increment(1);
        #[cfg(feature = "hot-path-tracing")]
        tracing::trace!("var_map.cas");
        let shard_id = self.shard_for(key);
        let mut inner = self.engine.shards()[shard_id].lock();
        let mut index = sync::lock(&self.indexes[shard_id]);

        let disk = *index.get(key).ok_or(DbError::KeyNotFound)?;
        let current = self
            .read_value_locked(&disk, &inner)
            .ok_or(DbError::KeyNotFound)?;
        if current.as_ref() != expected {
            return Err(DbError::CasMismatch);
        }

        let (new_disk_loc, _gsn) =
            inner.append_entry(shard_id as u8, key.as_bytes(), new_value, false)?;
        inner.add_dead_bytes(
            disk.file_id,
            crate::entry::entry_size(size_of::<K>(), disk.len),
        );
        let _old = index.insert(*key, new_disk_loc);
        debug_assert!(_old.is_some(), "cas: key must exist in index");

        drop(index);
        drop(inner);
        self.hook.on_write(
            key,
            if H::NEEDS_OLD_VALUE {
                Some(&*current)
            } else {
                None
            },
            Some(new_value),
        );
        Ok(())
    }

    /// Atomically read-modify-write. Returns `Some(new_value)` if key existed, `None` otherwise.
    /// The closure receives the current value and returns a new ByteView.
    /// The closure must not be heavy (shard lock is held).
    ///
    /// **Caveat:** Holds the shard lock while reading the current value. On a
    /// block-cache miss this performs disk I/O under the lock, blocking all
    /// writes to the same shard. Pre-warm the cache to avoid latency spikes.
    pub fn update(&self, key: &K, f: impl FnOnce(&[u8]) -> ByteView) -> DbResult<Option<ByteView>> {
        self.update_inner(key, f, false)
    }

    /// Like [`update()`](Self::update), but returns `Some(old_value)` instead of the new one.
    pub fn fetch_update(
        &self,
        key: &K,
        f: impl FnOnce(&[u8]) -> ByteView,
    ) -> DbResult<Option<ByteView>> {
        self.update_inner(key, f, true)
    }

    pub(crate) fn try_update_inner(
        &self,
        key: &K,
        f: impl FnOnce(&[u8]) -> DbResult<Option<ByteView>>,
        return_old: bool,
    ) -> DbResult<Option<ByteView>> {
        metrics::counter!("armdb.ops", "op" => "update", "tree" => "var_map").increment(1);
        #[cfg(feature = "hot-path-tracing")]
        tracing::trace!("var_map.update");
        let shard_id = self.shard_for(key);
        let mut inner = self.engine.shards()[shard_id].lock();
        let mut index = sync::lock(&self.indexes[shard_id]);

        let disk = match index.get(key) {
            Some(d) => *d,
            None => return Ok(None),
        };

        let current = match self.read_value_locked(&disk, &inner) {
            Some(v) => v,
            None => return Ok(None),
        };

        let new_value = match f(&current)? {
            Some(v) => v,
            None => return Ok(Some(current)),
        };

        let (new_disk_loc, _gsn) =
            inner.append_entry(shard_id as u8, key.as_bytes(), &new_value, false)?;
        inner.add_dead_bytes(
            disk.file_id,
            crate::entry::entry_size(size_of::<K>(), disk.len),
        );
        let _old = index.insert(*key, new_disk_loc);
        debug_assert!(_old.is_some(), "update: key must exist in index");

        drop(index);
        drop(inner);
        self.hook.on_write(
            key,
            if H::NEEDS_OLD_VALUE {
                Some(&*current)
            } else {
                None
            },
            Some(&*new_value),
        );
        Ok(Some(if return_old { current } else { new_value }))
    }

    fn update_inner(
        &self,
        key: &K,
        f: impl FnOnce(&[u8]) -> ByteView,
        return_old: bool,
    ) -> DbResult<Option<ByteView>> {
        self.try_update_inner(key, |bytes| Ok(Some(f(bytes))), return_old)
    }

    /// Check if a key exists.
    pub fn contains(&self, key: &K) -> bool {
        let index = sync::lock(&self.indexes[self.shard_for(key)]);
        index.contains_key(key)
    }

    /// Encoded value byte length for `key`, or `None` if absent.
    /// Reads only the in-memory index entry (`DiskLoc::len`); no disk I/O.
    pub fn entry_len(&self, key: &K) -> Option<u32> {
        let index = sync::lock(&self.indexes[self.shard_for(key)]);
        index.get(key).map(|disk| disk.len)
    }

    /// Total number of entries across all shards.
    pub fn len(&self) -> usize {
        self.entry_count.load(AtomicOrdering::Relaxed)
    }

    pub fn is_empty(&self) -> bool {
        self.len() == 0
    }

    /// Write hint files for all active shard files. Call during graceful shutdown.
    pub fn sync_hints(&self) -> DbResult<()> {
        for shard in self.engine.shards().iter() {
            shard.write_active_hint(size_of::<K>())?;
        }
        Ok(())
    }

    /// Pre-populate the block cache with blocks containing live values.
    ///
    /// Walks the index to collect unique block offsets, sorts them for sequential I/O,
    /// then reads each block into the cache. Only blocks with live data are loaded.
    pub fn warmup(&self) -> DbResult<()> {
        use std::collections::BTreeSet;

        let mut blocks: BTreeSet<(u8, u32, u64)> = BTreeSet::new();
        for index in self.indexes.iter() {
            let map = sync::lock(index);
            for disk in map.values() {
                let block_offset = disk.offset as u64 & !4095;
                blocks.insert((disk.shard_id, disk.file_id, block_offset));
            }
        }

        for (shard_id, file_id, block_offset) in &blocks {
            let key = BlockKey {
                shard_id: *shard_id,
                file_id: *file_id,
                block_offset: *block_offset,
            };
            if self.cache.get(&key).is_some() {
                continue;
            }
            let shard = &self.engine.shards()[*shard_id as usize];
            let (buf, is_full_block) = shard.read_block(*file_id, *block_offset)?;
            if is_full_block {
                self.cache.insert(key, Arc::new(buf));
            }
        }

        Ok(())
    }

    /// Iterate all entries and optionally mutate them. Call once at startup.
    ///
    /// - `Keep` — no change (fires `on_init` if `H::NEEDS_INIT`); not counted
    /// - `Update(new_value)` — replace value (hook-free write, fires `on_init`)
    /// - `Delete` — remove entry (hook-free tombstone, no hooks)
    ///
    /// `on_write` is NEVER fired during migrate — see `docs/hooks.md` in the armdb crate.
    /// Returns the number of mutated entries.
    pub fn migrate(
        &self,
        f: impl Fn(&K, &[u8]) -> crate::MigrateAction<ByteView>,
    ) -> DbResult<usize> {
        use crate::MigrateAction;

        let mut count = 0;
        for i in 0..self.engine.shards().len() {
            let keys: Vec<K> = {
                let index = sync::lock(&self.indexes[i]);
                index.keys().copied().collect()
            };
            for key in keys {
                let value = match self.get(&key) {
                    Some(v) => v,
                    None => {
                        tracing::warn!(
                            key = ?key.as_bytes(),
                            "var_map migrate: skipping entry — value read failed"
                        );
                        continue;
                    }
                };
                match f(&key, &value) {
                    MigrateAction::Keep => {
                        if H::NEEDS_INIT {
                            self.hook.on_init(&key, &value);
                        }
                    }
                    MigrateAction::Update(new_value) => {
                        // Keys were collected from indexes[i], so they always
                        // hash to shard i — no need to recompute via shard_for.
                        {
                            let mut inner = self.engine.shards()[i].lock();
                            let mut index = sync::lock(&self.indexes[i]);
                            self.put_locked(i, &mut inner, &mut index, &key, &new_value)?;
                        }
                        if H::NEEDS_INIT {
                            self.hook.on_init(&key, &new_value);
                        }
                        count += 1;
                    }
                    MigrateAction::Delete => {
                        let mut inner = self.engine.shards()[i].lock();
                        let mut index = sync::lock(&self.indexes[i]);
                        self.delete_locked(i, &mut inner, &mut index, &key)?;
                        count += 1;
                    }
                }
            }
        }

        tracing::info!(mutations = count, "var_map migration complete");
        Ok(count)
    }

    /// Replay `on_init` for every live entry. Used when no migration runs
    /// (Db calls this after `run_migration` returns `false`).
    ///
    /// Lock order: snapshot keys under per-shard index lock, release the
    /// lock, then call `self.get(key)` which re-acquires the index lock
    /// briefly to copy the `DiskLoc` and reads the value WITHOUT holding
    /// the index lock. This matches the lock order of public writes
    /// (shard → index) and avoids the inverted order that would deadlock
    /// against an active writer.
    pub(crate) fn replay_init(&self) {
        if !H::NEEDS_INIT {
            return;
        }
        let mut count = 0usize;
        for i in 0..self.engine.shards().len() {
            let keys: Vec<K> = {
                let index = sync::lock(&self.indexes[i]);
                index.keys().copied().collect()
            };
            for key in keys {
                let value = match self.get(&key) {
                    Some(v) => v,
                    None => {
                        tracing::warn!(
                            key = ?key.as_bytes(),
                            "var_map replay_init: skipping entry — value read failed"
                        );
                        continue;
                    }
                };
                self.hook.on_init(&key, &value);
                count += 1;
            }
        }
        tracing::debug!(replayed = count, "var_map replay_init complete");
    }

    /// Atomically execute multiple operations on a single shard.
    /// All keys must route to the same shard as `shard_key`.
    /// The closure must be short — shard lock is held for its duration.
    pub fn atomic<R>(
        &self,
        shard_key: &K,
        f: impl FnOnce(&mut VarMapShard<'_, K, H>) -> DbResult<R>,
    ) -> DbResult<R> {
        let shard_id = self.shard_for(shard_key);
        let inner = self.engine.shards()[shard_id].lock();
        let index = sync::lock(&self.indexes[shard_id]);
        let mut shard = VarMapShard {
            tree: self,
            inner,
            index,
            shard_id,
        };
        f(&mut shard)
    }

    pub(crate) fn indexes(&self) -> &[Mutex<HashMap<K, DiskLoc>>] {
        &self.indexes
    }

    fn put_locked(
        &self,
        shard_id: usize,
        inner: &mut ShardInner,
        index: &mut HashMap<K, DiskLoc>,
        key: &K,
        value: &[u8],
    ) -> DbResult<()> {
        let (disk_loc, _gsn) = inner.append_entry(shard_id as u8, key.as_bytes(), value, false)?;

        if let Some(old_disk) = index.insert(*key, disk_loc) {
            inner.add_dead_bytes(
                old_disk.file_id,
                crate::entry::entry_size(size_of::<K>(), old_disk.len),
            );
        } else {
            self.entry_count.fetch_add(1, AtomicOrdering::Relaxed);
        }

        Ok(())
    }

    fn insert_locked(
        &self,
        shard_id: usize,
        inner: &mut ShardInner,
        index: &mut HashMap<K, DiskLoc>,
        key: &K,
        value: &[u8],
    ) -> DbResult<()> {
        if index.contains_key(key) {
            return Err(DbError::KeyExists);
        }

        let (disk_loc, _gsn) = inner.append_entry(shard_id as u8, key.as_bytes(), value, false)?;
        index.insert(*key, disk_loc);
        self.entry_count.fetch_add(1, AtomicOrdering::Relaxed);
        Ok(())
    }

    fn delete_locked(
        &self,
        shard_id: usize,
        inner: &mut ShardInner,
        index: &mut HashMap<K, DiskLoc>,
        key: &K,
    ) -> DbResult<bool> {
        if !index.contains_key(key) {
            return Ok(false);
        }

        inner.append_entry(shard_id as u8, key.as_bytes(), &[], true)?;

        if let Some(old_disk) = index.remove(key) {
            inner.add_dead_bytes(
                old_disk.file_id,
                crate::entry::entry_size(size_of::<K>(), old_disk.len),
            );
            self.entry_count.fetch_sub(1, AtomicOrdering::Relaxed);
            Ok(true)
        } else {
            Ok(false)
        }
    }

    fn read_value_cached_inner(&self, disk: &DiskLoc) -> DbResult<ByteView> {
        let len = disk.len as usize;
        let start = (disk.offset & 4095) as usize;

        // Large values (>2 blocks) bypass the cached path. This check MUST
        // sit before the cache lookup: a warm first block would otherwise
        // route a large value into `extract_from_block`, which only supports
        // two blocks and would panic on `next[..second_len]` when
        // `second_len > 4096`. StaleDiskLoc propagates so VarMap::get's
        // per-shard-index retry loop takes a fresh DiskLoc snapshot.
        if start + len > 8192 {
            let shard = &self.engine.shards()[disk.shard_id as usize];
            let inner = shard.lock();
            return self.read_value_locked_result(disk, &inner);
        }

        let block_offset = disk.offset as u64 & !4095;
        let cache_key = BlockKey {
            shard_id: disk.shard_id,
            file_id: disk.file_id,
            block_offset,
        };

        if let Some(block) = self.cache.get(&cache_key) {
            metrics::counter!("armdb.cache.hit").increment(1);
            return Self::extract_from_block(&block, start, len, || {
                self.get_or_read_block(disk.shard_id, disk.file_id, block_offset + 4096)
            });
        }

        {
            let shard = &self.engine.shards()[disk.shard_id as usize];
            let inner = shard.lock();
            if inner.active.file_id == disk.file_id
                && let Some(bytes) = inner.write_buf.read(disk.offset as u64, len)
            {
                return Ok(ByteView::new(bytes));
            }
        }

        metrics::counter!("armdb.cache.miss").increment(1);
        let block = self.get_or_read_block(disk.shard_id, disk.file_id, block_offset)?;
        Self::extract_from_block(&block, start, len, || {
            self.get_or_read_block(disk.shard_id, disk.file_id, block_offset + 4096)
        })
    }

    fn extract_from_block(
        block: &AlignedBuf,
        start: usize,
        len: usize,
        next_block: impl FnOnce() -> DbResult<Arc<AlignedBuf>>,
    ) -> DbResult<ByteView> {
        debug_assert!(
            start + len <= 8192,
            "extract_from_block supports at most 2 blocks (8192 bytes)"
        );
        if start + len <= 4096 {
            Ok(ByteView::new(&block[start..start + len]))
        } else {
            let next = next_block()?;
            let first_part = &block[start..];
            let second_len = len - first_part.len();
            let mut combined = Vec::with_capacity(len);
            combined.extend_from_slice(first_part);
            combined.extend_from_slice(&next[..second_len]);
            Ok(ByteView::from_vec(combined))
        }
    }

    fn get_or_read_block(
        &self,
        shard_id: u8,
        file_id: u32,
        block_offset: u64,
    ) -> DbResult<Arc<AlignedBuf>> {
        let key = BlockKey {
            shard_id,
            file_id,
            block_offset,
        };
        if let Some(cached) = self.cache.get(&key) {
            return Ok(cached);
        }
        let shard = &self.engine.shards()[shard_id as usize];
        let (buf, is_full_block) = shard.read_block(file_id, block_offset)?;
        let arc = Arc::new(buf);
        if is_full_block {
            self.cache.insert(key, arc.clone());
        }
        Ok(arc)
    }

    /// Read value when shard lock is already held, propagating `DbError`.
    /// Used by both `read_value_locked` (Option wrapper) and the large-value
    /// fallback in `read_value_cached_inner`.
    fn read_value_locked_result(&self, disk: &DiskLoc, inner: &ShardInner) -> DbResult<ByteView> {
        let len = disk.len as usize;

        // 1. Write buffer (for unflushed data)
        if inner.active.file_id == disk.file_id
            && let Some(bytes) = inner.write_buf.read(disk.offset as u64, len)
        {
            return Ok(ByteView::new(bytes));
        }

        // 2. Block cache (lock-free, single-block fast path)
        let block_offset = disk.offset as u64 & !4095;
        let start = (disk.offset & 4095) as usize;
        if start + len <= 4096 {
            let cache_key = BlockKey {
                shard_id: disk.shard_id,
                file_id: disk.file_id,
                block_offset,
            };
            if let Some(block) = self.cache.get(&cache_key) {
                return Ok(ByteView::new(&block[start..start + len]));
            }

            // 2b. Cache miss — read block under held lock, populate cache
            let (buf, is_full_block) = inner.read_block_locked(disk.file_id, block_offset)?;
            let arc = Arc::new(buf);
            if is_full_block {
                self.cache.insert(cache_key, arc.clone());
            }
            return Ok(ByteView::new(&arc[start..start + len]));
        }

        // 3. Multi-block: disk read via encryption-aware helper (no cache).
        let bytes = inner.read_value_from_disk_locked(disk)?;
        Ok(ByteView::new(&bytes))
    }

    /// Read value when shard lock is already held. Used by CAS/update.
    ///
    /// Thin `Option` wrapper around `read_value_locked_result` that preserves
    /// the existing call-site contract: `StaleDiskLoc` is logged at error
    /// level and collapsed to `None`; other errors collapse to `None` silently.
    fn read_value_locked(&self, disk: &DiskLoc, inner: &ShardInner) -> Option<ByteView> {
        match self.read_value_locked_result(disk, inner) {
            Ok(v) => Some(v),
            Err(DbError::StaleDiskLoc) => {
                tracing::error!(
                    file_id = disk.file_id,
                    shard_id = disk.shard_id,
                    "stale DiskLoc under shard lock - programming bug",
                );
                None
            }
            Err(_) => None,
        }
    }

    pub fn shard_for(&self, key: &K) -> usize {
        crate::shard_for_key(key, self.shard_prefix_bits, self.engine.shards().len())
    }
}

#[cfg(feature = "replication")]
impl<K: Key + Send + Sync + Hash + Eq, H: WriteHook<K>> crate::replication::ReplicationTarget
    for VarMap<K, H>
{
    fn apply_entry(
        &self,
        _shard_inner: &mut crate::shard::ShardInner,
        shard_id: u8,
        file_id: u32,
        entry_offset: u64,
        header: &crate::entry::EntryHeader,
        key: &[u8],
        _value: &[u8],
    ) -> DbResult<crate::replication::ApplyOutcome> {
        use crate::replication::ApplyOutcome;

        let key = K::from_bytes(key);

        let disk = DiskLoc::new(
            shard_id,
            file_id,
            (entry_offset + size_of::<crate::entry::EntryHeader>() as u64 + size_of::<K>() as u64)
                as u32,
            header.value_len,
        );

        if header.is_tombstone() {
            let old = sync::lock(&self.indexes[self.shard_for(&key)]).remove(&key);
            match old {
                Some(old_disk) => {
                    self.entry_count.fetch_sub(1, AtomicOrdering::Relaxed);
                    Ok(ApplyOutcome::TombstoneRemoved(old_disk))
                }
                None => Ok(ApplyOutcome::Inserted), // no-op tombstone — no dead bytes
            }
        } else {
            let old = sync::lock(&self.indexes[self.shard_for(&key)]).insert(key, disk);
            match old {
                Some(old_disk) => Ok(ApplyOutcome::Replaced(old_disk)),
                None => {
                    self.entry_count.fetch_add(1, AtomicOrdering::Relaxed);
                    Ok(ApplyOutcome::Inserted)
                }
            }
        }
    }

    fn try_apply_entry(
        &self,
        shard_inner: &mut crate::shard::ShardInner,
        shard_id: u8,
        file_id: u32,
        entry_offset: u64,
        header: &crate::entry::EntryHeader,
        raw_after_header: &[u8],
    ) -> DbResult<crate::replication::ApplyOutcome> {
        use crate::replication::ApplyOutcome;

        if raw_after_header.len() < size_of::<K>() + header.value_len as usize {
            return Ok(ApplyOutcome::NotMatched);
        }
        let key = &raw_after_header[..size_of::<K>()];
        let value = &raw_after_header[size_of::<K>()..size_of::<K>() + header.value_len as usize];
        let crc = crate::entry::compute_crc32(header.gsn, header.value_len, key, value);
        if crc != header.crc32 {
            return Ok(ApplyOutcome::NotMatched);
        }
        self.apply_entry(
            shard_inner,
            shard_id,
            file_id,
            entry_offset,
            header,
            key,
            value,
        )
    }

    fn key_len(&self) -> usize {
        size_of::<K>()
    }
}

#[cfg(feature = "replication")]
impl<K: Key + Send + Sync + Hash + Eq, H: WriteHook<K>> VarMap<K, H> {
    /// Install SPSC replication producers into every shard and start a
    /// `ReplicationServer` bound to `bind_addr`.
    ///
    /// # Single-call contract
    ///
    /// Each call installs fresh SPSC producers, replacing any previously
    /// installed ones. Call this at most once per `VarMap` instance — a
    /// second call will orphan the in-flight producer of any active streaming
    /// connection on the first server, which will then observe an empty ring
    /// buffer and silently stop forwarding entries.
    pub fn start_replication_server(
        &self,
        bind_addr: std::net::SocketAddr,
        signal: crate::shutdown::ShutdownSignal,
    ) -> crate::error::DbResult<crate::replication::ReplicationServer> {
        let consumers = self.install_replication_producers()?;
        crate::replication::ReplicationServer::start(
            bind_addr,
            self.engine.shards().clone(),
            consumers,
            self.engine.config().max_file_size,
            signal,
        )
    }

    fn install_replication_producers(
        &self,
    ) -> crate::error::DbResult<Vec<rtrb::Consumer<crate::replication::ReplicationEntry>>> {
        const SPSC_CAPACITY: usize = 4096;
        let shards = self.engine.shards();
        let mut consumers = Vec::with_capacity(shards.len());
        for shard in shards.iter() {
            let (p, c) = rtrb::RingBuffer::new(SPSC_CAPACITY);
            shard.set_replication_producer(p);
            consumers.push(c);
        }
        Ok(consumers)
    }

    /// Start a `ReplicationClient` that streams entries from `leader_addr`
    /// into `registry`. Symmetric to [`Self::start_replication_server`].
    ///
    /// `key_len` is derived from `size_of::<K>()`.
    pub fn start_replication_client(
        &self,
        leader_addr: std::net::SocketAddr,
        registry: std::sync::Arc<crate::replication::ReplicationRegistry>,
        signal: crate::shutdown::ShutdownSignal,
    ) -> crate::error::DbResult<crate::replication::ReplicationClient> {
        crate::replication::ReplicationClient::start(
            leader_addr,
            self.engine.shards().clone(),
            registry,
            size_of::<K>() as u16,
            signal,
        )
    }
}

#[cfg(feature = "replication")]
impl<K, H> VarMap<K, H>
where
    K: Key + Send + Sync + Hash + Eq + 'static,
    H: WriteHook<K> + Send + Sync + 'static,
{
    /// Wrap a shared handle to this map as a `Box<dyn ReplicationTarget>`.
    ///
    /// The returned box holds an `Arc` clone — the caller retains full read
    /// access to the original map through the `Arc` while the registry owns
    /// the box. This is the intended pattern for follower-side wiring:
    ///
    /// ```ignore
    /// let follower = Arc::new(VarMap::<[u8; 8]>::open(path, cfg)?);
    /// let registry = ReplicationRegistry::new(follower.as_replication_target());
    /// // `follower` remains usable for .get() etc.
    /// ```
    pub fn as_replication_target(
        self: &std::sync::Arc<Self>,
    ) -> Box<dyn crate::replication::ReplicationTarget> {
        Box::new(std::sync::Arc::clone(self))
    }
}

/// Handle for atomic multi-key operations within a single shard.
/// Obtained via [`VarMap::atomic`]. The shard + index locks are held for the
/// lifetime of this struct — keep the closure short.
pub struct VarMapShard<'a, K: Key + Send + Sync + Hash + Eq, H: WriteHook<K> = NoHook> {
    tree: &'a VarMap<K, H>,
    inner: MutexGuard<'a, ShardInner>,
    index: MutexGuard<'a, HashMap<K, DiskLoc>>,
    shard_id: usize,
}

impl<K: Key + Send + Sync + Hash + Eq, H: WriteHook<K>> VarMapShard<'_, K, H> {
    pub fn put(&mut self, key: &K, value: &[u8]) -> DbResult<()> {
        self.check_shard(key)?;
        self.tree
            .put_locked(self.shard_id, &mut self.inner, &mut self.index, key, value)
    }

    pub fn insert(&mut self, key: &K, value: &[u8]) -> DbResult<()> {
        self.check_shard(key)?;
        self.tree
            .insert_locked(self.shard_id, &mut self.inner, &mut self.index, key, value)
    }

    pub fn delete(&mut self, key: &K) -> DbResult<bool> {
        self.check_shard(key)?;
        self.tree
            .delete_locked(self.shard_id, &mut self.inner, &mut self.index, key)
    }

    pub fn get(&self, key: &K) -> Option<ByteView> {
        let disk = *self.index.get(key)?;
        self.tree.read_value_locked(&disk, &self.inner)
    }

    pub fn get_or_err(&self, key: &K) -> DbResult<ByteView> {
        self.get(key).ok_or(DbError::KeyNotFound)
    }

    pub fn contains(&self, key: &K) -> bool {
        self.index.contains_key(key)
    }

    fn check_shard(&self, key: &K) -> DbResult<()> {
        if self.tree.shard_for(key) != self.shard_id {
            return Err(DbError::ShardMismatch);
        }
        Ok(())
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::Config;
    use crate::compaction::compact_shard;
    use std::sync::atomic::{AtomicUsize, Ordering as AtomicOrdering};
    use tempfile::tempdir;

    /// Test hook with counters for on_write / on_init calls and the last observed new value for
    /// on_write. NEEDS_INIT and NEEDS_OLD_VALUE are parameterized by const generics.
    #[derive(Default)]
    struct CountingHook<const NEEDS_INIT: bool, const NEEDS_OLD: bool> {
        writes: AtomicUsize,
        writes_with_old: AtomicUsize,
        inits: AtomicUsize,
        last_write_new: crate::sync::Mutex<Option<Vec<u8>>>,
        last_init_value: crate::sync::Mutex<Option<Vec<u8>>>,
    }

    impl<const NEEDS_INIT: bool, const NEEDS_OLD: bool> WriteHook<[u8; 8]>
        for CountingHook<NEEDS_INIT, NEEDS_OLD>
    {
        const NEEDS_OLD_VALUE: bool = NEEDS_OLD;
        const NEEDS_INIT: bool = NEEDS_INIT;

        fn on_write(&self, _key: &[u8; 8], old: Option<&[u8]>, new: Option<&[u8]>) {
            self.writes.fetch_add(1, AtomicOrdering::Relaxed);
            if old.is_some() {
                self.writes_with_old.fetch_add(1, AtomicOrdering::Relaxed);
            }
            *crate::sync::lock(&self.last_write_new) = new.map(<[u8]>::to_vec);
        }

        fn on_init(&self, _key: &[u8; 8], value: &[u8]) {
            self.inits.fetch_add(1, AtomicOrdering::Relaxed);
            *crate::sync::lock(&self.last_init_value) = Some(value.to_vec());
        }
    }

    fn open_test_map_hooked<const NEEDS_INIT: bool, const NEEDS_OLD: bool>(
        dir: &std::path::Path,
        hook: CountingHook<NEEDS_INIT, NEEDS_OLD>,
    ) -> VarMap<[u8; 8], CountingHook<NEEDS_INIT, NEEDS_OLD>> {
        let mut cfg = Config::test();
        cfg.shard_count = 1;
        cfg.max_file_size = 8192;
        cfg.write_buffer_size = 8192;
        cfg.compaction_threshold = 0.0;
        VarMap::open_hooked(dir, cfg, hook).expect("open hooked test map")
    }

    fn open_test_map(dir: &std::path::Path) -> VarMap<[u8; 8]> {
        let mut cfg = Config::test();
        cfg.shard_count = 1;
        cfg.max_file_size = 8192;
        cfg.write_buffer_size = 8192;
        cfg.compaction_threshold = 0.0;
        VarMap::open(dir, cfg).expect("open test map")
    }

    fn put_until_compactable(map: &VarMap<[u8; 8]>, key: [u8; 8]) -> DiskLoc {
        // Capture the FIRST put's DiskLoc — file 0 accumulates 100% dead bytes
        // as later overwrites move the live pointer to other files, so
        // compaction at threshold=0.0 erases it. The captured DiskLoc is
        // genuinely stale afterward. Each put is 280 bytes; with
        // max_file_size=8192, ~30 puts cross the first rotation boundary.
        // Use 65 puts so the snap file is sealed as immutable AND fully dead.
        map.put(&key, &[0u8; 256]).expect("first put");
        let snap = *sync::lock(&map.indexes[0]).get(&key).expect("indexed");
        for i in 1..65u8 {
            map.put(&key, &[i; 256]).expect("overwrite");
        }
        map.put(&key, b"final-value-payload-XX").expect("final put");
        snap
    }

    #[test]
    fn var_map_len_tracks_mutations() {
        let dir = tempdir().unwrap();
        let map: VarMap<[u8; 8]> = VarMap::open(dir.path(), Config::default()).unwrap();
        assert_eq!(map.len(), 0);
        assert!(map.is_empty());

        let k1 = [1u8; 8];
        let k2 = [2u8; 8];
        map.put(&k1, b"a").unwrap();
        assert_eq!(map.len(), 1);

        map.put(&k2, b"b").unwrap();
        assert_eq!(map.len(), 2);

        // Overwrite — len stays the same
        map.put(&k1, b"c").unwrap();
        assert_eq!(map.len(), 2);

        map.delete(&k1).unwrap();
        assert_eq!(map.len(), 1);

        map.delete(&k2).unwrap();
        assert_eq!(map.len(), 0);
        assert!(map.is_empty());

        // Delete non-existent — no change
        map.delete(&k1).unwrap();
        assert_eq!(map.len(), 0);
    }

    #[test]
    fn var_map_len_survives_reopen() {
        let dir = tempdir().unwrap();
        {
            let map: VarMap<[u8; 8]> = VarMap::open(dir.path(), Config::default()).unwrap();
            for i in 0u64..50 {
                map.put(&i.to_le_bytes(), b"val").unwrap();
            }
            assert_eq!(map.len(), 50);
        }
        // Reopen
        let map: VarMap<[u8; 8]> = VarMap::open(dir.path(), Config::default()).unwrap();
        assert_eq!(map.len(), 50);
    }

    #[test]
    fn read_value_cached_inner_returns_stale_after_compaction() {
        let dir = tempdir().unwrap();
        let map = open_test_map(dir.path());

        let key = 7u64.to_be_bytes();
        let snap = put_until_compactable(&map, key);

        let shard = &map.engine.shards()[snap.shard_id as usize];
        let _ = compact_shard(shard, &map, 0.0).expect("compaction");

        match map.read_value_cached_inner(&snap) {
            Err(DbError::StaleDiskLoc) => {}
            Ok(v) => panic!("expected StaleDiskLoc, got Ok({:?})", v.as_bytes()),
            Err(e) => panic!("expected StaleDiskLoc, got Err({e})"),
        }
    }

    #[test]
    fn get_during_compaction_returns_some() {
        let dir = tempdir().unwrap();
        let map = open_test_map(dir.path());

        let key = 11u64.to_be_bytes();
        let _snap = put_until_compactable(&map, key);
        let shard = &map.engine.shards()[0];
        let _ = compact_shard(shard, &map, 0.0).expect("compaction");

        let v = map.get(&key).expect("post-compaction get");
        assert_eq!(v.as_bytes(), b"final-value-payload-XX");
    }

    #[test]
    fn get_or_read_block_returns_stale_for_unknown_file_id() {
        let dir = tempdir().unwrap();
        let map = open_test_map(dir.path());

        match map.get_or_read_block(0, 9999, 0) {
            Err(DbError::StaleDiskLoc) => {}
            Ok(_) => panic!("expected StaleDiskLoc, got Ok"),
            Err(e) => panic!("expected StaleDiskLoc, got Err({e})"),
        }
    }

    #[test]
    fn retry_limit_returns_none_on_persistent_stale() {
        let dir = tempdir().unwrap();
        let map = open_test_map(dir.path());

        let key = 99u64.to_be_bytes();
        map.put(&key, b"payload").expect("put");

        // Force every read_value_cached_inner call to see Stale by manually
        // clearing inner.immutable AND moving active.file_id out of the way.
        // The HashMap still points at the original (now-unreachable) file_id,
        // so each retry hits Shard::read_block which returns StaleDiskLoc.
        let snap = *sync::lock(&map.indexes[0]).get(&key).expect("indexed");

        {
            let shard = &map.engine.shards()[snap.shard_id as usize];
            let mut inner = shard.lock();
            inner.immutable = Vec::new();
            inner.active.file_id = u32::MAX;
        }

        assert!(
            map.get(&key).is_none(),
            "MAX_STALE_RETRIES must terminate the retry loop and return None"
        );
    }

    #[test]
    fn var_map_replay_init_fires_on_init_per_live_key_raw() {
        let dir = tempdir().unwrap();
        let map = open_test_map_hooked::<true, false>(dir.path(), CountingHook::default());

        for i in 0u64..5 {
            map.put(&i.to_be_bytes(), &[i as u8; 16]).expect("put");
        }
        map.delete(&3u64.to_be_bytes()).expect("delete");

        map.hook.writes.store(0, AtomicOrdering::Relaxed);
        map.hook.inits.store(0, AtomicOrdering::Relaxed);

        map.replay_init();

        assert_eq!(map.hook.inits.load(AtomicOrdering::Relaxed), 4);
        assert_eq!(map.hook.writes.load(AtomicOrdering::Relaxed), 0);
    }

    #[test]
    fn var_map_replay_init_no_hook_is_noop() {
        let dir = tempdir().unwrap();
        let map = open_test_map(dir.path());

        for i in 0u64..3 {
            map.put(&i.to_be_bytes(), &[i as u8; 8]).expect("put");
        }
        map.replay_init();
        assert!(map.get(&0u64.to_be_bytes()).is_some());
    }

    #[test]
    fn var_map_migrate_keep_fires_on_init_not_on_write_raw() {
        use crate::MigrateAction;
        let dir = tempdir().unwrap();
        let map = open_test_map_hooked::<true, false>(dir.path(), CountingHook::default());

        for i in 0u64..4 {
            map.put(&i.to_be_bytes(), &[i as u8; 16]).expect("put");
        }
        map.hook.writes.store(0, AtomicOrdering::Relaxed);
        map.hook.inits.store(0, AtomicOrdering::Relaxed);

        let mutated = map.migrate(|_, _| MigrateAction::Keep).expect("migrate");

        assert_eq!(mutated, 0);
        assert_eq!(map.hook.inits.load(AtomicOrdering::Relaxed), 4);
        assert_eq!(map.hook.writes.load(AtomicOrdering::Relaxed), 0);
    }

    #[test]
    fn var_map_migrate_update_fires_on_init_with_new_value_raw() {
        use crate::MigrateAction;
        let dir = tempdir().unwrap();
        let map = open_test_map_hooked::<true, false>(dir.path(), CountingHook::default());

        let key = 42u64.to_be_bytes();
        map.put(&key, b"old-value").expect("put");
        map.hook.writes.store(0, AtomicOrdering::Relaxed);
        map.hook.inits.store(0, AtomicOrdering::Relaxed);

        let new = ByteView::new(b"new-value");
        let mutated = map
            .migrate(move |_, _| MigrateAction::Update(new.clone()))
            .expect("migrate");

        assert_eq!(mutated, 1);
        assert_eq!(map.hook.inits.load(AtomicOrdering::Relaxed), 1);
        let g = crate::sync::lock(&map.hook.last_init_value);
        assert_eq!(g.as_deref(), Some(b"new-value".as_ref()));
        drop(g);
        assert_eq!(map.hook.writes.load(AtomicOrdering::Relaxed), 0);
        assert_eq!(map.get(&key).unwrap().as_bytes(), b"new-value");
    }

    #[test]
    fn var_map_migrate_delete_fires_no_hooks_raw() {
        use crate::MigrateAction;
        let dir = tempdir().unwrap();
        let map = open_test_map_hooked::<true, false>(dir.path(), CountingHook::default());

        let key = 7u64.to_be_bytes();
        map.put(&key, b"x").expect("put");
        map.hook.writes.store(0, AtomicOrdering::Relaxed);
        map.hook.inits.store(0, AtomicOrdering::Relaxed);

        let mutated = map.migrate(|_, _| MigrateAction::Delete).expect("migrate");

        assert_eq!(mutated, 1);
        assert_eq!(map.hook.inits.load(AtomicOrdering::Relaxed), 0);
        assert_eq!(map.hook.writes.load(AtomicOrdering::Relaxed), 0);
        assert!(map.get(&key).is_none());
    }

    #[test]
    fn var_map_public_put_still_fires_on_write_once() {
        let dir = tempdir().unwrap();
        let map = open_test_map_hooked::<true, false>(dir.path(), CountingHook::default());

        map.put(&1u64.to_be_bytes(), b"v").expect("put");
        assert_eq!(map.hook.writes.load(AtomicOrdering::Relaxed), 1);
    }

    #[test]
    fn var_map_migrate_no_init_hook_is_silent_for_keep_and_update() {
        use crate::MigrateAction;
        let dir = tempdir().unwrap();
        // NEEDS_INIT = false: on_init must never be called.
        let map = open_test_map_hooked::<false, false>(dir.path(), CountingHook::default());

        for i in 0u64..3 {
            map.put(&i.to_be_bytes(), &[i as u8; 16]).expect("put");
        }
        map.hook.writes.store(0, AtomicOrdering::Relaxed);
        map.hook.inits.store(0, AtomicOrdering::Relaxed);

        map.migrate(|_, _| MigrateAction::Keep)
            .expect("migrate keep");
        assert_eq!(map.hook.inits.load(AtomicOrdering::Relaxed), 0);
        assert_eq!(map.hook.writes.load(AtomicOrdering::Relaxed), 0);

        let new = ByteView::new(b"new");
        map.migrate(move |_, _| MigrateAction::Update(new.clone()))
            .expect("migrate update");
        assert_eq!(map.hook.inits.load(AtomicOrdering::Relaxed), 0);
        assert_eq!(map.hook.writes.load(AtomicOrdering::Relaxed), 0);
    }

    #[test]
    fn var_map_atomic_does_not_fire_hooks() {
        let dir = tempdir().unwrap();
        let map = open_test_map_hooked::<true, false>(dir.path(), CountingHook::default());

        let key = 1u64.to_be_bytes();
        map.atomic(&key, |shard| {
            shard.put(&key, b"a")?;
            shard.delete(&key)?;
            Ok(())
        })
        .expect("atomic");

        assert_eq!(map.hook.writes.load(AtomicOrdering::Relaxed), 0);
        assert_eq!(map.hook.inits.load(AtomicOrdering::Relaxed), 0);
    }

    /// `_result` returns Ok when the value sits in the active write buffer.
    #[test]
    fn read_value_locked_result_ok_from_write_buf() {
        let dir = tempdir().unwrap();
        let map = open_test_map(dir.path());

        let key = 1u64.to_be_bytes();
        let payload = b"in-write-buffer-value";
        map.put(&key, payload).expect("put");

        let disk = *sync::lock(&map.indexes[0]).get(&key).expect("indexed");
        let shard = &map.engine.shards()[disk.shard_id as usize];
        let inner = shard.lock();
        let v = map
            .read_value_locked_result(&disk, &inner)
            .expect("write-buf read must succeed");
        assert_eq!(v.as_bytes(), payload);
    }

    /// `_result` returns Ok when the value is on an immutable file and the
    /// entry sits within a single 4 KiB block.
    #[test]
    fn read_value_locked_result_ok_from_disk_immutable() {
        let dir = tempdir().unwrap();
        let map = open_test_map(dir.path());

        let key = 2u64.to_be_bytes();
        let payload = b"single-block-immutable";
        map.put(&key, payload).expect("put");
        // Each put is 280 bytes; with max_file_size=8192, need >30 puts to
        // cross the rotation boundary. Use keys 100..135 to avoid overwriting key 2.
        for i in 100u64..135 {
            map.put(&i.to_be_bytes(), &[i as u8; 256]).expect("rotator");
        }

        let disk = *sync::lock(&map.indexes[0]).get(&key).expect("indexed");
        // `open_test_map` uses default CacheConfig (max_size = 0) — cache is
        // disabled, so step 2 returns None and the read exercises step 3 (disk).

        let shard = &map.engine.shards()[disk.shard_id as usize];
        let inner = shard.lock();
        assert_ne!(
            disk.file_id, inner.active.file_id,
            "test setup failed: key=2 entry is still in the active file's write buffer",
        );
        let v = map
            .read_value_locked_result(&disk, &inner)
            .expect("disk read must succeed");
        assert_eq!(v.as_bytes(), payload);
    }

    /// `_result` propagates `StaleDiskLoc` from the disk read.
    #[test]
    fn read_value_locked_result_propagates_stale_disk_loc() {
        let dir = tempdir().unwrap();
        let map = open_test_map(dir.path());

        let key = 3u64.to_be_bytes();
        let snap = put_until_compactable(&map, key);

        let shard = &map.engine.shards()[snap.shard_id as usize];
        let _ = compact_shard(shard, &map, 0.0).expect("compaction");

        let inner = shard.lock();
        match map.read_value_locked_result(&snap, &inner) {
            Err(DbError::StaleDiskLoc) => {}
            Ok(v) => panic!("expected StaleDiskLoc, got Ok({:?})", v.as_bytes()),
            Err(e) => panic!("expected StaleDiskLoc, got Err({e})"),
        }
    }

    /// Backward-compat: Option wrapper still returns None on StaleDiskLoc.
    /// No tracing assertion: the None is the load-bearing invariant.
    #[test]
    fn read_value_locked_returns_none_on_stale() {
        let dir = tempdir().unwrap();
        let map = open_test_map(dir.path());

        let key = 4u64.to_be_bytes();
        let snap = put_until_compactable(&map, key);

        let shard = &map.engine.shards()[snap.shard_id as usize];
        let _ = compact_shard(shard, &map, 0.0).expect("compaction");

        let inner = shard.lock();
        assert!(map.read_value_locked(&snap, &inner).is_none());
    }

    /// Pins the "size check sits before cache lookup" invariant for VarMap.
    /// Without the early `start + len > 8192` check, the cached path would
    /// hit `extract_from_block` and panic on `next[..second_len]`.
    #[test]
    fn large_value_with_first_block_cached_uses_fallback() {
        let dir = tempdir().unwrap();
        let mut cfg = Config::test();
        cfg.shard_count = 1;
        cfg.max_file_size = 128 * 1024;
        cfg.write_buffer_size = 128 * 1024;
        cfg.compaction_threshold = 0.0;
        // Enable the block cache so `cache.insert` actually takes effect
        // (default CacheConfig has max_size = 0, making the cache a no-op).
        cfg.cache.max_size = 1 << 20;
        let map: VarMap<[u8; 8]> = VarMap::open(dir.path(), cfg).expect("open");

        let key = 42u64.to_be_bytes();
        let payload: Vec<u8> = (0..20_000u32).map(|i| i as u8).collect();
        map.put(&key, &payload).expect("put large");
        map.engine.shards()[0]
            .rotate_active_for_test(8)
            .expect("rotate");

        let disk = *sync::lock(&map.indexes[0]).get(&key).expect("indexed");
        let start = (disk.offset & 4095) as usize;
        assert!(
            start + disk.len as usize > 8192,
            "test precondition: large value must span >2 blocks",
        );

        let block_offset = disk.offset as u64 & !4095;
        let cache_key = BlockKey {
            shard_id: disk.shard_id,
            file_id: disk.file_id,
            block_offset,
        };
        map.cache
            .insert(cache_key, Arc::new(AlignedBuf::zeroed(4096)));
        assert!(
            map.cache.get(&cache_key).is_some(),
            "cache must be enabled for warm-cache scenario",
        );

        let v = map
            .read_value_cached_inner(&disk)
            .expect("large value must read via locked fallback");
        assert_eq!(v.as_bytes(), payload.as_slice());
    }

    #[test]
    fn extract_from_block_single_block() {
        let mut block = AlignedBuf::zeroed(4096);
        for (i, byte) in block.iter_mut().enumerate() {
            *byte = i as u8;
        }
        let v = VarMap::<[u8; 8]>::extract_from_block(&block, 100, 50, || {
            panic!("next_block must not be called for single-block reads")
        })
        .expect("ok");
        let expected: Vec<u8> = (100u8..150u8).collect();
        assert_eq!(v.as_bytes(), expected.as_slice());
    }

    #[test]
    fn extract_from_block_two_blocks_exact() {
        let mut first = AlignedBuf::zeroed(4096);
        for byte in first.iter_mut() {
            *byte = 0xAA;
        }
        let mut second = AlignedBuf::zeroed(4096);
        for byte in second.iter_mut() {
            *byte = 0xBB;
        }
        let v = VarMap::<[u8; 8]>::extract_from_block(&first, 4095, 4097, || Ok(Arc::new(second)))
            .expect("ok");
        let bytes = v.as_bytes();
        assert_eq!(bytes.len(), 4097);
        assert_eq!(bytes[0], 0xAA);
        assert_eq!(bytes[1], 0xBB);
        assert_eq!(bytes[4096], 0xBB);
    }

    #[test]
    fn extract_from_block_two_blocks_partial() {
        let mut first = AlignedBuf::zeroed(4096);
        for byte in first.iter_mut() {
            *byte = 0x11;
        }
        let mut second = AlignedBuf::zeroed(4096);
        for byte in second.iter_mut() {
            *byte = 0x22;
        }
        let v = VarMap::<[u8; 8]>::extract_from_block(&first, 4000, 200, || Ok(Arc::new(second)))
            .expect("ok");
        let bytes = v.as_bytes();
        assert_eq!(bytes.len(), 200);
        assert!(bytes[..96].iter().all(|b| *b == 0x11));
        assert!(bytes[96..].iter().all(|b| *b == 0x22));
    }

    fn open_large_value_map(dir: &std::path::Path) -> VarMap<[u8; 8]> {
        let mut cfg = Config::test();
        cfg.shard_count = 1;
        cfg.max_file_size = 128 * 1024;
        cfg.write_buffer_size = 128 * 1024;
        cfg.compaction_threshold = 0.0;
        VarMap::open(dir, cfg).expect("open large-value test map")
    }

    fn build_large_payload(seed: u8) -> Vec<u8> {
        (0..50_000u32)
            .map(|i| (i as u8).wrapping_add(seed))
            .collect()
    }

    /// `open_large_value_map` uses default CacheConfig (max_size = 0), so
    /// the block cache is disabled and the read goes through the large-value
    /// locked fallback → disk read.
    #[test]
    fn large_value_read_via_locked_fallback() {
        let dir = tempdir().unwrap();
        let map = open_large_value_map(dir.path());

        let key = 100u64.to_be_bytes();
        let payload = build_large_payload(0);
        map.put(&key, &payload).expect("put large");
        map.engine.shards()[0]
            .rotate_active_for_test(8)
            .expect("rotate");

        let v = map
            .get(&key)
            .expect("read must succeed via locked fallback");
        assert_eq!(v.as_bytes(), payload.as_slice());
    }

    #[cfg(feature = "encryption")]
    #[test]
    fn large_value_read_encrypted() {
        let dir = tempdir().unwrap();
        let mut cfg = Config::test();
        cfg.shard_count = 1;
        cfg.max_file_size = 128 * 1024;
        cfg.write_buffer_size = 128 * 1024;
        cfg.compaction_threshold = 0.0;
        cfg.encryption_key = Some([7u8; 32]);
        // Cache stays at default (max_size = 0, disabled), so the locked
        // fallback always routes through pread_value_encrypted.
        let map: VarMap<[u8; 8]> = VarMap::open(dir.path(), cfg).expect("open enc");

        let key = 101u64.to_be_bytes();
        let payload = build_large_payload(0xAB);
        map.put(&key, &payload).expect("put encrypted large");
        map.engine.shards()[0]
            .rotate_active_for_test(8)
            .expect("rotate");

        let v = map
            .get(&key)
            .expect("encrypted large read must succeed via pread_value_encrypted");
        assert_eq!(v.as_bytes(), payload.as_slice());
    }

    #[test]
    fn large_value_stale_disk_loc_deterministic() {
        let dir = tempdir().unwrap();
        let map = open_large_value_map(dir.path());

        let key = 102u64.to_be_bytes();
        let payload = build_large_payload(0x42);
        map.put(&key, &payload).expect("first large put");

        let snap = *sync::lock(&map.indexes[0]).get(&key).expect("indexed");

        // Rotate the active file so the large entry moves to an immutable file.
        map.engine.shards()[0]
            .rotate_active_for_test(8)
            .expect("rotate after large put");

        // Overwrite many times to move the live pointer off the original file
        // (making it 100% dead so compaction erases it).
        for i in 1..20u8 {
            map.put(&key, &[i; 256]).expect("overwrite");
        }
        map.put(&key, b"live-after-compaction").expect("final put");

        let shard = &map.engine.shards()[snap.shard_id as usize];
        let _ = compact_shard(shard, &map, 0.0).expect("compaction");

        match map.read_value_cached_inner(&snap) {
            Err(DbError::StaleDiskLoc) => {}
            Ok(v) => panic!("expected StaleDiskLoc, got Ok({:?})", v.as_bytes()),
            Err(e) => panic!("expected StaleDiskLoc, got Err({e})"),
        }

        let v = map
            .get(&key)
            .expect("public path must retry and return live value");
        assert_eq!(v.as_bytes(), b"live-after-compaction");
    }

    /// `_result` step 2: single-block cache fast path.
    #[test]
    fn read_value_locked_result_ok_from_cache_single_block() {
        let dir = tempdir().unwrap();
        let mut cfg = Config::test();
        cfg.shard_count = 1;
        cfg.max_file_size = 8192;
        cfg.write_buffer_size = 8192;
        cfg.compaction_threshold = 0.0;
        cfg.cache.max_size = 1 << 20;
        let map: VarMap<[u8; 8]> = VarMap::open(dir.path(), cfg).expect("open");

        let key = 9u64.to_be_bytes();
        let payload = b"small-single-block-value";
        map.put(&key, payload).expect("put");
        // Need enough writes to push write_offset past max_file_size=8192.
        for i in 100u64..135 {
            map.put(&i.to_be_bytes(), &[i as u8; 256]).expect("rotator");
        }

        let disk = *sync::lock(&map.indexes[0]).get(&key).expect("indexed");

        let start = (disk.offset & 4095) as usize;
        let len = disk.len as usize;
        assert!(
            start + len <= 4096,
            "test precondition: value must fit in a single block",
        );

        // Pre-warm the cache BEFORE acquiring the shard lock.
        // get_or_read_block → shard.read_block → sync::lock(&inner), so it
        // must not be called while the shard lock is already held.
        let block_offset = disk.offset as u64 & !4095;
        let cache_key = BlockKey {
            shard_id: disk.shard_id,
            file_id: disk.file_id,
            block_offset,
        };
        let block = map
            .get_or_read_block(disk.shard_id, disk.file_id, block_offset)
            .expect("read block");
        map.cache.insert(cache_key, block);
        assert!(
            map.cache.get(&cache_key).is_some(),
            "cache must contain the block for step 2 to fire",
        );

        let shard = &map.engine.shards()[disk.shard_id as usize];
        let inner = shard.lock();
        assert_ne!(
            disk.file_id, inner.active.file_id,
            "test setup failed: key entry is still in the active write buffer",
        );

        let v = map
            .read_value_locked_result(&disk, &inner)
            .expect("cache-hit read must succeed");
        assert_eq!(v.as_bytes(), payload);
    }

    /// End-to-end roundtrip with file_id above u16::MAX.
    ///
    /// Uses write_buffer_size=128 KiB so a single 512-byte entry fits in one
    /// flush cycle; max_file_size=4096 forces per-entry rotation; cache
    /// enabled so the fast path is also exercised.  After bumping
    /// next_file_id to 70_000 and rotating, the active file gets an id that
    /// exceeds u16::MAX.  The entry is flushed + rotated to make it
    /// immutable, then `get` must return the correct bytes via the locked
    /// disk-read path.  Before the file_id u32 widening this would either
    /// panic or return wrong bytes because the high bits were truncated.
    #[test]
    fn var_map_get_with_file_id_above_u16() {
        let dir = tempdir().unwrap();
        let mut cfg = Config::test();
        cfg.shard_count = 1;
        cfg.max_file_size = 128 * 1024;
        cfg.write_buffer_size = 128 * 1024;
        cfg.compaction_threshold = 0.0;
        cfg.cache.max_size = 1 << 20;
        let map: VarMap<[u8; 8]> = VarMap::open(dir.path(), cfg).expect("open");

        let shard = &map.engine.shards()[0];

        // Bump file id well past u16::MAX and rotate so the next active file
        // carries the new id.
        shard.set_next_file_id(70_000);
        shard.rotate_active_for_test(8).expect("first rotate");
        assert!(
            shard.active_file_id() >= 70_000,
            "active_file_id should be >= 70_000 after rotation"
        );

        let key = 42u64.to_be_bytes();
        let value = vec![0xC3u8; 512];
        map.put(&key, &value).expect("put");

        // Confirm the DiskLoc has file_id > u16::MAX.
        {
            let disk = *sync::lock(&map.indexes[0]).get(&key).expect("indexed");
            assert!(
                disk.file_id > u16::MAX as u32,
                "DiskLoc.file_id must be above u16::MAX, got {}",
                disk.file_id,
            );
        }

        // Flush the write buffer and rotate so the entry lands on an
        // immutable file — reads will go through Shard::read_block.
        shard.flush().expect("flush");
        shard.rotate_active_for_test(8).expect("second rotate");

        let got = map.get(&key).expect("get must return Some");
        assert_eq!(got.as_bytes(), value.as_slice());
    }
}