move pure WAL implementation to wal.rs

2 files changed, 355 insertions, 354 deletions

diff --git a/src/lib.rs b/src/lib.rs
index a2b88ad..7635dab 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,355 +1,2 @@
 #[macro_use] extern crate scan_fmt;
-use std::collections::BinaryHeap;
-
-#[repr(u8)]
-enum WALRingType {
-    #[allow(dead_code)]
-    Null = 0x0,
-    Full,
-    First,
-    Middle,
-    Last
-}
-
-#[repr(packed)]
-struct WALRingBlob {
-    crc32: u32,
-    rsize: u32,
-    rtype: WALRingType,
-    // payload follows
-}
-
-pub type WALBytes = Box<[u8]>;
-pub type WALFileId = u64;
-pub type WALPos = u64;
-
-#[derive(Eq, PartialEq, Copy, Clone, Debug)]
-pub struct WALRingId {
-    start: WALPos,
-    end: WALPos
-}
-
-impl Ord for WALRingId {
-    fn cmp(&self, other: &WALRingId) -> std::cmp::Ordering {
-        other.start.cmp(&self.start).then_with(|| other.end.cmp(&self.end))
-    }
-}
-
-impl PartialOrd for WALRingId {
-    fn partial_cmp(&self, other: &WALRingId) -> Option<std::cmp::Ordering> {
-        Some(self.cmp(other))
-    }
-}
-
-/// the state for a WAL writer
-struct WALState {
-    /// the first file id of WAL
-    first_fid: WALFileId,
-    /// the next position for a record, addressed in the entire WAL space
-    next: WALPos,
-    /// number of bits for a file
-    file_nbit: u64,
-}
-
-pub trait WALFile {
-    /// Initialize the file space in [offset, offset + length) to zero.
-    fn allocate(&self, offset: WALPos, length: usize) -> Result<(), ()>;
-    /// Truncate a file to a specified length.
-    fn truncate(&self, length: usize) -> Result<(), ()>;
-    /// Write data with offset.
-    fn write(&self, offset: WALPos, data: WALBytes);
-    /// Read data with offset.
-    fn read(&self, offset: WALPos, length: usize) -> WALBytes;
-}
-
-pub trait WALStore {
-    /// Open a file given the filename, create the file if not exists when `touch` is `true`.
-    fn open_file(&self, filename: &str, touch: bool) -> Option<Box<dyn WALFile>>;
-    /// Unlink a file given the filename.
-    fn remove_file(&self, filename: &str) -> Result<(), ()>;
-    /// Enumerate all WAL files.
-    fn enumerate_files(&self) -> Box<[String]>;
-    /// Apply (redo) the payload during recovery.
-    fn apply_payload(&self, payload: WALBytes);
-}
-
-/// The middle layer that manages WAL file handles and invokes public trait functions to actually
-/// manipulate files and their contents.
-struct WALFilePool<F: WALStore> {
-    store: F,
-    handles: lru::LruCache<WALFileId, Box<dyn WALFile>>,
-    file_nbit: u64,
-    file_size: u64,
-    block_nbit: u64,
-}
-
-impl<F: WALStore> WALFilePool<F> {
-    fn new(store: F, file_nbit: u8, block_nbit: u8, cache_size: usize) -> Self {
-        let file_nbit = file_nbit as u64;
-        let block_nbit = block_nbit as u64;
-        WALFilePool {
-            store,
-            handles: lru::LruCache::new(cache_size),
-            file_nbit,
-            file_size: 1 << (file_nbit as u64),
-            block_nbit,
-        }
-    }
-
-    fn get_fname(fid: WALFileId) -> String {
-        format!("{:08x}.log", fid)
-    }
-
-    fn get_file(&mut self, fid: u64, touch: bool) -> &'static dyn WALFile {
-        let h = match self.handles.get(&fid) {
-            Some(h) => &**h,
-            None => {
-                self.handles.put(fid, self.store.open_file(&Self::get_fname(fid), touch).unwrap());
-                &**self.handles.get(&fid).unwrap()
-            }
-        };
-        unsafe {&*(h as *const dyn WALFile)}
-    }
-
-    fn get_fid(&mut self, fname: &str) -> WALFileId {
-        scan_fmt!(fname, "{x}.log", [hex WALFileId]).unwrap()
-    }
-
-    // TODO: evict stale handles
-    fn write(&mut self, writes: Vec<(WALPos, WALBytes)>) {
-        // pre-allocate the file space
-        let mut fid = writes[0].0 >> self.file_nbit;
-        let mut alloc_start = writes[0].0 & (self.file_size - 1);
-        let mut alloc_end = alloc_start + writes[0].1.len() as u64;
-        let mut h = self.get_file(fid, true);
-        for (off, w) in &writes[1..] {
-            let next_fid = off >> self.file_nbit;
-            if next_fid != fid {
-                h.allocate(alloc_start, (alloc_end - alloc_start) as usize).unwrap();
-                h = self.get_file(next_fid, true);
-                alloc_start = 0;
-                alloc_end = alloc_start + w.len() as u64;
-                fid = next_fid;
-            } else {
-                alloc_end += w.len() as u64;
-            }
-        }
-        h.allocate(alloc_start, (alloc_end - alloc_start) as usize).unwrap();
-        for (off, w) in writes.into_iter() {
-            self.get_file(off >> self.file_nbit, true).write(off & (self.file_size - 1), w);
-        }
-    }
-
-    fn remove_file(&self, fid: u64) -> Result<(), ()> {
-        self.store.remove_file(&Self::get_fname(fid))
-    }
-
-    fn reset(&mut self) { self.handles.clear() }
-}
-
-pub struct WALWriter<F: WALStore> {
-    state: WALState,
-    file_pool: WALFilePool<F>,
-    block_buffer: WALBytes,
-    block_size: u32,
-    next_complete: WALPos,
-    io_complete: BinaryHeap<WALRingId>
-}
-
-impl<F: WALStore> WALWriter<F> {
-    fn new(state: WALState, file_pool: WALFilePool<F>) -> Self {
-        let mut b = Vec::new();
-        let block_size = 1 << file_pool.block_nbit as u32;
-        //let block_nbit = state.block_nbit;
-        //let block_size = 1 << (block_nbit as u32);
-        //let file_nbit = state.file_nbit;
-        //let file_size = 1 << (file_nbit as u64);
-        b.resize(block_size as usize, 0);
-        WALWriter{
-            state,
-            file_pool,
-            block_buffer: b.into_boxed_slice(),
-            block_size,
-            next_complete: 0,
-            io_complete: BinaryHeap::new(),
-        }
-    }
-
-    /// Submit a sequence of records to WAL; WALStore/WALFile callbacks are invoked before the
-    /// function returns.  The caller then has the knowledge of WAL writes so it should defer
-    /// actual data writes after WAL writes.
-    pub fn grow<T: AsRef<[WALBytes]>>(&mut self, records: T) -> Box<[WALRingId]> {
-        let mut res = Vec::new();
-        let mut writes = Vec::new();
-        let msize = std::mem::size_of::<WALRingBlob>() as u32;
-        // the global offest of the begining of the block
-        // the start of the unwritten data
-        let mut bbuff_start = self.state.next as u32 & (self.block_size - 1);
-        // the end of the unwritten data
-        let mut bbuff_cur = bbuff_start;
-
-        for _rec in records.as_ref() {
-            let mut rec = &_rec[..];
-            let mut rsize = rec.len() as u32;
-            let mut started = false;
-            while rsize > 0 {
-                let remain = self.block_size - bbuff_cur;
-                if remain > msize {
-                    let d = remain - msize;
-                    let ring_start = self.state.next + (bbuff_cur - bbuff_start) as u64;
-                    let blob = unsafe {std::mem::transmute::<*mut u8, &mut WALRingBlob>(
-                        (&mut self.block_buffer[bbuff_cur as usize..]).as_mut_ptr())};
-                    bbuff_cur += msize;
-                    if d >= rsize {
-                        // the remaining rec fits in the block
-                        let payload = rec;
-                        blob.crc32 = crc::crc32::checksum_ieee(payload);
-                        blob.rsize = rsize;
-                        blob.rtype = if started {WALRingType::Last} else {WALRingType::Full};
-                        &mut self.block_buffer[
-                            bbuff_cur as usize..
-                            bbuff_cur as usize + payload.len()].copy_from_slice(payload);
-                        bbuff_cur += rsize;
-                        rsize = 0;
-                    } else {
-                        // the remaining block can only accommodate partial rec
-                        let payload = &rec[..d as usize];
-                        blob.crc32 = crc::crc32::checksum_ieee(payload);
-                        blob.rsize = d;
-                        blob.rtype = if started {WALRingType::Middle} else {
-                            started = true;
-                            WALRingType::First
-                        };
-                        &mut self.block_buffer[
-                            bbuff_cur as usize..
-                            bbuff_cur as usize + payload.len()].copy_from_slice(payload);
-                        bbuff_cur += d;
-                        rsize -= d;
-                        rec = &rec[d as usize..];
-                    }
-                    let ring_end = self.state.next + (bbuff_cur - bbuff_start) as u64;
-                    res.push(WALRingId{start: ring_start, end: ring_end});
-                } else {
-                    // add padding space by moving the point to the end of the block
-                    bbuff_cur = self.block_size;
-                }
-                if bbuff_cur == self.block_size {
-                    writes.push((self.state.next,
-                             self.block_buffer[bbuff_start as usize..]
-                                .to_vec().into_boxed_slice()));
-                    self.state.next += (self.block_size - bbuff_start) as u64;
-                    bbuff_start = 0;
-                    bbuff_cur = 0;
-                }
-            }
-        }
-        if bbuff_cur > bbuff_start {
-            writes.push((self.state.next,
-                     self.block_buffer[bbuff_start as usize..bbuff_cur as usize]
-                        .to_vec().into_boxed_slice()));
-            self.state.next += (bbuff_cur - bbuff_start) as u64;
-        }
-        self.file_pool.write(writes);
-        res.into_boxed_slice()
-    }
-
-    /// Inform the WALWriter that data writes (specified by a slice of (offset, length) tuples) are
-    /// complete so that it could automatically remove obsolete WAL files.
-    pub fn peel<T: AsRef<[WALRingId]>>(&mut self, records: T) {
-        let msize = std::mem::size_of::<WALRingBlob>() as u64;
-        let block_size = self.block_size as u64;
-        for rec in records.as_ref() {
-            self.io_complete.push(*rec)
-        }
-        let orig_fid = self.state.first_fid;
-        while let Some(s) = self.io_complete.peek().and_then(|&e| Some(e.start)) {
-            if s != self.next_complete {
-                break
-            }
-            let mut m = self.io_complete.pop().unwrap();
-            let block_remain = block_size - (m.end & (block_size - 1));
-            if block_remain <= msize as u64 {
-                m.end += block_remain
-            }
-            self.next_complete = m.end
-        }
-        let next_fid = self.next_complete >> self.state.file_nbit;
-        for fid in orig_fid..next_fid {
-            self.file_pool.remove_file(fid).unwrap();
-        }
-        self.state.first_fid = next_fid;
-    }
-}
-
-pub struct WALLoader<F: WALStore> {
-    file_pool: WALFilePool<F>,
-}
-
-impl<F: WALStore> WALLoader<F> {
-    pub fn new(store: F, file_nbit: u8, block_nbit: u8, cache_size: usize) -> Self {
-        let file_pool = WALFilePool::new(store, file_nbit, block_nbit, cache_size);
-        WALLoader{ file_pool }
-    }
-
-    pub fn recover(mut self) -> WALWriter<F> {
-        let block_size = 1 << self.file_pool.block_nbit;
-        let msize = std::mem::size_of::<WALRingBlob>() as u32;
-        let mut logfiles = self.file_pool.store.enumerate_files();
-        // TODO: use regex to filter out invalid files
-        // TODO: check for missing logfiles
-        logfiles.sort();
-        let mut chunks = None;
-        for fname in logfiles.iter() {
-            let fid = self.file_pool.get_fid(fname);
-            let f = self.file_pool.get_file(fid, false);
-            let mut off = 0;
-            while block_size - (off & (block_size - 1)) > msize as u64 {
-                let header_raw = f.read(off, msize as usize);
-                off += msize as u64;
-                let header = unsafe {
-                    std::mem::transmute::<*const u8, &WALRingBlob>(header_raw.as_ptr())};
-                let rsize = header.rsize;
-                match header.rtype {
-                    WALRingType::Full => {
-                        assert!(chunks.is_none());
-                        let payload = f.read(off, rsize as usize);
-                        off += rsize as u64;
-                        self.file_pool.store.apply_payload(payload);
-                    },
-                    WALRingType::First => {
-                        assert!(chunks.is_none());
-                        chunks = Some(vec![f.read(off, rsize as usize)]);
-                        off += rsize as u64;
-                    },
-                    WALRingType::Middle => {
-                        chunks.as_mut().unwrap().push(f.read(off, rsize as usize));
-                        off += rsize as u64;
-                    },
-                    WALRingType::Last => {
-                        chunks.as_mut().unwrap().push(f.read(off, rsize as usize));
-                        off += rsize as u64;
-
-                        let _chunks = chunks.take().unwrap();
-                        let mut payload = Vec::new();
-                        payload.resize(_chunks.iter().fold(0, |acc, v| acc + v.len()), 0);
-                        let mut ps = &mut payload[..];
-                        for c in _chunks {
-                            ps[..c.len()].copy_from_slice(&*c);
-                            ps = &mut ps[c.len()..];
-                        }
-                        self.file_pool.store.apply_payload(payload.into_boxed_slice());
-                    },
-                    WALRingType::Null => break,
-                }
-            }
-            f.truncate(0).unwrap();
-            self.file_pool.remove_file(fid).unwrap();
-        }
-        self.file_pool.reset();
-        WALWriter::new(WALState {
-            first_fid: 0,
-            next: 0,
-            file_nbit: self.file_pool.file_nbit,
-        }, self.file_pool)
-    }
-}
+pub mod wal;
diff --git a/src/wal.rs b/src/wal.rs
new file mode 100644
index 0000000..05ffd01
--- /dev/null
+++ b/src/wal.rs
@@ -0,0 +1,354 @@
+use std::collections::BinaryHeap;
+
+#[repr(u8)]
+enum WALRingType {
+    #[allow(dead_code)]
+    Null = 0x0,
+    Full,
+    First,
+    Middle,
+    Last
+}
+
+#[repr(packed)]
+struct WALRingBlob {
+    crc32: u32,
+    rsize: u32,
+    rtype: WALRingType,
+    // payload follows
+}
+
+pub type WALBytes = Box<[u8]>;
+pub type WALFileId = u64;
+pub type WALPos = u64;
+
+#[derive(Eq, PartialEq, Copy, Clone, Debug)]
+pub struct WALRingId {
+    start: WALPos,
+    end: WALPos
+}
+
+impl Ord for WALRingId {
+    fn cmp(&self, other: &WALRingId) -> std::cmp::Ordering {
+        other.start.cmp(&self.start).then_with(|| other.end.cmp(&self.end))
+    }
+}
+
+impl PartialOrd for WALRingId {
+    fn partial_cmp(&self, other: &WALRingId) -> Option<std::cmp::Ordering> {
+        Some(self.cmp(other))
+    }
+}
+
+/// the state for a WAL writer
+struct WALState {
+    /// the first file id of WAL
+    first_fid: WALFileId,
+    /// the next position for a record, addressed in the entire WAL space
+    next: WALPos,
+    /// number of bits for a file
+    file_nbit: u64,
+}
+
+pub trait WALFile {
+    /// Initialize the file space in [offset, offset + length) to zero.
+    fn allocate(&self, offset: WALPos, length: usize) -> Result<(), ()>;
+    /// Truncate a file to a specified length.
+    fn truncate(&self, length: usize) -> Result<(), ()>;
+    /// Write data with offset.
+    fn write(&self, offset: WALPos, data: WALBytes);
+    /// Read data with offset.
+    fn read(&self, offset: WALPos, length: usize) -> WALBytes;
+}
+
+pub trait WALStore {
+    /// Open a file given the filename, create the file if not exists when `touch` is `true`.
+    fn open_file(&self, filename: &str, touch: bool) -> Option<Box<dyn WALFile>>;
+    /// Unlink a file given the filename.
+    fn remove_file(&self, filename: &str) -> Result<(), ()>;
+    /// Enumerate all WAL files.
+    fn enumerate_files(&self) -> Box<[String]>;
+    /// Apply (redo) the payload during recovery.
+    fn apply_payload(&self, payload: WALBytes);
+}
+
+/// The middle layer that manages WAL file handles and invokes public trait functions to actually
+/// manipulate files and their contents.
+struct WALFilePool<F: WALStore> {
+    store: F,
+    handles: lru::LruCache<WALFileId, Box<dyn WALFile>>,
+    file_nbit: u64,
+    file_size: u64,
+    block_nbit: u64,
+}
+
+impl<F: WALStore> WALFilePool<F> {
+    fn new(store: F, file_nbit: u8, block_nbit: u8, cache_size: usize) -> Self {
+        let file_nbit = file_nbit as u64;
+        let block_nbit = block_nbit as u64;
+        WALFilePool {
+            store,
+            handles: lru::LruCache::new(cache_size),
+            file_nbit,
+            file_size: 1 << (file_nbit as u64),
+            block_nbit,
+        }
+    }
+
+    fn get_fname(fid: WALFileId) -> String {
+        format!("{:08x}.log", fid)
+    }
+
+    fn get_file(&mut self, fid: u64, touch: bool) -> &'static dyn WALFile {
+        let h = match self.handles.get(&fid) {
+            Some(h) => &**h,
+            None => {
+                self.handles.put(fid, self.store.open_file(&Self::get_fname(fid), touch).unwrap());
+                &**self.handles.get(&fid).unwrap()
+            }
+        };
+        unsafe {&*(h as *const dyn WALFile)}
+    }
+
+    fn get_fid(&mut self, fname: &str) -> WALFileId {
+        scan_fmt!(fname, "{x}.log", [hex WALFileId]).unwrap()
+    }
+
+    // TODO: evict stale handles
+    fn write(&mut self, writes: Vec<(WALPos, WALBytes)>) {
+        // pre-allocate the file space
+        let mut fid = writes[0].0 >> self.file_nbit;
+        let mut alloc_start = writes[0].0 & (self.file_size - 1);
+        let mut alloc_end = alloc_start + writes[0].1.len() as u64;
+        let mut h = self.get_file(fid, true);
+        for (off, w) in &writes[1..] {
+            let next_fid = off >> self.file_nbit;
+            if next_fid != fid {
+                h.allocate(alloc_start, (alloc_end - alloc_start) as usize).unwrap();
+                h = self.get_file(next_fid, true);
+                alloc_start = 0;
+                alloc_end = alloc_start + w.len() as u64;
+                fid = next_fid;
+            } else {
+                alloc_end += w.len() as u64;
+            }
+        }
+        h.allocate(alloc_start, (alloc_end - alloc_start) as usize).unwrap();
+        for (off, w) in writes.into_iter() {
+            self.get_file(off >> self.file_nbit, true).write(off & (self.file_size - 1), w);
+        }
+    }
+
+    fn remove_file(&self, fid: u64) -> Result<(), ()> {
+        self.store.remove_file(&Self::get_fname(fid))
+    }
+
+    fn reset(&mut self) { self.handles.clear() }
+}
+
+pub struct WALWriter<F: WALStore> {
+    state: WALState,
+    file_pool: WALFilePool<F>,
+    block_buffer: WALBytes,
+    block_size: u32,
+    next_complete: WALPos,
+    io_complete: BinaryHeap<WALRingId>
+}
+
+impl<F: WALStore> WALWriter<F> {
+    fn new(state: WALState, file_pool: WALFilePool<F>) -> Self {
+        let mut b = Vec::new();
+        let block_size = 1 << file_pool.block_nbit as u32;
+        //let block_nbit = state.block_nbit;
+        //let block_size = 1 << (block_nbit as u32);
+        //let file_nbit = state.file_nbit;
+        //let file_size = 1 << (file_nbit as u64);
+        b.resize(block_size as usize, 0);
+        WALWriter{
+            state,
+            file_pool,
+            block_buffer: b.into_boxed_slice(),
+            block_size,
+            next_complete: 0,
+            io_complete: BinaryHeap::new(),
+        }
+    }
+
+    /// Submit a sequence of records to WAL; WALStore/WALFile callbacks are invoked before the
+    /// function returns.  The caller then has the knowledge of WAL writes so it should defer
+    /// actual data writes after WAL writes.
+    pub fn grow<T: AsRef<[WALBytes]>>(&mut self, records: T) -> Box<[WALRingId]> {
+        let mut res = Vec::new();
+        let mut writes = Vec::new();
+        let msize = std::mem::size_of::<WALRingBlob>() as u32;
+        // the global offest of the begining of the block
+        // the start of the unwritten data
+        let mut bbuff_start = self.state.next as u32 & (self.block_size - 1);
+        // the end of the unwritten data
+        let mut bbuff_cur = bbuff_start;
+
+        for _rec in records.as_ref() {
+            let mut rec = &_rec[..];
+            let mut rsize = rec.len() as u32;
+            let mut started = false;
+            while rsize > 0 {
+                let remain = self.block_size - bbuff_cur;
+                if remain > msize {
+                    let d = remain - msize;
+                    let ring_start = self.state.next + (bbuff_cur - bbuff_start) as u64;
+                    let blob = unsafe {std::mem::transmute::<*mut u8, &mut WALRingBlob>(
+                        (&mut self.block_buffer[bbuff_cur as usize..]).as_mut_ptr())};
+                    bbuff_cur += msize;
+                    if d >= rsize {
+                        // the remaining rec fits in the block
+                        let payload = rec;
+                        blob.crc32 = crc::crc32::checksum_ieee(payload);
+                        blob.rsize = rsize;
+                        blob.rtype = if started {WALRingType::Last} else {WALRingType::Full};
+                        &mut self.block_buffer[
+                            bbuff_cur as usize..
+                            bbuff_cur as usize + payload.len()].copy_from_slice(payload);
+                        bbuff_cur += rsize;
+                        rsize = 0;
+                    } else {
+                        // the remaining block can only accommodate partial rec
+                        let payload = &rec[..d as usize];
+                        blob.crc32 = crc::crc32::checksum_ieee(payload);
+                        blob.rsize = d;
+                        blob.rtype = if started {WALRingType::Middle} else {
+                            started = true;
+                            WALRingType::First
+                        };
+                        &mut self.block_buffer[
+                            bbuff_cur as usize..
+                            bbuff_cur as usize + payload.len()].copy_from_slice(payload);
+                        bbuff_cur += d;
+                        rsize -= d;
+                        rec = &rec[d as usize..];
+                    }
+                    let ring_end = self.state.next + (bbuff_cur - bbuff_start) as u64;
+                    res.push(WALRingId{start: ring_start, end: ring_end});
+                } else {
+                    // add padding space by moving the point to the end of the block
+                    bbuff_cur = self.block_size;
+                }
+                if bbuff_cur == self.block_size {
+                    writes.push((self.state.next,
+                             self.block_buffer[bbuff_start as usize..]
+                                .to_vec().into_boxed_slice()));
+                    self.state.next += (self.block_size - bbuff_start) as u64;
+                    bbuff_start = 0;
+                    bbuff_cur = 0;
+                }
+            }
+        }
+        if bbuff_cur > bbuff_start {
+            writes.push((self.state.next,
+                     self.block_buffer[bbuff_start as usize..bbuff_cur as usize]
+                        .to_vec().into_boxed_slice()));
+            self.state.next += (bbuff_cur - bbuff_start) as u64;
+        }
+        self.file_pool.write(writes);
+        res.into_boxed_slice()
+    }
+
+    /// Inform the WALWriter that data writes (specified by a slice of (offset, length) tuples) are
+    /// complete so that it could automatically remove obsolete WAL files.
+    pub fn peel<T: AsRef<[WALRingId]>>(&mut self, records: T) {
+        let msize = std::mem::size_of::<WALRingBlob>() as u64;
+        let block_size = self.block_size as u64;
+        for rec in records.as_ref() {
+            self.io_complete.push(*rec)
+        }
+        let orig_fid = self.state.first_fid;
+        while let Some(s) = self.io_complete.peek().and_then(|&e| Some(e.start)) {
+            if s != self.next_complete {
+                break
+            }
+            let mut m = self.io_complete.pop().unwrap();
+            let block_remain = block_size - (m.end & (block_size - 1));
+            if block_remain <= msize as u64 {
+                m.end += block_remain
+            }
+            self.next_complete = m.end
+        }
+        let next_fid = self.next_complete >> self.state.file_nbit;
+        for fid in orig_fid..next_fid {
+            self.file_pool.remove_file(fid).unwrap();
+        }
+        self.state.first_fid = next_fid;
+    }
+}
+
+pub struct WALLoader<F: WALStore> {
+    file_pool: WALFilePool<F>,
+}
+
+impl<F: WALStore> WALLoader<F> {
+    pub fn new(store: F, file_nbit: u8, block_nbit: u8, cache_size: usize) -> Self {
+        let file_pool = WALFilePool::new(store, file_nbit, block_nbit, cache_size);
+        WALLoader{ file_pool }
+    }
+
+    pub fn recover(mut self) -> WALWriter<F> {
+        let block_size = 1 << self.file_pool.block_nbit;
+        let msize = std::mem::size_of::<WALRingBlob>() as u32;
+        let mut logfiles = self.file_pool.store.enumerate_files();
+        // TODO: use regex to filter out invalid files
+        // TODO: check for missing logfiles
+        logfiles.sort();
+        let mut chunks = None;
+        for fname in logfiles.iter() {
+            let fid = self.file_pool.get_fid(fname);
+            let f = self.file_pool.get_file(fid, false);
+            let mut off = 0;
+            while block_size - (off & (block_size - 1)) > msize as u64 {
+                let header_raw = f.read(off, msize as usize);
+                off += msize as u64;
+                let header = unsafe {
+                    std::mem::transmute::<*const u8, &WALRingBlob>(header_raw.as_ptr())};
+                let rsize = header.rsize;
+                match header.rtype {
+                    WALRingType::Full => {
+                        assert!(chunks.is_none());
+                        let payload = f.read(off, rsize as usize);
+                        off += rsize as u64;
+                        self.file_pool.store.apply_payload(payload);
+                    },
+                    WALRingType::First => {
+                        assert!(chunks.is_none());
+                        chunks = Some(vec![f.read(off, rsize as usize)]);
+                        off += rsize as u64;
+                    },
+                    WALRingType::Middle => {
+                        chunks.as_mut().unwrap().push(f.read(off, rsize as usize));
+                        off += rsize as u64;
+                    },
+                    WALRingType::Last => {
+                        chunks.as_mut().unwrap().push(f.read(off, rsize as usize));
+                        off += rsize as u64;
+
+                        let _chunks = chunks.take().unwrap();
+                        let mut payload = Vec::new();
+                        payload.resize(_chunks.iter().fold(0, |acc, v| acc + v.len()), 0);
+                        let mut ps = &mut payload[..];
+                        for c in _chunks {
+                            ps[..c.len()].copy_from_slice(&*c);
+                            ps = &mut ps[c.len()..];
+                        }
+                        self.file_pool.store.apply_payload(payload.into_boxed_slice());
+                    },
+                    WALRingType::Null => break,
+                }
+            }
+            f.truncate(0).unwrap();
+            self.file_pool.remove_file(fid).unwrap();
+        }
+        self.file_pool.reset();
+        WALWriter::new(WALState {
+            first_fid: 0,
+            next: 0,
+            file_nbit: self.file_pool.file_nbit,
+        }, self.file_pool)
+    }
+}