gibblox-cache 0.0.1

#![cfg_attr(not(feature = "std"), no_std)]

extern crate alloc;

use alloc::{boxed::Box, collections::BTreeMap, format, string::String, sync::Arc, vec, vec::Vec};
use async_trait::async_trait;
use core::fmt;
use futures_channel::oneshot;
use gibblox_core::{
    BlockReader, BlockReaderConfigIdentity, GibbloxError, GibbloxErrorKind, GibbloxResult,
    ReadContext, derive_block_identity_id, derive_config_identity_id,
};
use tracing::{debug, trace};

const CACHE_MAGIC: [u8; 7] = *b"GIBBLX!";
const CACHE_VERSION: u8 = 2;
const CACHE_PREFIX_LEN: usize = 28;
const DEFAULT_FLUSH_BLOCKS: u32 = 64;
const ZERO_CHUNK_LEN: usize = 4096;

/// Snapshot of cache behavior and population.
#[derive(Clone, Copy, Debug)]
#[cfg_attr(target_arch = "wasm32", derive(serde::Serialize))]
pub struct CacheStats {
    pub total_hits: u64,
    pub total_misses: u64,
    pub cached_blocks: u64,
    pub total_blocks: u64,
}

impl CacheStats {
    pub fn hit_rate(&self) -> f64 {
        let total = self.total_hits + self.total_misses;
        if total == 0 {
            return 0.0;
        }
        (self.total_hits as f64 / total as f64) * 100.0
    }

    pub fn fill_rate(&self) -> f64 {
        if self.total_blocks == 0 {
            return 0.0;
        }
        (self.cached_blocks as f64 / self.total_blocks as f64) * 100.0
    }
}

/// Backend I/O abstraction for a single cache file.
///
/// Implementations are expected to be internally synchronized; the cache wrapper may call these
/// methods concurrently from multiple tasks.
#[async_trait]
pub trait CacheOps: Send + Sync {
    /// Read bytes at a fixed offset. Returns the number of bytes read.
    async fn read_at(&self, offset: u64, out: &mut [u8]) -> GibbloxResult<usize>;

    /// Write all bytes at a fixed offset.
    async fn write_at(&self, offset: u64, data: &[u8]) -> GibbloxResult<()>;

    /// Resize the underlying cache file.
    async fn set_len(&self, len: u64) -> GibbloxResult<()>;

    /// Persist pending data and metadata changes.
    async fn flush(&self) -> GibbloxResult<()>;
}

#[async_trait]
impl<T> CacheOps for Arc<T>
where
    T: CacheOps + ?Sized,
{
    async fn read_at(&self, offset: u64, out: &mut [u8]) -> GibbloxResult<usize> {
        (**self).read_at(offset, out).await
    }

    async fn write_at(&self, offset: u64, data: &[u8]) -> GibbloxResult<()> {
        (**self).write_at(offset, data).await
    }

    async fn set_len(&self, len: u64) -> GibbloxResult<()> {
        (**self).set_len(len).await
    }

    async fn flush(&self) -> GibbloxResult<()> {
        (**self).flush().await
    }
}

/// Read-only block reader wrapper that consults a local file-style cache.
///
/// The cache file contains a compact header, a per-block validity bitmap, and raw backing bytes.
/// Misses are fetched from the inner reader, written into the data region, and marked valid.
///
/// The cache flushes automatically after writing `flush_every_blocks` new blocks to ensure
/// persistence across sessions. There is no dirty flag; the bitmap is the authoritative
/// source of validity.
pub struct CachedBlockReader<S, C> {
    inner: S,
    cache: C,
    block_size: u32,
    total_blocks: u64,
    bitmap_offset: u64,
    data_offset: u64,
    flush_every_blocks: u32,
    state: async_lock::Mutex<CacheState>,
    in_flight: async_lock::Mutex<InFlight>,
    mutation_lock: async_lock::Mutex<()>,
}

impl<S, C> CachedBlockReader<S, C>
where
    S: BlockReader,
    C: CacheOps,
{
    /// Construct a cached reader using the default flush policy.
    pub async fn new(inner: S, cache: C) -> GibbloxResult<Self> {
        Self::with_flush_block_limit(inner, cache, DEFAULT_FLUSH_BLOCKS).await
    }

    /// Construct a cached reader with a custom flush threshold.
    ///
    /// The cache will flush after writing `flush_every_blocks` newly cached blocks.
    /// Lower values provide better crash resilience at the cost of more frequent I/O.
    /// A value of 1 flushes after every write batch (maximally safe but slowest).
    pub async fn with_flush_block_limit(
        inner: S,
        cache: C,
        flush_every_blocks: u32,
    ) -> GibbloxResult<Self> {
        if flush_every_blocks == 0 {
            return Err(GibbloxError::with_message(
                GibbloxErrorKind::InvalidInput,
                "flush block limit must be non-zero",
            ));
        }
        let block_size = inner.block_size();
        if block_size == 0 || !block_size.is_power_of_two() {
            return Err(GibbloxError::with_message(
                GibbloxErrorKind::InvalidInput,
                "block size must be non-zero power of two",
            ));
        }
        let total_blocks = inner.total_blocks().await?;
        let layout = CacheLayout::new(block_size, total_blocks)?;
        let identity = cached_reader_identity_string(&inner);
        let opened = open_or_initialize_cache(&cache, &layout, &identity).await?;
        let valid_blocks = count_set_bits(&opened.valid);
        debug!(
            block_size,
            total_blocks,
            identity = %identity,
            valid_blocks,
            flush_every_blocks,
            "cached block reader initialized"
        );
        trace!(
            block_size,
            total_blocks,
            bitmap_offset = opened.mapping.bitmap_offset,
            data_offset = opened.mapping.data_offset,
            flush_every_blocks,
            "cached block reader initialized"
        );

        Ok(Self {
            inner,
            cache,
            block_size,
            total_blocks,
            bitmap_offset: opened.mapping.bitmap_offset,
            data_offset: opened.mapping.data_offset,
            flush_every_blocks,
            state: async_lock::Mutex::new(CacheState {
                valid: opened.valid,
                blocks_since_flush: 0,
                total_hits: 0,
                total_misses: 0,
                last_stats_log: 0,
                cached_blocks: valid_blocks,
            }),
            in_flight: async_lock::Mutex::new(InFlight::new()),
            mutation_lock: async_lock::Mutex::new(()),
        })
    }

    /// Force a cache flush and clear the dirty bit if there are pending writes.
    pub async fn flush_cache(&self) -> GibbloxResult<()> {
        let _guard = self.mutation_lock.lock().await;
        debug!("cache flush requested explicitly");
        self.flush_locked_if_needed(true).await
    }

    fn block_size_usize(&self) -> usize {
        self.block_size as usize
    }

    fn validate_range(&self, lba: u64, blocks: u64) -> GibbloxResult<()> {
        let end = lba.checked_add(blocks).ok_or_else(|| {
            GibbloxError::with_message(GibbloxErrorKind::OutOfRange, "lba overflow")
        })?;
        if end > self.total_blocks {
            return Err(GibbloxError::with_message(
                GibbloxErrorKind::OutOfRange,
                "requested range exceeds total blocks",
            ));
        }
        Ok(())
    }

    fn blocks_from_len(&self, len: usize) -> GibbloxResult<u64> {
        if len == 0 {
            return Ok(0);
        }
        if !len.is_multiple_of(self.block_size_usize()) {
            return Err(GibbloxError::with_message(
                GibbloxErrorKind::InvalidInput,
                "buffer length must align to block size",
            ));
        }
        Ok((len / self.block_size_usize()) as u64)
    }

    fn data_offset_for_block(&self, block_idx: u64) -> GibbloxResult<u64> {
        let block_bytes = block_idx
            .checked_mul(self.block_size as u64)
            .ok_or_else(|| {
                GibbloxError::with_message(GibbloxErrorKind::OutOfRange, "block offset overflow")
            })?;
        self.data_offset.checked_add(block_bytes).ok_or_else(|| {
            GibbloxError::with_message(GibbloxErrorKind::OutOfRange, "data offset overflow")
        })
    }

    async fn snapshot_segments(&self, lba: u64, blocks: u64) -> Vec<(u64, u64, bool)> {
        let guard = self.state.lock().await;
        let mut segments = Vec::new();
        let mut block = lba;
        while block < lba + blocks {
            let hit = bit_is_set(&guard.valid, block);
            let mut len = 1u64;
            while block + len < lba + blocks && bit_is_set(&guard.valid, block + len) == hit {
                len += 1;
            }
            segments.push((block, len, hit));
            block += len;
        }
        segments
    }

    async fn invalidate_range(&self, start_block: u64, blocks: u64) {
        if blocks == 0 {
            return;
        }
        let mut guard = self.state.lock().await;
        let removed = clear_bits_and_count_removed(&mut guard.valid, start_block, blocks);
        guard.cached_blocks = guard.cached_blocks.saturating_sub(removed);
    }

    /// Return a snapshot of cache statistics.
    pub async fn get_stats(&self) -> CacheStats {
        let guard = self.state.lock().await;
        CacheStats {
            total_hits: guard.total_hits,
            total_misses: guard.total_misses,
            cached_blocks: guard.cached_blocks,
            total_blocks: self.total_blocks,
        }
    }

    async fn fill_from_cache(
        &self,
        lba: u64,
        blocks: u64,
        buf: &mut [u8],
    ) -> GibbloxResult<Vec<u64>> {
        let mut missing = Vec::new();
        let bs = self.block_size_usize();
        let segments = self.snapshot_segments(lba, blocks).await;

        for (start_block, len_blocks, hit) in segments {
            if !hit {
                missing.extend(start_block..start_block + len_blocks);
                continue;
            }

            let block_offset = (start_block - lba) as usize;
            let byte_start = block_offset.checked_mul(bs).ok_or_else(|| {
                GibbloxError::with_message(GibbloxErrorKind::OutOfRange, "buffer offset overflow")
            })?;
            let byte_len = (len_blocks as usize).checked_mul(bs).ok_or_else(|| {
                GibbloxError::with_message(GibbloxErrorKind::OutOfRange, "buffer length overflow")
            })?;
            let byte_end = byte_start.checked_add(byte_len).ok_or_else(|| {
                GibbloxError::with_message(GibbloxErrorKind::OutOfRange, "buffer end overflow")
            })?;
            let out = &mut buf[byte_start..byte_end];

            let data_offset = self.data_offset_for_block(start_block)?;
            let full = read_exact_at(&self.cache, data_offset, out).await?;
            if !full {
                self.invalidate_range(start_block, len_blocks).await;
                missing.extend(start_block..start_block + len_blocks);
            }
        }

        trace!(
            lba,
            blocks,
            missing = missing.len(),
            "cache read pass complete"
        );

        Ok(missing)
    }

    async fn mark_in_flight(&self, blocks: &[u64]) -> (Vec<u64>, Vec<oneshot::Receiver<()>>) {
        let mut to_fetch = Vec::new();
        let mut waiters = Vec::new();
        let mut guard = self.in_flight.lock().await;
        for block in blocks {
            match guard.waiters.get_mut(block) {
                Some(pending) => {
                    let (tx, rx) = oneshot::channel();
                    pending.push(tx);
                    waiters.push(rx);
                }
                None => {
                    guard.waiters.insert(*block, Vec::new());
                    to_fetch.push(*block);
                }
            }
        }
        trace!(
            requested = blocks.len(),
            to_fetch = to_fetch.len(),
            waiting = waiters.len(),
            "updated in-flight block registry"
        );
        (to_fetch, waiters)
    }

    async fn take_waiters(&self, start_block: u64, len_blocks: u64) -> Vec<oneshot::Sender<()>> {
        let mut guard = self.in_flight.lock().await;
        let mut senders = Vec::new();
        for block in start_block..(start_block + len_blocks) {
            if let Some(mut pending) = guard.waiters.remove(&block) {
                senders.append(&mut pending);
            }
        }
        senders
    }

    fn coalesce(blocks: &[u64]) -> Vec<(u64, u64)> {
        if blocks.is_empty() {
            return Vec::new();
        }
        let mut ranges = Vec::new();
        let mut start = blocks[0];
        let mut len = 1u64;
        for pair in blocks.windows(2) {
            if let [prev, curr] = pair {
                if *curr == *prev + 1 {
                    len += 1;
                } else {
                    ranges.push((start, len));
                    start = *curr;
                    len = 1;
                }
            }
        }
        ranges.push((start, len));
        ranges
    }

    async fn mark_valid_and_collect_bitmap_write(
        &self,
        start_block: u64,
        blocks: u64,
    ) -> GibbloxResult<(u64, Vec<u8>, bool)> {
        let end_block = start_block.checked_add(blocks).ok_or_else(|| {
            GibbloxError::with_message(GibbloxErrorKind::OutOfRange, "block range overflow")
        })?;
        let first_byte = (start_block / 8) as usize;
        let last_byte = ((end_block - 1) / 8) as usize;

        let mut guard = self.state.lock().await;
        let newly_cached = set_bits_and_count_new(&mut guard.valid, start_block, blocks);
        guard.cached_blocks = guard.cached_blocks.saturating_add(newly_cached);
        guard.blocks_since_flush = guard.blocks_since_flush.saturating_add(blocks as u32);
        let should_flush = guard.blocks_since_flush >= self.flush_every_blocks;
        let chunk = guard.valid[first_byte..=last_byte].to_vec();
        let bitmap_offset = self.bitmap_offset + first_byte as u64;
        Ok((bitmap_offset, chunk, should_flush))
    }

    async fn flush_locked_if_needed(&self, force: bool) -> GibbloxResult<()> {
        let (should_flush, blocks_written, valid_blocks) = {
            let guard = self.state.lock().await;
            let should = force || guard.blocks_since_flush >= self.flush_every_blocks;
            (should, guard.blocks_since_flush, guard.cached_blocks)
        };
        if !should_flush {
            return Ok(());
        }

        debug!(
            force,
            blocks_since_last_flush = blocks_written,
            valid_blocks,
            "flushing cache data and metadata"
        );

        self.cache.flush().await?;

        let mut guard = self.state.lock().await;
        guard.blocks_since_flush = 0;
        debug!("cache flush completed successfully");
        Ok(())
    }

    async fn fetch_and_populate(
        &self,
        ranges: &[(u64, u64)],
        ctx: ReadContext,
    ) -> GibbloxResult<()> {
        let bs = self.block_size_usize();
        for (start_block, len_blocks) in ranges {
            trace!(
                start_block,
                len_blocks, "fetching missing range from inner source"
            );
            let expected_bytes = (*len_blocks as usize).checked_mul(bs).ok_or_else(|| {
                GibbloxError::with_message(GibbloxErrorKind::OutOfRange, "range too large")
            })?;
            let mut buf = vec![0u8; expected_bytes];
            let result = async {
                let read = self.inner.read_blocks(*start_block, &mut buf, ctx).await?;
                if read != expected_bytes {
                    return Err(GibbloxError::with_message(
                        GibbloxErrorKind::Io,
                        "inner source returned short read",
                    ));
                }

                let _mutation_guard = self.mutation_lock.lock().await;

                let data_offset = self.data_offset_for_block(*start_block)?;
                self.cache.write_at(data_offset, &buf).await?;

                let (bitmap_offset, bitmap_chunk, should_flush) = self
                    .mark_valid_and_collect_bitmap_write(*start_block, *len_blocks)
                    .await?;
                self.cache.write_at(bitmap_offset, &bitmap_chunk).await?;
                trace!(
                    start_block,
                    len_blocks,
                    bytes = expected_bytes,
                    should_flush,
                    "cache range populated"
                );
                if should_flush {
                    self.flush_locked_if_needed(false).await?;
                }

                Ok(())
            }
            .await;

            let senders = self.take_waiters(*start_block, *len_blocks).await;
            for sender in senders {
                let _ = sender.send(());
            }
            result?;
        }
        Ok(())
    }

    /// Ensure blocks [lba, lba+blocks) are cached without copying data out.
    ///
    /// This fetches any missing blocks from the inner reader and populates the
    /// cache without allocating or returning a caller buffer.
    ///
    /// Already-cached blocks are skipped (bitmap check only). The provided
    /// ReadContext propagates priority hints to the inner reader.
    pub async fn ensure_cached(
        &self,
        lba: u64,
        blocks: u64,
        ctx: ReadContext,
    ) -> GibbloxResult<()> {
        if blocks == 0 {
            return Ok(());
        }
        self.validate_range(lba, blocks)?;

        // Fast-path cache probe for readahead/background callers.
        // This avoids large temporary allocations and cache file reads when
        // all requested blocks are already marked valid.
        let missing = self.missing_blocks_from_bitmap(lba, blocks).await;
        if missing.is_empty() {
            return Ok(()); // All cached, early return
        }

        // Mark in-flight for deduplication
        let (to_fetch, waiters) = self.mark_in_flight(&missing).await;

        // Fetch missing ranges if we're the first to request them
        if !to_fetch.is_empty() {
            self.fetch_and_populate(&Self::coalesce(&to_fetch), ctx)
                .await?;
        }

        // Wait for any overlapping in-flight fetches
        for waiter in waiters {
            let _ = waiter.await;
        }

        Ok(())
    }

    async fn missing_blocks_from_bitmap(&self, lba: u64, blocks: u64) -> Vec<u64> {
        let mut missing = Vec::new();
        let guard = self.state.lock().await;
        for block in lba..(lba + blocks) {
            if !bit_is_set(&guard.valid, block) {
                missing.push(block);
            }
        }
        missing
    }
}

#[async_trait]
impl<S, C> BlockReader for CachedBlockReader<S, C>
where
    S: BlockReader,
    C: CacheOps,
{
    fn block_size(&self) -> u32 {
        self.block_size
    }

    async fn total_blocks(&self) -> GibbloxResult<u64> {
        Ok(self.total_blocks)
    }

    fn write_identity(&self, out: &mut dyn fmt::Write) -> fmt::Result {
        write_cached_identity(&self.inner, out)
    }

    async fn read_blocks(
        &self,
        lba: u64,
        buf: &mut [u8],
        ctx: ReadContext,
    ) -> GibbloxResult<usize> {
        let blocks = self.blocks_from_len(buf.len())?;
        if blocks == 0 {
            return Ok(0);
        }
        self.validate_range(lba, blocks)?;

        let missing = self.fill_from_cache(lba, blocks, buf).await?;

        // Update hit/miss statistics and log periodically
        {
            let mut guard = self.state.lock().await;
            if missing.is_empty() {
                guard.total_hits += blocks;
            } else {
                guard.total_hits += blocks - missing.len() as u64;
                guard.total_misses += missing.len() as u64;
            }
            let total_reads = guard.total_hits + guard.total_misses;
            if total_reads > 0 && (total_reads - guard.last_stats_log) >= 1000 {
                guard.last_stats_log = total_reads;
                let hit_rate = (guard.total_hits as f64 / total_reads as f64) * 100.0;
                debug!(
                    total_hits = guard.total_hits,
                    total_misses = guard.total_misses,
                    hit_rate = format!("{:.1}%", hit_rate),
                    "cache statistics"
                );
            }
        }

        if missing.is_empty() {
            trace!(lba, blocks, "cache hit");
            return Ok(buf.len());
        }

        trace!(lba, blocks, missing = missing.len(), "cache miss");

        let (to_fetch, waiters) = self.mark_in_flight(&missing).await;
        if !to_fetch.is_empty() {
            self.fetch_and_populate(&Self::coalesce(&to_fetch), ctx)
                .await?;
        }
        for waiter in waiters {
            let _ = waiter.await;
        }

        let final_missing = self.fill_from_cache(lba, blocks, buf).await?;
        if !final_missing.is_empty() {
            return Err(GibbloxError::with_message(
                GibbloxErrorKind::Io,
                "cache fetch did not populate all blocks",
            ));
        }
        Ok(buf.len())
    }
}

fn write_cached_identity<S: BlockReader + ?Sized>(
    inner: &S,
    out: &mut dyn fmt::Write,
) -> fmt::Result {
    out.write_str("cached:(")?;
    inner.write_identity(out)?;
    out.write_str(")")
}

fn write_cached_config_identity<C: BlockReaderConfigIdentity + ?Sized>(
    config: &C,
    out: &mut dyn fmt::Write,
) -> fmt::Result {
    out.write_str("cached:(")?;
    config.write_identity(out)?;
    out.write_str(")")
}

pub fn cached_reader_identity_string<S: BlockReader + ?Sized>(inner: &S) -> String {
    let mut identity = String::new();
    let _ = write_cached_identity(inner, &mut identity);
    identity
}

pub fn cached_config_identity_string<C: BlockReaderConfigIdentity + ?Sized>(config: &C) -> String {
    let mut identity = String::new();
    let _ = write_cached_config_identity(config, &mut identity);
    identity
}

pub fn derive_cached_reader_identity_id<S: BlockReader + ?Sized>(
    inner: &S,
    total_blocks: u64,
) -> u32 {
    derive_block_identity_id(inner.block_size(), total_blocks, |writer| {
        write_cached_identity(inner, writer)
    })
}

pub fn derive_cached_config_identity_id<C: BlockReaderConfigIdentity + ?Sized>(config: &C) -> u32 {
    struct CachedConfigIdentity<'a, C: BlockReaderConfigIdentity + ?Sized>(&'a C);

    impl<C: BlockReaderConfigIdentity + ?Sized> BlockReaderConfigIdentity
        for CachedConfigIdentity<'_, C>
    {
        fn write_identity(&self, out: &mut dyn fmt::Write) -> fmt::Result {
            write_cached_config_identity(self.0, out)
        }
    }

    derive_config_identity_id(&CachedConfigIdentity(config))
}

struct CacheLayout {
    block_size: u32,
    total_blocks: u64,
    bitmap_bytes: u64,
    data_bytes: u64,
}

impl CacheLayout {
    fn new(block_size: u32, total_blocks: u64) -> GibbloxResult<Self> {
        if block_size == 0 || !block_size.is_power_of_two() {
            return Err(GibbloxError::with_message(
                GibbloxErrorKind::InvalidInput,
                "block size must be non-zero power of two",
            ));
        }
        let bitmap_bytes = total_blocks.div_ceil(8);
        if bitmap_bytes > usize::MAX as u64 {
            return Err(GibbloxError::with_message(
                GibbloxErrorKind::OutOfRange,
                "bitmap exceeds addressable memory",
            ));
        }
        let data_bytes = total_blocks.checked_mul(block_size as u64).ok_or_else(|| {
            GibbloxError::with_message(GibbloxErrorKind::OutOfRange, "cache too large")
        })?;

        Ok(Self {
            block_size,
            total_blocks,
            bitmap_bytes,
            data_bytes,
        })
    }

    fn bitmap_len_usize(&self) -> usize {
        self.bitmap_bytes as usize
    }
}

struct CacheMapping {
    bitmap_offset: u64,
    data_offset: u64,
    total_len: u64,
}

impl CacheMapping {
    fn new(layout: &CacheLayout, identity_len: usize) -> GibbloxResult<Self> {
        if identity_len > u32::MAX as usize {
            return Err(GibbloxError::with_message(
                GibbloxErrorKind::InvalidInput,
                "identity too large",
            ));
        }
        let bitmap_offset = (CACHE_PREFIX_LEN as u64)
            .checked_add(identity_len as u64)
            .ok_or_else(|| {
                GibbloxError::with_message(GibbloxErrorKind::OutOfRange, "metadata overflow")
            })?;
        let data_offset = bitmap_offset
            .checked_add(layout.bitmap_bytes)
            .ok_or_else(|| {
                GibbloxError::with_message(GibbloxErrorKind::OutOfRange, "bitmap overflow")
            })?;
        let total_len = data_offset.checked_add(layout.data_bytes).ok_or_else(|| {
            GibbloxError::with_message(GibbloxErrorKind::OutOfRange, "cache length overflow")
        })?;
        Ok(Self {
            bitmap_offset,
            data_offset,
            total_len,
        })
    }
}

struct CacheHeader {
    block_size: u32,
    total_blocks: u64,
    identity_len: u32,
}

impl CacheHeader {
    fn encode_prefix(&self) -> [u8; CACHE_PREFIX_LEN] {
        let mut out = [0u8; CACHE_PREFIX_LEN];
        out[0..7].copy_from_slice(&CACHE_MAGIC);
        out[7] = CACHE_VERSION;
        out[8] = 0; // reserved
        out[9] = 0; // reserved
        out[10..12].copy_from_slice(&0u16.to_le_bytes()); // reserved
        out[12..16].copy_from_slice(&self.block_size.to_le_bytes());
        out[16..24].copy_from_slice(&self.total_blocks.to_le_bytes());
        out[24..28].copy_from_slice(&self.identity_len.to_le_bytes());
        out
    }

    fn decode_prefix(prefix: &[u8; CACHE_PREFIX_LEN]) -> Option<Self> {
        if prefix[0..7] != CACHE_MAGIC {
            return None;
        }
        if prefix[7] != CACHE_VERSION {
            return None;
        }
        let block_size = u32::from_le_bytes([prefix[12], prefix[13], prefix[14], prefix[15]]);
        let total_blocks = u64::from_le_bytes([
            prefix[16], prefix[17], prefix[18], prefix[19], prefix[20], prefix[21], prefix[22],
            prefix[23],
        ]);
        let identity_len = u32::from_le_bytes([prefix[24], prefix[25], prefix[26], prefix[27]]);
        Some(Self {
            block_size,
            total_blocks,
            identity_len,
        })
    }
}

struct OpenedCache {
    mapping: CacheMapping,
    valid: Vec<u8>,
}

async fn open_or_initialize_cache<C: CacheOps>(
    cache: &C,
    layout: &CacheLayout,
    identity: &str,
) -> GibbloxResult<OpenedCache> {
    debug!(
        block_size = layout.block_size,
        total_blocks = layout.total_blocks,
        identity = %identity,
        "opening cache state"
    );
    let mut prefix = [0u8; CACHE_PREFIX_LEN];
    let have_prefix = read_exact_at(cache, 0, &mut prefix).await?;
    if !have_prefix {
        debug!("cache prefix missing; initializing new cache file");
        return initialize_cache(cache, layout, identity).await;
    }

    let Some(header) = CacheHeader::decode_prefix(&prefix) else {
        debug!("cache header invalid; reinitializing cache file");
        return initialize_cache(cache, layout, identity).await;
    };
    if header.block_size != layout.block_size || header.total_blocks != layout.total_blocks {
        debug!(
            stored_block_size = header.block_size,
            stored_total_blocks = header.total_blocks,
            expected_block_size = layout.block_size,
            expected_total_blocks = layout.total_blocks,
            "cache geometry mismatch; reinitializing cache file"
        );
        return initialize_cache(cache, layout, identity).await;
    }

    let identity_len = header.identity_len as usize;
    let mapping = CacheMapping::new(layout, identity_len)?;
    let mut stored_identity = vec![0u8; identity_len];
    if !read_exact_at(cache, CACHE_PREFIX_LEN as u64, &mut stored_identity).await? {
        debug!("cache identity bytes missing; reinitializing cache file");
        return initialize_cache(cache, layout, identity).await;
    }
    let stored_identity_str = String::from_utf8_lossy(&stored_identity);
    if stored_identity.as_slice() != identity.as_bytes() {
        debug!(
            stored_identity = %stored_identity_str,
            expected_identity = %identity,
            "cache identity mismatch; reinitializing cache file"
        );
        return initialize_cache(cache, layout, identity).await;
    }

    cache.set_len(mapping.total_len).await?;

    let mut valid = vec![0u8; layout.bitmap_len_usize()];
    if !read_exact_at(cache, mapping.bitmap_offset, &mut valid).await? {
        debug!("cache bitmap missing; reinitializing cache file");
        return initialize_cache(cache, layout, identity).await;
    }

    let valid_blocks = valid.iter().map(|b| b.count_ones()).sum::<u32>();
    debug!(valid_blocks, "cache opened successfully with existing data");

    Ok(OpenedCache { mapping, valid })
}

async fn initialize_cache<C: CacheOps>(
    cache: &C,
    layout: &CacheLayout,
    identity: &str,
) -> GibbloxResult<OpenedCache> {
    let identity_len = identity.len();
    let mapping = CacheMapping::new(layout, identity_len)?;

    cache.set_len(0).await?;
    cache.set_len(mapping.total_len).await?;

    let header = CacheHeader {
        block_size: layout.block_size,
        total_blocks: layout.total_blocks,
        identity_len: identity_len as u32,
    }
    .encode_prefix();

    cache.write_at(0, &header).await?;
    cache
        .write_at(CACHE_PREFIX_LEN as u64, identity.as_bytes())
        .await?;
    write_zero_region(cache, mapping.bitmap_offset, layout.bitmap_bytes).await?;
    cache.flush().await?;

    debug!(
        block_size = layout.block_size,
        total_blocks = layout.total_blocks,
        identity = %identity,
        identity_len,
        "cache file initialized with clean state"
    );

    Ok(OpenedCache {
        mapping,
        valid: vec![0u8; layout.bitmap_len_usize()],
    })
}

async fn read_exact_at<C: CacheOps>(
    cache: &C,
    mut offset: u64,
    out: &mut [u8],
) -> GibbloxResult<bool> {
    let mut filled = 0usize;
    while filled < out.len() {
        let read = cache.read_at(offset, &mut out[filled..]).await?;
        if read == 0 {
            return Ok(false);
        }
        filled = filled.checked_add(read).ok_or_else(|| {
            GibbloxError::with_message(GibbloxErrorKind::OutOfRange, "read size overflow")
        })?;
        offset = offset.checked_add(read as u64).ok_or_else(|| {
            GibbloxError::with_message(GibbloxErrorKind::OutOfRange, "read offset overflow")
        })?;
    }
    Ok(true)
}

async fn write_zero_region<C: CacheOps>(cache: &C, mut offset: u64, len: u64) -> GibbloxResult<()> {
    if len == 0 {
        return Ok(());
    }
    let mut remaining = len;
    let zero = [0u8; ZERO_CHUNK_LEN];
    while remaining > 0 {
        let write_len = remaining.min(ZERO_CHUNK_LEN as u64) as usize;
        cache.write_at(offset, &zero[..write_len]).await?;
        offset = offset.checked_add(write_len as u64).ok_or_else(|| {
            GibbloxError::with_message(GibbloxErrorKind::OutOfRange, "zero-fill offset overflow")
        })?;
        remaining -= write_len as u64;
    }
    Ok(())
}

struct InFlight {
    waiters: BTreeMap<u64, Vec<oneshot::Sender<()>>>,
}

impl InFlight {
    fn new() -> Self {
        Self {
            waiters: BTreeMap::new(),
        }
    }
}

struct CacheState {
    valid: Vec<u8>,
    blocks_since_flush: u32,
    total_hits: u64,
    total_misses: u64,
    last_stats_log: u64,
    cached_blocks: u64,
}

fn bit_is_set(bits: &[u8], idx: u64) -> bool {
    let byte_idx = (idx / 8) as usize;
    let mask = 1u8 << (idx % 8);
    bits[byte_idx] & mask != 0
}

fn bit_mask(start_bit: u8, end_bit: u8) -> u8 {
    let width = (end_bit - start_bit + 1) as u16;
    let ones = if width >= 8 {
        u16::from(u8::MAX)
    } else {
        (1u16 << width) - 1
    };
    (ones as u8) << start_bit
}

fn set_bits_and_count_new(bits: &mut [u8], start: u64, len: u64) -> u64 {
    let mut idx = start;
    let end = start + len;
    let mut newly_set = 0u64;
    while idx < end {
        let byte_idx = (idx / 8) as usize;
        let start_bit = (idx % 8) as u8;
        let span = ((8 - start_bit as u64).min(end - idx)) as u8;
        let end_bit = start_bit + span - 1;
        let mask = bit_mask(start_bit, end_bit);
        let before = bits[byte_idx];
        let after = before | mask;
        bits[byte_idx] = after;
        newly_set += (after.count_ones() - before.count_ones()) as u64;
        idx += span as u64;
    }
    newly_set
}

fn clear_bits_and_count_removed(bits: &mut [u8], start: u64, len: u64) -> u64 {
    let mut idx = start;
    let end = start + len;
    let mut removed = 0u64;
    while idx < end {
        let byte_idx = (idx / 8) as usize;
        let start_bit = (idx % 8) as u8;
        let span = ((8 - start_bit as u64).min(end - idx)) as u8;
        let end_bit = start_bit + span - 1;
        let mask = bit_mask(start_bit, end_bit);
        let before = bits[byte_idx];
        let after = before & !mask;
        bits[byte_idx] = after;
        removed += (before.count_ones() - after.count_ones()) as u64;
        idx += span as u64;
    }
    removed
}

fn count_set_bits(bits: &[u8]) -> u64 {
    bits.iter().map(|byte| byte.count_ones() as u64).sum()
}

/// In-memory cache file implementation useful for tests and embedded callers.
pub struct MemoryCacheOps {
    state: async_lock::Mutex<Vec<u8>>,
}

impl MemoryCacheOps {
    pub fn new() -> Self {
        Self {
            state: async_lock::Mutex::new(Vec::new()),
        }
    }
}

impl Default for MemoryCacheOps {
    fn default() -> Self {
        Self::new()
    }
}

#[async_trait]
impl CacheOps for MemoryCacheOps {
    async fn read_at(&self, offset: u64, out: &mut [u8]) -> GibbloxResult<usize> {
        if out.is_empty() {
            return Ok(0);
        }
        let guard = self.state.lock().await;
        let start = match usize::try_from(offset) {
            Ok(v) => v,
            Err(_) => return Ok(0),
        };
        if start >= guard.len() {
            return Ok(0);
        }
        let available = guard.len() - start;
        let copy_len = available.min(out.len());
        out[..copy_len].copy_from_slice(&guard[start..start + copy_len]);
        Ok(copy_len)
    }

    async fn write_at(&self, offset: u64, data: &[u8]) -> GibbloxResult<()> {
        if data.is_empty() {
            return Ok(());
        }
        let mut guard = self.state.lock().await;
        let start = usize::try_from(offset).map_err(|_| {
            GibbloxError::with_message(GibbloxErrorKind::OutOfRange, "offset exceeds memory cache")
        })?;
        let end = start.checked_add(data.len()).ok_or_else(|| {
            GibbloxError::with_message(GibbloxErrorKind::OutOfRange, "write overflow")
        })?;
        if end > guard.len() {
            guard.resize(end, 0);
        }
        guard[start..end].copy_from_slice(data);
        Ok(())
    }

    async fn set_len(&self, len: u64) -> GibbloxResult<()> {
        let len = usize::try_from(len).map_err(|_| {
            GibbloxError::with_message(GibbloxErrorKind::OutOfRange, "length exceeds memory cache")
        })?;
        let mut guard = self.state.lock().await;
        guard.resize(len, 0);
        Ok(())
    }

    async fn flush(&self) -> GibbloxResult<()> {
        Ok(())
    }
}