Blocktree/crates/btlib/src/sectored_buf.rs

use log::error;
use positioned_io::Size;
use std::io::{self, Read, Seek, SeekFrom, Write};

use crate::{
    bterr, suppress_err_if_non_zero, BlockError, BlockMeta, BoxInIoErr, Decompose, MetaAccess,
    Positioned, ReadDual, ReadExt, Result, Sectored, SeekFromExt, SizeExt, Split, TryCompose,
    TrySeek, WriteDual, EMPTY_SLICE,
};

pub use private::SectoredBuf;

mod private {
    use super::*;

    /// A stream which buffers writes and read such that the inner stream only sees reads and writes
    /// of sector length buffers.
    pub struct SectoredBuf<T> {
        inner: T,
        buf: Vec<u8>,
        /// The offset into the inner stream which the zero offset byte in `buf` corresponds to.
        buf_start: usize,
        /// Indicates if the contents of `buf` have been written to, and so whether `buf` needs to
        /// be written back to `inner` before it is refilled.
        dirty: bool,
        /// The current position of this stream, expressed as an offset into the inner stream.
        pos: usize,
    }

    impl SectoredBuf<()> {
        pub fn new() -> SectoredBuf<()> {
            SectoredBuf {
                inner: (),
                pos: 0,
                buf: Vec::new(),
                buf_start: 0,
                dirty: false,
            }
        }
    }

    /// Returns the slice of the internal buffer which is ready to be read from.
    /// If the buffer all the bytes in the buffer have been consumed, then the buffer is refilled.
    /// The returned slice will be empty if and only if there are no additional bytes in the
    /// inner stream.
    macro_rules! readable_slice {
        ($self:expr) => {{
            let pos = $self.buf_pos();
            let end = $self.buf_end();
            if pos == end {
                match $self.fill_internal_buf() {
                    Ok(nread) => {
                        if nread > 0 {
                            Ok(&$self.buf[..$self.buf_end()])
                        } else {
                            Ok(&$self.buf[..0])
                        }
                    }
                    Err(err) => Err(err),
                }
            } else {
                Ok(&$self.buf[pos..end])
            }
        }};
    }

    impl<T> SectoredBuf<T> {
        /// Returns a reference to the inner stream.
        pub fn get_ref(&self) -> &T {
            &self.inner
        }

        /// Returns a mutable reference to the inner stream.
        pub fn get_mut(&mut self) -> &mut T {
            &mut self.inner
        }

        /// Returns the offset into the internal buffer that corresponds to the current position.
        fn buf_pos(&self) -> usize {
            let buf_pos = self.pos - self.buf_start;
            debug_assert!(buf_pos <= self.buf.len());
            buf_pos
        }
    }

    impl<T: AsRef<BlockMeta>> SectoredBuf<T> {
        fn len(&self) -> usize {
            self.inner
                .as_ref()
                .body
                .secrets()
                .unwrap()
                .size
                .try_into()
                .unwrap()
        }

        /// Returns one more than the last index in the internal buffer which can be read.
        fn buf_end(&self) -> usize {
            let len = self.len();
            let sect_sz = self.sector_sz();
            let limit = len.min(self.buf_start + sect_sz);
            limit - self.buf_start
        }
    }

    impl<T: Read + Seek + AsRef<BlockMeta>> SectoredBuf<T> {
        /// Fills the internal buffer by reading from the inner stream at the current position
        /// and updates `self.buf_start` with the position read from.
        fn fill_internal_buf(&mut self) -> Result<usize> {
            self.buf_start = self.inner.stream_position()?.try_into().box_err()?;
            let read_bytes = if self.buf_start < self.len() {
                let read_bytes = self.inner.fill_buf(&mut self.buf)?;
                if read_bytes < self.buf.len() {
                    return Err(bterr!(BlockError::IncorrectSize {
                        expected: self.buf.len(),
                        actual: read_bytes,
                    }));
                }
                read_bytes
            } else {
                0
            };
            Ok(read_bytes)
        }
    }

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

    impl<T> Split<SectoredBuf<&'static [u8]>, T> for SectoredBuf<T> {
        fn split(self) -> (SectoredBuf<&'static [u8]>, T) {
            let new_self = SectoredBuf {
                inner: EMPTY_SLICE,
                buf: self.buf,
                buf_start: self.buf_start,
                dirty: self.dirty,
                pos: self.pos,
            };
            (new_self, self.inner)
        }

        fn combine(left: SectoredBuf<&'static [u8]>, right: T) -> Self {
            SectoredBuf {
                inner: right,
                buf: left.buf,
                buf_start: left.buf_start,
                dirty: left.dirty,
                pos: left.pos,
            }
        }
    }

    impl<T> Decompose<T> for SectoredBuf<T> {
        fn into_inner(self) -> T {
            self.inner
        }
    }

    impl<T: Sectored + Read + Seek + AsRef<BlockMeta>> TryCompose<T, SectoredBuf<T>>
        for SectoredBuf<()>
    {
        type Error = crate::Error;
        fn try_compose(self, inner: T) -> Result<SectoredBuf<T>> {
            let sect_sz = inner.sector_sz();
            let mut sectored = SectoredBuf {
                inner,
                buf: self.buf,
                buf_start: 0,
                dirty: false,
                pos: 0,
            };
            sectored.buf.resize(sect_sz, 0);
            sectored.inner.seek(SeekFrom::Start(0))?;
            sectored.fill_internal_buf()?;
            Ok(sectored)
        }
    }

    impl<T> Sectored for SectoredBuf<T> {
        fn sector_sz(&self) -> usize {
            self.buf.len()
        }
    }

    impl<T: Seek + Read + Write + MetaAccess> Write for SectoredBuf<T> {
        fn write(&mut self, mut src: &[u8]) -> io::Result<usize> {
            let src_len_start = src.len();
            let mut dest = {
                let buf_pos = self.buf_pos();
                &mut self.buf[buf_pos..]
            };
            while !src.is_empty() {
                if dest.is_empty() {
                    suppress_err_if_non_zero!(src_len_start - src.len(), self.flush());
                    dest = &mut self.buf[..];
                }
                let sz = src.len().min(dest.len());
                dest[..sz].copy_from_slice(&src[..sz]);
                dest = &mut dest[sz..];
                src = &src[sz..];
                self.dirty = sz > 0;
                self.pos += sz;
                self.inner.mut_meta_body().access_secrets(|secrets| {
                    secrets.size = secrets.size.max(self.pos as u64);
                    Ok(())
                })?;
            }
            Ok(src_len_start - src.len())
        }

        fn flush(&mut self) -> io::Result<()> {
            if !self.dirty {
                return Ok(());
            }

            // Write out the contents of the buffer.
            let sect_sz: u64 = self.sector_sz().try_into().box_err()?;
            let inner_pos = self.inner.stream_position()?;
            let inner_pos_usize: usize = inner_pos.try_into().box_err()?;
            let is_new_sector = self.pos > inner_pos_usize;
            let is_full = (self.buf.len() - self.buf_pos()) == 0;
            let (seek_to, fill_internal_buf) = if is_new_sector {
                if is_full {
                    (inner_pos + sect_sz, true)
                } else {
                    (inner_pos, true)
                }
            } else {
                // The contents of the buffer were previously read from inner, so we write the
                // updated contents to the same offset.
                let sect_start: u64 = self.buf_start.try_into().box_err()?;
                self.inner.seek(SeekFrom::Start(sect_start))?;
                if is_full {
                    (inner_pos, true)
                } else {
                    // This is the one case were we don't have to refill the internal buffer.
                    (inner_pos - sect_sz, false)
                }
            };
            self.inner.write_all(&self.buf)?;
            self.inner.flush()?;

            // Seek to the next position.
            self.inner.seek(SeekFrom::Start(seek_to))?;
            if fill_internal_buf {
                self.fill_internal_buf()?;
            }

            self.dirty = false;
            Ok(())
        }
    }

    impl<T: Read + Seek + AsRef<BlockMeta>> Read for SectoredBuf<T> {
        fn read(&mut self, mut dest: &mut [u8]) -> io::Result<usize> {
            if self.pos == self.len() {
                return Ok(0);
            }

            let dest_len_start = dest.len();
            let mut src = readable_slice!(self)?;
            while !dest.is_empty() {
                if src.is_empty() {
                    src = suppress_err_if_non_zero!(
                        dest_len_start - dest.len(),
                        readable_slice!(self)
                    );
                    // If `src` is still empty, then we've reached the end of the stream.
                    if src.is_empty() {
                        break;
                    }
                }
                let sz = src.len().min(dest.len());
                dest[..sz].copy_from_slice(&src[..sz]);
                dest = &mut dest[sz..];
                src = &src[sz..];
                self.pos += sz;
            }
            Ok(dest_len_start - dest.len())
        }
    }

    impl<T: Seek + Read + AsRef<BlockMeta>> SectoredBuf<T> {
        /// Seeks this stream to the given position.
        /// If a seek to a different sector is needed then `pre_seek` is called before this seek
        /// is performed. This can be used to flush buffered data, or to prevent the seek if a
        /// flush can't be performed.
        fn seek_impl<F: FnOnce(&mut Self) -> io::Result<()>>(
            &mut self,
            seek_from: SeekFrom,
            pre_seek: F,
        ) -> io::Result<u64> {
            let pos = self.pos as u64;
            let pos_new = seek_from.abs(|| Ok(pos), || self.size_or_err())?;
            let len = self.len();
            if pos_new > len as u64 {
                return Err(io::Error::new(
                    io::ErrorKind::InvalidInput,
                    format!("can't seek to {pos_new}, only {len} bytes total"),
                ));
            }
            let sect_sz = self.sector_sz() as u64;
            let sect_index = pos / sect_sz;
            let sect_index_new = pos_new / sect_sz;
            if sect_index != sect_index_new || sect_sz == pos - self.buf_start as u64 {
                pre_seek(self)?;
                let seek_to = sect_index_new * sect_sz;
                self.inner.seek(SeekFrom::Start(seek_to))?;
                self.fill_internal_buf()?;
            }
            self.pos = pos_new.try_into().box_err()?;
            Ok(pos_new)
        }
    }

    impl<T: Seek + Read + Write + MetaAccess> Seek for SectoredBuf<T> {
        fn seek(&mut self, seek_from: SeekFrom) -> io::Result<u64> {
            self.seek_impl(seek_from, |sich| sich.flush())
        }
    }

    impl<T: Read + Seek + AsRef<BlockMeta>> TrySeek for SectoredBuf<T> {
        fn try_seek(&mut self, seek_from: SeekFrom) -> io::Result<()> {
            self.seek_impl(seek_from, |sich| {
                if sich.dirty {
                    Err(io::Error::new(
                        io::ErrorKind::Unsupported,
                        "SectoredBuf::try_seek failed because it has unwritten data",
                    ))
                } else {
                    Ok(())
                }
            })?;
            Ok(())
        }
    }

    impl<U, T: AsRef<U>> AsRef<U> for SectoredBuf<T> {
        fn as_ref(&self) -> &U {
            self.inner.as_ref()
        }
    }

    impl<U, T: AsMut<U>> AsMut<U> for SectoredBuf<T> {
        fn as_mut(&mut self) -> &mut U {
            self.inner.as_mut()
        }
    }

    impl<T: AsRef<BlockMeta>> Size for SectoredBuf<T> {
        /// Returns the size of the block, which the value stored in the block's metadata, not
        /// the length of the inner stream.
        fn size(&self) -> io::Result<Option<u64>> {
            Ok(Some(self.inner.as_ref().body.secrets()?.size))
        }
    }

    impl<T: Read + Write + Seek + MetaAccess> WriteDual for SectoredBuf<T> {
        fn write_from<R: Read>(&mut self, mut read: R, mut count: usize) -> io::Result<usize> {
            let pos_start = self.pos;
            let mut dest = {
                let pos = self.buf_pos();
                &mut self.buf[pos..]
            };
            let dest_len = dest.len();
            dest = &mut dest[..dest_len.min(count)];
            while count > 0 {
                if dest.is_empty() {
                    suppress_err_if_non_zero!(self.pos - pos_start, self.flush());
                    dest = &mut self.buf[..];
                    let dest_len = dest.len();
                    dest = &mut dest[..dest_len.min(count)];
                }
                let nread = suppress_err_if_non_zero!(self.pos - pos_start, read.read(dest));
                if 0 == nread {
                    break;
                }
                self.dirty = true;
                dest = &mut dest[nread..];
                self.pos += nread;
                count -= nread;
                self.inner.mut_meta_body().access_secrets(|secrets| {
                    secrets.size = secrets.size.max(self.pos as u64);
                    Ok(())
                })?;
            }
            Ok(self.pos - pos_start)
        }
    }

    impl<T: Read + Seek + AsRef<BlockMeta>> ReadDual for SectoredBuf<T> {
        fn read_into<W: Write>(&mut self, mut write: W, mut count: usize) -> io::Result<usize> {
            let pos_start = self.pos;
            let mut src = readable_slice!(self)?;
            src = &src[..src.len().min(count)];
            while count > 0 {
                if src.is_empty() {
                    src = suppress_err_if_non_zero!(self.pos - pos_start, readable_slice!(self));
                    src = &src[..src.len().min(count)];
                    // If `src` is still empty, then we've reached the end of the stream.
                    if src.is_empty() {
                        break;
                    }
                }
                let written = write.write(src)?;
                src = &src[written..];
                self.pos += written;
                count -= written;
            }
            Ok(self.pos - pos_start)
        }
    }

    impl<T> Positioned for SectoredBuf<T> {
        fn pos(&self) -> usize {
            self.pos
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    use crate::test_helpers::{
        integer_array, read_check, write_fill, BtCursor, Randomizer, SectoredCursor,
        SECTOR_SZ_DEFAULT,
    };

    fn make_sectored_buf(sect_sz: usize, sect_ct: usize) -> SectoredBuf<SectoredCursor<Vec<u8>>> {
        SectoredBuf::new()
            .try_compose(SectoredCursor::new(vec![0u8; sect_sz * sect_ct], sect_sz))
            .expect("compose for sectored buffer failed")
    }

    #[test]
    fn sectored_buf_fill_inner() {
        const SECT_SZ: usize = SECTOR_SZ_DEFAULT;
        const SECT_CT: usize = 16;
        let mut sectored = make_sectored_buf(SECT_SZ, SECT_CT);
        let sect_sz = sectored.sector_sz();
        assert_eq!(0, sect_sz % 16);
        let chunk_sz = sect_sz / 16;
        let chunk_ct = SECT_CT * 16;
        write_fill(&mut sectored, chunk_sz, chunk_ct);
    }

    #[test]
    fn sectored_buf_write_read_sequential() {
        const SECT_SZ: usize = SECTOR_SZ_DEFAULT;
        const SECT_CT: usize = 16;
        let mut sectored = make_sectored_buf(SECT_SZ, SECT_CT);
        let sect_sz = sectored.sector_sz();
        assert_eq!(0, sect_sz % 16);
        let chunk_sz = sect_sz / 16;
        // We subtract one here so that the underlying buffer is not completely filled. This
        // exercises the length limiting capability of the sectored buffer.
        let chunk_ct = SECT_CT * 16 - 1;
        write_fill(&mut sectored, chunk_sz, chunk_ct);
        sectored.seek(SeekFrom::Start(0)).expect("seek failed");
        read_check(&mut sectored, chunk_sz, chunk_ct);
    }

    #[test]
    fn sectored_buf_len_preserved() {
        const SECT_SZ: usize = SECTOR_SZ_DEFAULT;
        const SECT_CT: usize = 16;
        let mut sectored = make_sectored_buf(SECT_SZ, SECT_CT);
        let expected = vec![42u8; 12];
        // We need to ensure that writing expected will not fill up the buffer in sectored.
        assert!(expected.len() < sectored.sector_sz());

        sectored.write_all(&expected).expect("write failed");
        sectored.flush().expect("flush failed");
        let inner = sectored.into_inner();
        let mut sectored = SectoredBuf::new()
            .try_compose(inner)
            .expect("failed to compose sectored buffer");
        let mut actual = vec![0u8; expected.len()];
        sectored
            .fill_buf(actual.as_mut_slice())
            .expect("failed to fill actual");

        assert_eq!(expected, actual);
    }

    #[test]
    fn sectored_buf_seek() {
        let sect_sz = 16usize;
        let sect_ct = 16usize;
        let cap = sect_sz * sect_ct - std::mem::size_of::<usize>();
        let source = {
            let mut source = Vec::with_capacity(cap);
            source.extend(
                std::iter::successors(Some(0u8), |n| if *n <= 254 { Some(*n + 1) } else { None })
                    .take(cap),
            );
            source
        };
        let mut sectored = make_sectored_buf(sect_sz, sect_ct);
        sectored.write(&source).expect("write failed");
        let mut buf = [0u8; 1];
        let end = cap.try_into().expect("cap cannot fit into a u8");
        for pos in (0..end).rev() {
            sectored
                .seek(SeekFrom::Start(pos as u64))
                .expect("seek failed");
            sectored.read(&mut buf).expect("read failed");
            assert_eq!(pos, buf[0]);
        }
    }

    /// Tests that data written can be read from the buffer without an intervening call to `flush`.
    #[test]
    fn sectored_buf_write_then_read() {
        const EXPECTED: &[u8] = b"alpha";
        let mut sectored = make_sectored_buf(4096, 1);
        sectored.write(EXPECTED).expect("write failed");
        sectored.seek(SeekFrom::Start(0)).expect("seek failed");
        let mut actual = [0u8; EXPECTED.len()];
        sectored.read(&mut actual).expect("read failed");
        assert_eq!(EXPECTED, actual);
    }

    #[test]
    fn sectored_buf_write_read_random() {
        const SECT_SZ: usize = 16;
        const SECT_CT: usize = 16;
        const CAP: usize = SECT_SZ * SECT_CT - std::mem::size_of::<usize>();
        let source = {
            let mut expected = Vec::with_capacity(CAP);
            expected.extend(
                std::iter::successors(Some(0u8), |n| if *n <= 254 { Some(*n + 1) } else { None })
                    .take(CAP),
            );
            expected
        };
        let indices: Vec<(usize, usize)> = {
            let rando = Randomizer::new([3u8; Randomizer::HASH.len()]);
            let rando2 = Randomizer::new([5u8; Randomizer::HASH.len()]);
            rando
                .zip(rando2)
                .take(SECT_CT)
                .map(|(mut first, mut second)| {
                    first %= source.len();
                    second &= source.len();
                    let low = first.min(second);
                    let high = first.max(second);
                    (low, high)
                })
                .collect()
        };

        let mut sectored = make_sectored_buf(SECT_SZ, SECT_CT);
        sectored
            .write_all(&[0u8; CAP])
            .expect("failed to fill sectored");
        sectored.flush().expect("flush failed");
        for (_k, (low, high)) in indices.iter().enumerate() {
            sectored
                .seek(SeekFrom::Start(*low as u64))
                .expect("seek failed");
            let src = &source[*low..*high];
            sectored.write(src).expect("write failed");
        }
        sectored.flush().expect("flush failed");
        let mut buf = vec![0u8; CAP];
        for (_k, (low, high)) in indices.iter().enumerate() {
            sectored
                .seek(SeekFrom::Start(*low as u64))
                .expect("seek failed");
            let actual = &mut buf[*low..*high];
            sectored.fill_buf(actual).expect("read failed");
            let expected = &source[*low..*high];
            assert_eq!(expected, actual);
        }
    }

    #[test]
    fn sectored_buf_read_past_end() {
        const LEN: usize = 32;
        let mut sectored = SectoredBuf::new()
            .try_compose(SectoredCursor::new([0u8; LEN], LEN))
            .expect("compose failed");
        const BUF_LEN: usize = LEN + 1;
        sectored.write(&[1u8; BUF_LEN - 1]).expect("write failed");
        sectored.seek(SeekFrom::Start(0)).expect("seek failed");
        let mut buf = [0u8; BUF_LEN];
        // Note that buf is one byte longer than the available capacity in the cursor.
        sectored.read(&mut buf).expect("read failed");
        assert_eq!(&[1u8; BUF_LEN - 1], &buf[..(BUF_LEN - 1)]);
        assert_eq!(0u8, buf[BUF_LEN - 1]);
    }

    /// Tests that the data written in try_compose is actually written back to the underlying stream.
    #[test]
    fn sectored_buf_write_back() {
        let mut sectored = SectoredBuf::new()
            .try_compose(SectoredCursor::new(vec![0u8; 24], 16))
            .expect("compose failed");
        let expected = [1u8; 8];
        sectored.write(&expected).expect("first write failed");
        sectored.write(&[2u8; 8]).expect("second write failed");
        sectored.seek(SeekFrom::Start(0)).expect("seek failed");
        let mut actual = [0u8; 8];
        sectored.read(&mut actual).expect("read failed");
        assert_eq!(expected, actual);
    }

    #[test]
    fn sectored_buf_write_past_end() {
        const LEN: usize = 8;
        let mut sectored = SectoredBuf::new()
            .try_compose(SectoredCursor::new(vec![0u8; 0], LEN))
            .expect("compos failed");
        let expected = [1u8; LEN + 1];
        sectored.write(&expected).expect("write failed");
        sectored.seek(SeekFrom::Start(0)).expect("seek failed");
        let mut actual = [0u8; LEN + 1];
        sectored.read(&mut actual).expect("read failed");
        assert_eq!(expected, actual);
    }

    #[test]
    fn read_into_count_limits_read_bytes() {
        const DATA: [u8; 8] = [0, 1, 2, 3, 4, 5, 6, 7];
        let mut sectored = SectoredBuf::new()
            .try_compose(SectoredCursor::new(Vec::new(), DATA.len()))
            .unwrap();

        sectored.write(&DATA).unwrap();
        sectored.rewind().unwrap();
        let mut actual = BtCursor::new([0u8; DATA.len()]);
        let read = sectored.read_into(&mut actual, DATA.len() - 1).unwrap();
        assert_eq!(DATA.len() - 1, read);

        assert_eq!([0, 1, 2, 3, 4, 5, 6, 0], actual.into_inner());
    }

    #[test]
    fn read_into_read_spans_multiple_sectors() {
        const DATA: [u8; 8] = [0, 1, 2, 3, 4, 5, 6, 7];
        let mut sectored = SectoredBuf::new()
            .try_compose(SectoredCursor::new(Vec::new(), DATA.len()))
            .unwrap();

        sectored.write(&DATA).unwrap();
        sectored.write(&DATA).unwrap();
        sectored.rewind().unwrap();
        const ACTUAL_LEN: usize = DATA.len() + DATA.len() / 2;
        let mut actual = BtCursor::new([0u8; ACTUAL_LEN]);
        let read = sectored.read_into(&mut actual, ACTUAL_LEN).unwrap();
        assert_eq!(ACTUAL_LEN, read);

        assert_eq!([0, 1, 2, 3, 4, 5, 6, 7, 0, 1, 2, 3], actual.into_inner());
    }

    /// Tests that a read asking for more bytes than the number available returns the number
    /// available.
    #[test]
    fn read_into_read_past_len() {
        const DATA: [u8; 8] = [0, 1, 2, 3, 4, 5, 6, 7];
        let mut sectored = SectoredBuf::new()
            .try_compose(SectoredCursor::new(Vec::new(), DATA.len()))
            .unwrap();

        sectored.write(&DATA).unwrap();
        sectored.rewind().unwrap();
        const ACTUAL_LEN: usize = DATA.len() + 1;
        let mut actual = BtCursor::new([0u8; ACTUAL_LEN]);
        let read = sectored.read_into(&mut actual, ACTUAL_LEN).unwrap();
        assert_eq!(DATA.len(), read);

        assert_eq!([0, 1, 2, 3, 4, 5, 6, 7, 0], actual.into_inner());
    }

    #[test]
    fn write_from_full_cursor() {
        const DATA: [u8; 8] = [0, 1, 2, 3, 4, 5, 6, 7];
        let mut sectored = SectoredBuf::new()
            .try_compose(SectoredCursor::new(Vec::new(), DATA.len()))
            .unwrap();

        let written = sectored
            .write_from(BtCursor::new(DATA), DATA.len())
            .unwrap();
        assert_eq!(DATA.len(), written);
        sectored.flush().unwrap();

        assert_eq!(&DATA, sectored.into_inner().into_inner().as_slice());
    }

    #[test]
    fn write_from_count_limits_bytes_read() {
        const DATA: [u8; 8] = [0, 1, 2, 3, 4, 5, 6, 7];
        let mut sectored = SectoredBuf::new()
            .try_compose(SectoredCursor::new(Vec::new(), DATA.len()))
            .unwrap();
        let mut cursor = BtCursor::new(DATA);

        let written = sectored.write_from(&mut cursor, DATA.len() / 2).unwrap();
        assert_eq!(DATA.len() / 2, written);
        sectored.flush().unwrap();

        assert_eq!(
            &[0, 1, 2, 3, 0, 0, 0, 0],
            sectored.into_inner().into_inner().as_slice()
        );
        let mut remaining = Vec::new();
        cursor.read_to_end(&mut remaining).unwrap();
        assert_eq!(&[4, 5, 6, 7], remaining.as_slice());
    }

    #[test]
    fn write_from_write_spans_multiple_sectors() {
        const SECT_SZ: usize = 4;
        const DATA: [u8; SECT_SZ + 1] = [0, 1, 2, 3, 4];
        let mut sectored = SectoredBuf::new()
            .try_compose(SectoredCursor::new(Vec::new(), SECT_SZ))
            .unwrap();

        let written = sectored
            .write_from(BtCursor::new(DATA), DATA.len())
            .unwrap();
        assert_eq!(DATA.len(), written);
        sectored.rewind().unwrap();
        let mut actual = Vec::new();
        sectored.read_to_end(&mut actual).unwrap();

        assert_eq!(&[0, 1, 2, 3, 4], actual.as_slice());
    }

    #[test]
    fn try_seek_to_second_sector() {
        const SECT_SZ: usize = 4;
        const DATA_LEN: usize = 2 * SECT_SZ;
        const DATA: [u8; DATA_LEN] = integer_array::<DATA_LEN>(0);
        let mut sectored = SectoredBuf::new()
            .try_compose(SectoredCursor::new(Vec::new(), SECT_SZ))
            .unwrap();

        let written = sectored
            .write_from(BtCursor::new(DATA), DATA.len())
            .unwrap();
        assert_eq!(DATA.len(), written);
        sectored.rewind().unwrap();
        const OFFSET: u64 = SECT_SZ as u64 + 1;
        sectored.try_seek(SeekFrom::Start(OFFSET)).unwrap();
        let mut actual = BtCursor::new(Vec::new());
        sectored.read_into(&mut actual, SECT_SZ).unwrap();

        const EXPECTED_LEN: usize = SECT_SZ - 1;
        const EXPECTED: [u8; EXPECTED_LEN] = integer_array::<EXPECTED_LEN>(OFFSET as u8);
        assert_eq!(&EXPECTED, actual.into_inner().as_slice());
    }

    #[test]
    fn seek_past_end_is_error() {
        const SECT_SZ: usize = 4;
        let mut sectored = SectoredBuf::new()
            .try_compose(SectoredCursor::new(Vec::new(), SECT_SZ))
            .unwrap();

        let result = sectored.seek(SeekFrom::Start(1));

        let matched = if let Err(err) = result {
            io::ErrorKind::InvalidInput == err.kind()
        } else {
            false
        };
        assert!(matched);
    }

    #[test]
    fn seek_to_zero_when_empty() {
        const SECT_SZ: usize = 4;
        let mut sectored = SectoredBuf::new()
            .try_compose(SectoredCursor::new(Vec::new(), SECT_SZ))
            .unwrap();

        let pos = sectored.seek(SeekFrom::Start(0)).unwrap();

        assert_eq!(0, pos);
    }

    #[test]
    fn read_into_consumes_remaining_sector() {
        // SECT_SZ % 2 is assumed be 0.
        const SECT_SZ: usize = 4;
        const DATA_LEN: usize = 2 * SECT_SZ;
        const DATA: [u8; DATA_LEN] = integer_array::<DATA_LEN>(0);
        let mut sectored = SectoredBuf::new()
            .try_compose(SectoredCursor::new(Vec::new(), SECT_SZ))
            .unwrap();

        sectored.write(&DATA).unwrap();
        const MID_FIRST: usize = SECT_SZ / 2;
        sectored.seek(SeekFrom::Start(MID_FIRST as u64)).unwrap();
        // Notice that the `count` argument plus the current `sectored.pos` equals `SECT_SZ`.
        // This will cause `pos / sect_sz` to increase by one, but without incrementing
        // `buf_start`. This creates a special case that `seek_impl` has to take into account.
        sectored
            .read_into(&mut BtCursor::new([0u8; MID_FIRST]), MID_FIRST)
            .unwrap();
        const MID_SECOND: u64 = 3 * SECT_SZ as u64 / 2;
        sectored.try_seek(SeekFrom::Start(MID_SECOND)).unwrap();

        const EXPECTED_LEN: usize = SECT_SZ / 2;
        const EXPECTED: [u8; EXPECTED_LEN] = integer_array::<EXPECTED_LEN>(MID_SECOND as u8);
        let mut actual = BtCursor::new(Vec::new());
        let nread = sectored.read_into(&mut actual, EXPECTED_LEN).unwrap();
        assert_eq!(EXPECTED_LEN, nread);
        assert_eq!(&EXPECTED, actual.into_inner().as_slice());
    }
}