#![allow(unused)]
use std::{
    collections::{BTreeMap, VecDeque},
    fmt::Debug,
    fs::File,
    marker::PhantomData,
    mem::size_of,
    ops::Range,
    sync::Arc,
};

mod allocator;
mod atomic_arc;
mod mapped;
mod transaction;

use allocator::{AllocatorState, GeneralPurposeAllocator, SlabListPointer, SlabPointer};
use atomic_arc::AtomicArc;
use memmap::{Mmap, MmapMut};
use transaction::TransactionHandle;
use zerocopy::{AsBytes, FromBytes, FromZeroes, Ref, Unaligned, LE};

const PAGE_SIZE: u64 = 4096;

type U64 = zerocopy::byteorder::U64<LE>;
type U32 = zerocopy::byteorder::U32<LE>;
type U16 = zerocopy::byteorder::U16<LE>;

#[derive(Clone, Copy, FromBytes, FromZeroes, AsBytes, Unaligned, Hash)]
#[repr(transparent)]
pub struct FilePointer<T> {
    inner: RawFilePointer,
    _phantom: PhantomData<*const T>,
}

impl<T> Debug for FilePointer<T> {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        self.inner.fmt(f)
    }
}

impl<T> PartialOrd for FilePointer<T> {
    fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
        self.inner.partial_cmp(&other.inner)
    }
}

impl<T> Ord for FilePointer<T> {
    fn cmp(&self, other: &Self) -> std::cmp::Ordering {
        self.inner.cmp(&other.inner)
    }
}

impl<T> PartialEq for FilePointer<T> {
    fn eq(&self, other: &Self) -> bool {
        self.inner == other.inner
    }
}

impl<T> Eq for FilePointer<T> {}

impl<T> FilePointer<T> {
    fn from_range(range: FileRange) -> Self {
        assert_eq!(range.len(), size_of::<T>() as u64);
        Self::new(range.start)
    }
}

impl<T> FilePointer<T> {
    pub fn new(inner: RawFilePointer) -> Self {
        Self {
            inner,
            _phantom: PhantomData,
        }
    }

    pub fn null() -> Self {
        Self::new(RawFilePointer::null())
    }

    pub fn is_null(self) -> bool {
        self.inner.is_null()
    }

    pub fn range(self) -> FileRange {
        self.inner.range(size_of::<T>() as u64)
    }

    pub fn into_raw(self) -> RawFilePointer {
        self.inner
    }
}

#[derive(Clone, Copy, FromBytes, FromZeroes, AsBytes, Unaligned, Hash, PartialEq, Eq)]
#[repr(transparent)]
pub struct RawFilePointer(U64);

impl PartialOrd for RawFilePointer {
    fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
        self.0.get().partial_cmp(&other.0.get())
    }
}

impl Ord for RawFilePointer {
    fn cmp(&self, other: &Self) -> std::cmp::Ordering {
        self.0.get().cmp(&other.0.get())
    }
}

impl Debug for RawFilePointer {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "0x{:x}", self.0.get())
    }
}

impl RawFilePointer {
    fn page(self) -> PagePointer {
        PagePointer(u32::try_from(self.0.get() / PAGE_SIZE).unwrap().into())
    }
    fn page_offset(self) -> (PagePointer, u16) {
        (self.page(), (self.0.get() % PAGE_SIZE) as u16)
    }
    fn from_page_and_offset(page: PagePointer, offset: u16) -> Self {
        debug_assert!(
            offset < PAGE_SIZE as u16,
            "offset 0x{offset:x} out for page bounds (0..0x{PAGE_SIZE:x})"
        );
        page.start() + offset as u64
    }
    fn null() -> Self {
        Self(U64::ZERO)
    }
    fn is_null(self) -> bool {
        self == Self::null()
    }
}

#[derive(Clone, Copy, FromBytes, FromZeroes, AsBytes, Unaligned, Debug, PartialEq, Eq)]
#[repr(transparent)]
pub struct PagePointer(U32);

impl PagePointer {
    fn start(self) -> RawFilePointer {
        RawFilePointer((self.0.get() as u64 * PAGE_SIZE).into())
    }
    fn range(self) -> FileRange {
        self.start().range(PAGE_SIZE)
    }
    fn nth(n: u32) -> Self {
        Self(n.into())
    }
    fn null() -> Self {
        Self::nth(0)
    }
}

#[derive(Clone, Copy, FromBytes, FromZeroes, AsBytes, Unaligned, PartialEq, Eq)]
#[repr(C)]
pub struct FileRange {
    start: RawFilePointer,
    len: U64,
}

impl Debug for FileRange {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "{:?}..{:?}", self.start, self.start + self.len.get())
    }
}

impl std::ops::Add<u64> for RawFilePointer {
    type Output = Self;

    fn add(self, rhs: u64) -> Self::Output {
        Self((self.0.get() + rhs).into())
    }
}

impl RawFilePointer {
    pub fn range(&self, len: u64) -> FileRange {
        FileRange {
            start: *self,
            len: U64::new(len),
        }
    }
}

impl FileRange {
    fn len(&self) -> u64 {
        self.len.get()
    }
    fn as_range(&self) -> Range<usize> {
        let start = self.start.0.get().try_into().unwrap();
        start..start + usize::try_from(self.len()).unwrap()
    }
}

#[derive(Clone, Copy, FromBytes, FromZeroes, AsBytes, Unaligned)]
#[repr(C)]
struct Header<R> {
    magic: [u8; 16],
    root: FilePointer<R>,
    allocator_state: AllocatorState,
}

impl<R> Default for Header<R> {
    fn default() -> Self {
        Self {
            magic: *b"cool db format 1",
            root: FilePointer::null(),
            allocator_state: AllocatorState {
                general: RawFilePointer::null(),
                slabs: SlabListPointer(FilePointer::new(
                    RawFilePointer::null() + size_of::<Header<R>>() as u64,
                )),
            },
        }
    }
}

struct Snapshot<R> {
    root: FilePointer<R>,
    map: Mmap,
}

impl<R> Snapshot<R> {
    fn read<T: FromBytes>(&self, at: FilePointer<T>) -> &T {
        self.read_range(at.range())
    }

    fn read_range<T: FromBytes>(&self, range: FileRange) -> &T {
        Ref::<_, T>::new(&self.map[range.as_range()])
            .unwrap()
            .into_ref()
    }

    fn read_raw(&self, range: FileRange) -> &[u8] {
        &self.map[range.as_range()]
    }
}

pub struct Reader<R> {
    state: Arc<AtomicArc<Snapshot<R>>>,
}

pub struct Db<R> {
    file: File,
    map: MmapMut,
    slabs: BTreeMap<u32, SlabPointer>,
    state: Arc<AtomicArc<Snapshot<R>>>,
    snapshots: VecDeque<SnapshotAndFreeList<R>>,
    _phantom: PhantomData<R>,
}

struct SnapshotAndFreeList<R> {
    snapshot: Arc<Snapshot<R>>,
    to_free: Vec<FileRange>,
}

impl<R> Db<R> {
    fn header(&self) -> &Header<R> {
        unsafe { self.reference_range_unchecked(Self::header_ptr().range()) }
    }

    fn header_mut(&mut self) -> &mut Header<R> {
        unsafe { self.modify_range_unchecked(Self::header_ptr().range()) }
    }

    // NOTE: only allowed before any data of `size` has been allocated
    fn add_slab(&mut self, size: u32) -> SlabPointer {
        let allocator_state = self.header().allocator_state;
        let slab = allocator_state.slabs.add_slab(self, size);
        assert!(self.slabs.insert(size, slab).is_none());
        slab
    }

    // NOTE: only allowed before any data of `size` has been allocated
    fn ensure_slab(&mut self, size: u32) -> SlabPointer {
        self.slabs
            .get(&size)
            .copied()
            .unwrap_or_else(|| self.add_slab(size))
    }

    fn transaction(&mut self, f: impl FnOnce(&mut TransactionHandle<R>) -> FilePointer<R>) {
        let mut handle = TransactionHandle::new(self);

        let root = f(&mut handle);

        let to_free = handle.to_free();

        let snapshot = self.update_root(root);

        self.snapshots
            .push_back(SnapshotAndFreeList { snapshot, to_free });
    }

    fn free_old_epochs(&mut self) {
        let mut snapshots = std::mem::take(&mut self.snapshots);

        while snapshots
            .front_mut()
            .is_some_and(|snapshot| Arc::get_mut(&mut snapshot.snapshot).is_some())
        {
            // if the snapshot is uniqe we are the only owner and can free the epoch.

            // println!("freeing epoch");

            let snapshot = snapshots.pop_front().unwrap();

            for allocation in snapshot.to_free {
                self.free(allocation);
            }
        }

        self.snapshots = snapshots;
    }

    fn header_ptr() -> FilePointer<Header<R>> {
        FilePointer::new(RawFilePointer(0.into()))
    }

    fn root_ptr() -> FilePointer<FilePointer<R>> {
        FilePointer::new(RawFilePointer((size_of::<Header<R>>() as u64).into()))
    }

    fn allocator_state_ptr() -> RawFilePointer {
        RawFilePointer((size_of::<Header<R>>() as u64 + size_of::<RawFilePointer>() as u64).into())
    }

    fn general_purpose_allocator() -> GeneralPurposeAllocator {
        GeneralPurposeAllocator {
            head_ptr: FilePointer::new(Self::allocator_state_ptr()),
        }
    }

    pub fn create_reader(&self) -> Reader<R> {
        Reader {
            state: self.state.clone(),
        }
    }

    fn update_root(&mut self, new_root: FilePointer<R>) -> Arc<Snapshot<R>> {
        // TODO: we could write some here + flush here for better consistency
        // e.g. a copy of the new root pointer

        // flush all data in file
        self.map.flush().unwrap();

        // update root pointer and immediately flush
        unsafe {
            self.write(Self::root_ptr(), new_root);
        }

        self.map
            .flush_range(
                Self::root_ptr().into_raw().0.get() as usize,
                size_of::<RawFilePointer>(),
            )
            .unwrap();

        // update data that readers see
        self.state.swap(Arc::new(Snapshot {
            root: new_root,
            map: self.create_readonly_map(),
        }))
    }

    // TODO: fix pls
    #[track_caller]
    unsafe fn copy(&mut self, from: FileRange, to: FileRange) {
        let (head, tail) = self.map.split_at_mut(to.start.0.get() as usize);
        tail[0..to.len() as usize].copy_from_slice(&head[from.as_range()]);
    }

    #[track_caller]
    unsafe fn read<T: FromBytes>(&self, at: FilePointer<T>) -> T {
        self.read_range(at.range())
    }

    #[track_caller]
    unsafe fn read_range<T: FromBytes>(&self, range: FileRange) -> T {
        assert!(!range.start.is_null(), "null pointer dereference");
        Ref::<_, T>::new(&self.map[range.as_range()])
            .unwrap()
            .read()
    }

    #[track_caller]
    unsafe fn write<T: AsBytes>(&mut self, at: FilePointer<T>, data: T) {
        self.write_range(at.range(), data)
    }

    #[track_caller]
    unsafe fn write_range<T: AsBytes>(&mut self, range: FileRange, data: T) {
        assert!(!range.start.is_null(), "null pointer dereference");
        Ref::<_, T>::new(&mut self.map[range.as_range()])
            .unwrap()
            .write(data)
    }

    #[track_caller]
    unsafe fn modify<T: FromBytes + AsBytes>(&mut self, at: FilePointer<T>) -> &mut T {
        self.modify_range(at.range())
    }

    #[track_caller]
    unsafe fn modify_range<T: FromBytes + AsBytes>(&mut self, range: FileRange) -> &mut T {
        assert!(!range.start.is_null(), "null pointer dereference");
        self.modify_range_unchecked(range)
    }

    unsafe fn modify_range_unchecked<T: FromBytes + AsBytes>(
        &mut self,
        range: FileRange,
    ) -> &mut T {
        Ref::<_, T>::new(&mut self.map[range.as_range()])
            .unwrap()
            .into_mut()
    }

    #[track_caller]
    unsafe fn reference<T: FromBytes>(&self, at: FilePointer<T>) -> &T {
        self.reference_range(at.range())
    }

    #[track_caller]
    unsafe fn reference_range<T: FromBytes>(&self, range: FileRange) -> &T {
        assert!(!range.start.is_null(), "null pointer dereference");
        self.reference_range_unchecked(range)
    }

    unsafe fn reference_range_unchecked<T: FromBytes>(&self, range: FileRange) -> &T {
        Ref::<_, T>::new(&self.map[range.as_range()])
            .unwrap()
            .into_ref()
    }

    fn remap(&mut self) {
        let map = unsafe { MmapMut::map_mut(&self.file) }.unwrap();
        self.map = map;
    }

    fn create_readonly_map(&self) -> Mmap {
        unsafe { Mmap::map(&self.file) }.unwrap()
    }

    fn add_pages(&mut self, n: u64) -> PagePointer {
        // println!("adding {n} page{}", if n == 1 { "" } else { "s" });
        let len = self.file.metadata().unwrap().len();
        self.file.set_len(len + PAGE_SIZE * n).unwrap();
        self.remap();
        PagePointer::nth((len / PAGE_SIZE).try_into().unwrap())
    }

    fn remove_pages(&mut self, n: u64) {
        let len = self.file.metadata().unwrap().len();
        self.file
            .set_len(len.checked_sub(PAGE_SIZE * n).unwrap())
            .unwrap();

        self.remap();
    }

    pub fn create(file: File, slabs: &[u32]) -> Self {
        // clear file
        file.set_len(0).unwrap();
        file.set_len(PAGE_SIZE).unwrap();

        let map = unsafe { MmapMut::map_mut(&file) }.unwrap();

        let mut db = Self {
            state: Arc::new(AtomicArc::new(Arc::new(Snapshot {
                root: FilePointer::null(),
                map: unsafe { Mmap::map(&file).unwrap() },
            }))),
            file,
            map,
            slabs: BTreeMap::new(),
            snapshots: VecDeque::new(),
            _phantom: PhantomData,
        };

        unsafe {
            *db.header_mut() = Header::default();
            db.init_allocator(slabs);
        }

        let _ = db.state.swap(Arc::new(Snapshot {
            root: db.header().root,
            map: unsafe { Mmap::map(&db.file).unwrap() },
        }));

        db
    }

    pub fn open(file: File) -> Self {
        let map = unsafe { MmapMut::map_mut(&file) }.unwrap();

        let mut db = Self {
            state: Arc::new(AtomicArc::new(Arc::new(Snapshot {
                root: FilePointer::null(),
                map: unsafe { Mmap::map(&file).unwrap() },
            }))),
            file,
            map,
            slabs: BTreeMap::new(),
            snapshots: VecDeque::new(),
            _phantom: PhantomData,
        };

        let _ = db.state.swap(Arc::new(Snapshot {
            root: db.header().root,
            map: unsafe { Mmap::map(&db.file).unwrap() },
        }));

        db
    }

    unsafe fn init_allocator(&mut self, slabs: &[u32]) {
        let allocator_state = self.header().allocator_state;
        allocator_state.slabs.init(
            self,
            (PAGE_SIZE - size_of::<Header<R>>() as u64)
                .try_into()
                .unwrap(),
        );

        for &size in slabs {
            self.ensure_slab(size);
        }
    }

    fn end_of_file(&self) -> RawFilePointer {
        RawFilePointer::null() + self.file.metadata().unwrap().len()
    }

    fn get_slab(&self, size: u64) -> Option<SlabPointer> {
        u32::try_from(size)
            .ok()
            .and_then(|size| self.slabs.get(&size))
            .copied()
    }

    // TODO: scrap the PAGE-wise allocation and make slab allocations allocations of the general allocator.
    pub fn allocate(&mut self, size: u64) -> FileRange {
        if let Some(slab) = self.get_slab(size) {
            slab.alloc(self)
        } else {
            Self::general_purpose_allocator().allocate(self, size)
        }
    }

    pub fn free(&mut self, range: FileRange) {
        if let Some(slab) = self.get_slab(range.len()) {
            slab.free(self, range)
        } else {
            Self::general_purpose_allocator().free(self, range)
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use mapped::ReaderTrait;
    use std::io::Write;
    use std::process::Stdio;

    #[derive(Debug, Clone, Copy)]
    enum Operation {
        Allocate { size: u64 },
        Free { index: usize },
    }

    fn causes_fragmentation(sequence: &[Operation], print: bool) -> bool {
        let mut db = Db::<()>::create(tempfile::tempfile().unwrap(), &[]);
        let allocator = Db::<()>::general_purpose_allocator();

        let mut ranges = Vec::new();

        for &operation in sequence {
            match operation {
                Operation::Allocate { size } => ranges.push(allocator.allocate(&mut db, size)),
                Operation::Free { index } => {
                    if ranges.get(index).is_some() {
                        allocator.free(&mut db, ranges.remove(index))
                    }
                }
            }
        }

        for range in ranges.drain(..) {
            allocator.free(&mut db, range);
        }

        fragmentation(&mut db, print) > 1
    }

    fn fragmentation<R>(db: &mut Db<R>, print: bool) -> usize {
        let allocator = Db::<()>::general_purpose_allocator();

        let mut next = unsafe { db.read(GeneralPurposeAllocator::next_ptr(allocator.head_ptr)) };

        let mut n = 0;
        while !next.is_null() {
            let size = GeneralPurposeAllocator::size(db, next);
            if print {
                println!("\x1b[34m[{n}]\x1b[m {:?}", next.into_raw().range(size));
            }
            next = unsafe { db.read(GeneralPurposeAllocator::next_ptr(next)) };

            n += 1;
        }

        n
    }

    #[test]
    fn debug_fragmentation() {
        use Operation::*;
        #[rustfmt::skip]
        let mut sequence = vec![Allocate { size: 1946 }, Allocate { size: 3252 }, Free { index: 0 }, Allocate { size: 7391 }, Allocate { size: 394 }, Allocate { size: 3726 }, Allocate { size: 1429 }, Allocate { size: 3188 }, Allocate { size: 6375 }, Allocate { size: 4453 }, Allocate { size: 2514 }, Allocate { size: 4754 }, Allocate { size: 6785 }, Allocate { size: 2751 }, Allocate { size: 4107 }, Allocate { size: 3509 }, Allocate { size: 5897 }, Allocate { size: 7081 }, Allocate { size: 2419 }, Allocate { size: 5400 }, Allocate { size: 7135 }, Free { index: 14 }, Allocate { size: 2130 }, Free { index: 18 }, Allocate { size: 3450 }, Allocate { size: 1296 }, Allocate { size: 8091 }, Allocate { size: 4646 }, Allocate { size: 3891 }, Free { index: 0 }, Allocate { size: 1087 }, Allocate { size: 101 }, Allocate { size: 5353 }, Allocate { size: 3381 }, Allocate { size: 6869 }, Free { index: 1 }, Allocate { size: 3750 }, Allocate { size: 1398 }, Free { index: 22 }, Allocate { size: 18 }, Free { index: 25 }, Allocate { size: 642 }, Free { index: 4 }, Allocate { size: 4 }, Allocate { size: 1898 }, Allocate { size: 5259 }, Free { index: 26 }, Allocate { size: 3151 }, Allocate { size: 4989 }, Allocate { size: 6493 }, Allocate { size: 551 }, Allocate { size: 706 }, Allocate { size: 4161 }, Free { index: 16 }, Allocate { size: 3422 }, Allocate { size: 3011 }, Allocate { size: 5149 }, Allocate { size: 4687 }, Allocate { size: 5 }, Free { index: 34 }, Allocate { size: 191 }, Allocate { size: 2851 }, Allocate { size: 3597 }, Free { index: 28 }, Allocate { size: 7037 }, Allocate { size: 4660 }, Allocate { size: 194 }, Allocate { size: 5537 }, Allocate { size: 3242 }, Allocate { size: 6298 }, Allocate { size: 1239 }, Allocate { size: 7025 }, Allocate { size: 3563 }, Allocate { size: 5039 }, Free { index: 40 }, Allocate { size: 4549 }, Allocate { size: 5362 }, Allocate { size: 3510 }, Free { index: 31 }, Allocate { size: 226 }, Allocate { size: 6904 }, Allocate { size: 4150 }, Allocate { size: 4914 }, Allocate { size: 2330 }, Allocate { size: 2499 }, Allocate { size: 6677 }, Allocate { size: 95 }, Allocate { size: 3726 }, Allocate { size: 3258 }, Free { index: 2 }, Allocate { size: 2129 }, Allocate { size: 3674 }, Allocate { size: 1542 }, Allocate { size: 2210 }, Free { index: 21 }, Allocate { size: 3914 }, Allocate { size: 3108 }, Allocate { size: 1979 }, Allocate { size: 2677 }, Allocate { size: 8140 }, Allocate { size: 7573 }, Allocate { size: 121 }, Free { index: 59 }, Allocate { size: 6467 }, Allocate { size: 262 }, Allocate { size: 7711 }, Allocate { size: 2450 }, Allocate { size: 4351 }, Allocate { size: 4282 }, Free { index: 39 }, Allocate { size: 4050 }, Allocate { size: 67 }, Allocate { size: 5560 }, Free { index: 51 }, Allocate { size: 6038 }, Allocate { size: 555 }, Allocate { size: 1852 }, Free { index: 78 }, Allocate { size: 698 }];

        let mut prev_sequence = sequence.clone();
        while causes_fragmentation(&sequence, false) {
            prev_sequence = sequence.clone();
            sequence.pop();
        }

        println!("{prev_sequence:?}");

        let mut sequence = prev_sequence.clone();
        loop {
            let mut removed_something = false;
            let mut i = 0;
            while i < sequence.len() {
                let mut new_sequence = sequence.clone();

                new_sequence.remove(i);

                if causes_fragmentation(&new_sequence, false) {
                    removed_something = true;
                    println!("removed {i} ({:?})", sequence[i]);
                    sequence = new_sequence;
                } else {
                    for item in &mut new_sequence {
                        if let Operation::Free { index } = item {
                            if *index > i {
                                *index -= 1;
                            }
                        }
                    }

                    if causes_fragmentation(&new_sequence, false) {
                        removed_something = true;
                        println!("removed {i} ({:?}) after adjusting frees", sequence[i]);
                        sequence = new_sequence;
                    }
                }

                i += 1;
            }

            if !removed_something {
                break;
            }
        }

        loop {
            let mut merged_something = false;
            let mut i = 0;
            while i < sequence.len() {
                let mut new_sequence = sequence.clone();

                let removed = new_sequence.remove(i);

                if let Operation::Allocate { size: removed_size } = removed {
                    if let Some(Operation::Allocate { size }) = new_sequence.get_mut(i) {
                        *size += removed_size;
                    }
                }

                if causes_fragmentation(&new_sequence, false) {
                    merged_something = true;
                    println!(
                        "merged {} and {} ({:?} and {:?})",
                        i,
                        i + 1,
                        sequence[i],
                        sequence[i + 1]
                    );
                    sequence = new_sequence;
                } else {
                    for item in &mut new_sequence {
                        if let Operation::Free { index } = item {
                            if *index > i {
                                *index -= 1;
                            }
                        }
                    }

                    if causes_fragmentation(&new_sequence, false) {
                        merged_something = true;
                        println!(
                            "merged {} and {} ({:?} and {:?}) after adjusting frees",
                            i,
                            i + 1,
                            sequence[i],
                            sequence[i + 1]
                        );
                        sequence = new_sequence;
                    }
                }

                i += 1;
            }

            if !merged_something {
                break;
            }
        }

        println!("{sequence:?}");

        dbg!(causes_fragmentation(&sequence, true));
    }

    #[test]
    fn it_works() {
        let mut db = Db::<()>::create(tempfile::tempfile().unwrap(), &[4, 7, 16]);

        let mut ranges = Vec::new();
        for i in 0..10000 {
            ranges.push(db.allocate(i % 32));
        }

        let n = ranges.len();
        for range in ranges.drain(n / 4..n * 3 / 4) {
            db.free(range);
        }

        for i in 0..10000 {
            ranges.push(db.allocate((4 * i) % 32));
        }

        for range in ranges.drain(..) {
            db.free(range);
        }

        // hexdump(db.map.as_bytes());
    }

    #[test]
    fn transactions_work() {
        #[derive(Clone, Copy, FromBytes, FromZeroes, AsBytes, Unaligned)]
        #[repr(C)]
        struct DataHeader {
            generation: U64,
            list: FilePointer<DataList>,
        }

        #[derive(Clone, Copy, FromBytes, FromZeroes, AsBytes, Unaligned)]
        #[repr(C)]
        struct DataList {
            next: FilePointer<DataList>,
            data: U64,
        }

        let mut db = Db::<DataHeader>::create(
            tempfile::tempfile().unwrap(),
            &[size_of::<DataHeader>() as u32, size_of::<DataList>() as u32],
        );

        let mut snapshots = Vec::new();

        for i in 0..10 {
            db.transaction(|transaction| {
                let root = transaction.root();

                let root = if root.is_null() {
                    let (root, data) = transaction.allocate::<DataHeader>();

                    *data = DataHeader {
                        generation: 0.into(),
                        list: FilePointer::null(),
                    };

                    root
                } else {
                    root
                };

                let &data = transaction.read::<DataHeader>(root);

                assert_eq!(data.generation.get(), i);

                let n = {
                    let mut next = data.list;
                    let mut n = 0;
                    while !next.is_null() {
                        next = transaction.read::<DataList>(next).next;
                        n += 1;
                    }
                    n
                };

                let next = if n >= 5 {
                    transaction.read::<DataList>(data.list).next
                } else {
                    data.list
                };

                let (elem_ptr, element) = transaction.allocate::<DataList>();

                element.next = next;
                element.data = i.into();

                let (root, data) = transaction.modify::<DataHeader>(root);
                data.list = elem_ptr;
                data.generation = (i + 1).into();

                root
            });

            snapshots.push(db.create_reader().state.get());
        }

        for (i, snapshot) in snapshots.iter().enumerate() {
            let root = snapshot.read::<DataHeader>(snapshot.root);

            assert_eq!(root.generation.get(), i as u64 + 1);

            let mut items = Vec::new();

            let mut ptr = root.list;

            while !ptr.is_null() {
                let element = snapshot.read::<DataList>(ptr);

                items.push(element.data.get());
                ptr = element.next;
            }

            assert_eq!(items.len(), (i + 1).min(5));
            assert_eq!(items[0], i as u64);

            for (expected, &is) in items.iter().skip(1).rev().enumerate() {
                assert_eq!(expected as u64, is);
            }
        }

        drop(snapshots);

        // hexdump(db.map.as_bytes());

        db.free_old_epochs();

        // hexdump(db.map.as_bytes());
    }

    fn hexdump(bytes: &[u8]) {
        let mut child = std::process::Command::new("hexdump")
            .arg("-C")
            .stdin(Stdio::piped())
            .stdout(Stdio::inherit())
            .spawn()
            .unwrap();

        let mut stdin = child.stdin.take().expect("failed to get stdin");

        stdin.write_all(bytes).unwrap();

        std::mem::drop(stdin);

        child.wait().unwrap();
    }
}