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/*! This crate provides a buffered reader capable of reading chunks of bytes of a data stream in reverse order. Its implementation is an adapted copy of [BufReader](https://doc.rust-lang.org/std/io/trait.BufRead.html) from the nightly `std::io`. # Usage ## Reading chunks of bytes in reverse order: ```rust extern crate rev_buf_reader; use rev_buf_reader::RevBufReader; use std::io::{self, Read}; fn main() { let data = [0, 1, 2, 3, 4, 5, 6, 7]; let inner = io::Cursor::new(&data); let mut reader = RevBufReader::new(inner); let mut buffer = [0, 0, 0]; assert_eq!(reader.read(&mut buffer).ok(), Some(3)); assert_eq!(buffer, [5, 6, 7]); let mut buffer = [0, 0, 0, 0, 0]; assert_eq!(reader.read(&mut buffer).ok(), Some(5)); assert_eq!(buffer, [0, 1, 2, 3, 4]); } ``` ## Reading text lines in reverse order: ```rust extern crate rev_buf_reader; use rev_buf_reader::RevBufReader; use std::io::{self, BufRead}; fn main() { let data = "a\nb\nc"; let inner = io::Cursor::new(&data); let reader = RevBufReader::new(inner); let mut lines = reader.lines(); assert_eq!(lines.next().unwrap().unwrap(), "c".to_string()); assert_eq!(lines.next().unwrap().unwrap(), "b".to_string()); assert_eq!(lines.next().unwrap().unwrap(), "a".to_string()); assert!(lines.next().is_none()); } ``` */ #![cfg_attr(feature = "iovec", feature(iovec))] #![cfg_attr(feature = "read_initializer", feature(read_initializer))] extern crate memchr; use std::io::prelude::*; use std::fmt; use std::io::{self, SeekFrom}; #[cfg(feature = "iovec")] use std::io::IoSliceMut; #[cfg(feature = "read_initializer")] use std::io::Initializer; use std::str; const DEFAULT_BUF_SIZE: usize = 8 * 1024; struct Guard<'a> { buf: &'a mut Vec<u8>, len: usize, } impl Drop for Guard<'_> { fn drop(&mut self) { unsafe { self.buf.set_len(self.len); } } } // The method `read_line` will append data into a `String` buffer, but we need to // be pretty careful when doing this. The implementation will just call // `.as_mut_vec()` and then delegate to a byte-oriented reading method, but we // must ensure that when returning we never leave `buf` in a state such that it // contains invalid UTF-8 in its bounds. // // To this end, we use an RAII guard (to protect against panics) which updates // the length of the string when it is dropped. This guard initially truncates // the string to the prior length and only after we've validated that the // new contents are valid UTF-8 do we allow it to set a longer length. // // The unsafety in this function is twofold: // // 1. We're looking at the raw bytes of `buf`, so we take on the burden of UTF-8 // checks. // 2. We're passing a raw buffer to the function `f`, and it is expected that // the function only *appends* bytes to the buffer. We'll get undefined // behavior if existing bytes are overwritten to have non-UTF-8 data. fn append_to_string<F>(buf: &mut String, f: F) -> io::Result<usize> where F: FnOnce(&mut Vec<u8>) -> io::Result<usize>, { unsafe { let mut g = Guard { len: buf.len(), buf: buf.as_mut_vec(), }; let ret = f(g.buf); if str::from_utf8(&g.buf[g.len..]).is_err() { ret.and_then(|_| { Err(io::Error::new( io::ErrorKind::InvalidData, "stream did not contain valid UTF-8", )) }) } else { g.len = g.buf.len(); ret } } } fn read_until<R: BufRead + ?Sized>(r: &mut R, delim: u8, buf: &mut Vec<u8>) -> io::Result<usize> { let mut read = loop { let mut first = [0u8; 1]; match r.read(&mut first) { Ok(n) => { buf.extend_from_slice(&first[..n]); break n; } Err(ref e) if e.kind() == io::ErrorKind::Interrupted => continue, Err(error) => return Err(error), } }; loop { let (done, used) = { let available = match r.fill_buf() { Ok(n) => n, Err(ref e) if e.kind() == io::ErrorKind::Interrupted => continue, Err(e) => return Err(e), }; match memchr::memrchr(delim, available) { Some(mut i) => { i += 1; // Skip actual detected delimiter buf.splice(..0, available[i..].iter().cloned()); (true, available.len() - i) } None => { buf.splice(..0, available.iter().cloned()); (false, available.len()) } } }; r.consume(used); read += used; if done || used == 0 { return Ok(read); } } } /// `RevBufReader` is a struct similar to `std::io::BufReader`, which adds /// buffering to any reader. But unlike `BufReader`, `RevBufReader` reads a /// data stream from the end to the start. The order of the bytes, however, /// remains the same. For example, when using `RevBufReader` to read a text file, /// we can read the same lines as we would by using `BufReader`, but starting /// from the last line until we get to the first one. /// /// In order to able to read a data stream in reverse order, it must implement /// both `std::io::Read` and `std::io::Seek`. /// /// [`Read`]: ../../std/io/trait.Read.html /// [`Seek`]: ../../std/io/trait.Seek.html /// /// # Examples /// /// ```no_run /// use rev_buf_reader::RevBufReader; /// use std::io::prelude::*; /// use std::fs::File; /// /// fn main() -> std::io::Result<()> { /// let f = File::open("log.txt")?; /// let mut reader = RevBufReader::new(f); /// /// let mut line = String::new(); /// let len = reader.read_line(&mut line)?; /// println!("Last line is {} bytes long", len); /// Ok(()) /// } /// ``` pub struct RevBufReader<R> { inner: R, buf: Box<[u8]>, pos: usize, cap: usize, } impl<R: Read + Seek> RevBufReader<R> { /// Creates a new `RevBufReader` with a default buffer capacity. The default is currently 8 KB, /// but may change in the future. /// /// # Examples /// /// ```no_run /// use rev_buf_reader::RevBufReader; /// use std::fs::File; /// /// fn main() -> std::io::Result<()> { /// let f = File::open("log.txt")?; /// let reader = RevBufReader::new(f); /// Ok(()) /// } /// ``` pub fn new(inner: R) -> RevBufReader<R> { RevBufReader::with_capacity(DEFAULT_BUF_SIZE, inner) } /// Creates a new `RevBufReader` with the specified buffer capacity. /// /// # Examples /// /// Creating a buffer with ten bytes of capacity: /// /// ```no_run /// use rev_buf_reader::RevBufReader; /// use std::fs::File; /// /// fn main() -> std::io::Result<()> { /// let f = File::open("log.txt")?; /// let reader = RevBufReader::with_capacity(10, f); /// Ok(()) /// } /// ``` pub fn with_capacity(capacity: usize, mut inner: R) -> RevBufReader<R> { unsafe { let mut buffer = Vec::with_capacity(capacity); buffer.set_len(capacity); #[cfg(feature = "read_initializer")] inner.initializer().initialize(&mut buffer); inner .seek(SeekFrom::End(0)) .expect("Cannot find the end of the stream."); RevBufReader { inner, buf: buffer.into_boxed_slice(), pos: 0, cap: 0, } } } /// Tries to seek `-length` bytes from the current position in the inner stream. /// It can fail because we may be trying to seek behind the start of the stream. /// If that's the case, we seek to the start of the stream, instead. It returns /// a result containing the absolute value of the actual offset that was sought. /// Other errors may occur during this operation, which will be passed to the caller. #[inline] fn checked_seek_back(&mut self, length: usize) -> io::Result<usize> { // It should be safe to assume that offset fits within an i64 as the alternative // means we managed to allocate 8 exbibytes and that's absurd. let offset = (self.cap + length) as i64; // This can fail if we're trying to seek to a negative offset. let checked_length = match self.inner.seek(SeekFrom::Current(-offset)) { Ok(_) => length, Err(error) => { let position = self.inner.seek(SeekFrom::Current(0))? as usize; if position > offset as usize { // In this case, the error is not due to seeking to a negative offset. return Err(error); } self.inner.seek(SeekFrom::Start(0))?; position.saturating_sub(self.cap) } }; self.cap = 0; Ok(checked_length) } } impl<R> RevBufReader<R> { /// Gets a reference to the underlying reader. /// /// It is inadvisable to directly read from the underlying reader. /// /// # Examples /// /// ```no_run /// use rev_buf_reader::RevBufReader; /// use std::fs::File; /// /// fn main() -> std::io::Result<()> { /// let f1 = File::open("log.txt")?; /// let reader = RevBufReader::new(f1); /// /// let f2 = reader.get_ref(); /// Ok(()) /// } /// ``` pub fn get_ref(&self) -> &R { &self.inner } /// Gets a mutable reference to the underlying reader. /// /// It is inadvisable to directly read from the underlying reader. /// /// # Examples /// /// ```no_run /// use rev_buf_reader::RevBufReader; /// use std::fs::File; /// /// fn main() -> std::io::Result<()> { /// let f1 = File::open("log.txt")?; /// let mut reader = RevBufReader::new(f1); /// /// let f2 = reader.get_mut(); /// Ok(()) /// } /// ``` pub fn get_mut(&mut self) -> &mut R { &mut self.inner } /// Returns a reference to the internally buffered data. /// /// Unlike `fill_buf`, this will not attempt to fill the buffer if it is empty. /// /// # Examples /// /// ```no_run /// use rev_buf_reader::RevBufReader; /// use std::io::BufRead; /// use std::fs::File; /// /// fn main() -> std::io::Result<()> { /// let f = File::open("log.txt")?; /// let mut reader = RevBufReader::new(f); /// assert!(reader.buffer().is_empty()); /// /// if reader.fill_buf()?.len() > 0 { /// assert!(!reader.buffer().is_empty()); /// } /// Ok(()) /// } /// ``` pub fn buffer(&self) -> &[u8] { &self.buf[0..self.pos] } /// Unwraps this `RevBufReader`, returning the underlying reader. /// /// Note that any leftover data in the internal buffer is lost. /// /// # Examples /// /// ```no_run /// use rev_buf_reader::RevBufReader; /// use std::fs::File; /// /// fn main() -> std::io::Result<()> { /// let f1 = File::open("log.txt")?; /// let reader = RevBufReader::new(f1); /// /// let f2 = reader.into_inner(); /// Ok(()) /// } /// ``` pub fn into_inner(self) -> R { self.inner } /// Invalidates all data in the internal buffer. #[inline] fn discard_buffer(&mut self) { self.pos = 0; self.cap = 0; } } impl<R: Seek> RevBufReader<R> { /// Seeks relative to the current position. If the new position lies within the buffer, /// the buffer will not be flushed, allowing for more efficient seeks. /// This method does not return the location of the underlying reader, so the caller /// must track this information themselves if it is required. pub fn seek_relative(&mut self, offset: i64) -> io::Result<()> { let pos = self.pos as u64; if offset < 0 { if let Some(new_pos) = pos.checked_sub((-offset) as u64) { self.pos = new_pos as usize; return Ok(()); } } else { if let Some(new_pos) = pos.checked_add(offset as u64) { if new_pos <= self.cap as u64 { self.pos = new_pos as usize; return Ok(()); } } } self.seek(SeekFrom::Current(offset)).map(|_| ()) } } impl<R: Read + Seek> Read for RevBufReader<R> { fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> { // If we don't have any buffered data and we're doing a massive read // (larger than our internal buffer), bypass our internal buffer // entirely. if self.pos == 0 && buf.len() >= self.buf.len() { let length = self.checked_seek_back(buf.len())?; self.inner .read_exact(&mut buf[..length]) .expect("Should be able to read the checked amount of data."); self.inner .seek(SeekFrom::Current(-(length as i64))) .expect("Unable to seek back to previous position."); return Ok(length); } let nread = { let rem = self.fill_buf()?; let offset = rem.len().saturating_sub(buf.len()); let mut rem = &rem[offset..]; rem.read(buf)? }; self.consume(nread); Ok(nread) } #[cfg(feature = "iovec")] fn read_vectored(&mut self, bufs: &mut [IoSliceMut<'_>]) -> io::Result<usize> { let total_len = bufs.iter().map(|b| b.len()).sum::<usize>(); if self.pos == self.cap && total_len >= self.buf.len() { let length = self.checked_seek_back(total_len)?; self.inner .read_vectored(bufs) .expect("Should be able to read the checked amount of data."); self.inner .seek(SeekFrom::Current(-(length as i64))) .expect("Unable to seek back to previous position."); return Ok(length); } let nread = { let rem = self.fill_buf()?; let offset = rem.len().saturating_sub(total_len); let mut rem = &rem[offset..]; rem.read_vectored(bufs)? }; self.consume(nread); Ok(nread) } // We can't skip unconditionally because of the large buffer case in read. #[cfg(feature = "read_initializer")] unsafe fn initializer(&self) -> Initializer { self.inner.initializer() } } impl<R: Read + Seek> BufRead for RevBufReader<R> { fn fill_buf(&mut self) -> io::Result<&[u8]> { // If we've reached the end of our internal buffer then we need to fetch // some more data from the underlying reader. if self.pos == 0 { let length = self.checked_seek_back(self.buf.len())?; self.inner .read_exact(&mut self.buf[..length]) .expect("Should be able to read the checked amount of data."); self.cap = length; self.pos = self.cap; } Ok(&self.buf[0..self.pos]) } fn consume(&mut self, amt: usize) { self.pos = self.pos.saturating_sub(amt); } fn read_until(&mut self, byte: u8, buf: &mut Vec<u8>) -> io::Result<usize> { read_until(self, byte, buf) } fn read_line(&mut self, buf: &mut String) -> io::Result<usize> { // Note that we are not calling the `.read_until` method here, but // rather our hardcoded implementation. For more details as to why, see // the comments in `read_to_end`. append_to_string(buf, |b| read_until(self, b'\n', b)) } } impl<R> fmt::Debug for RevBufReader<R> where R: fmt::Debug, { fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result { fmt.debug_struct("RevBufReader") .field("reader", &self.inner) .field("buffer", &format_args!("{}/{}", self.pos, self.buf.len())) .finish() } } impl<R: Seek> Seek for RevBufReader<R> { /// Seek to an offset, in bytes, in the underlying reader. /// /// The position used for seeking with `SeekFrom::Current(_)` is the /// position the underlying reader would be at if the `RevBufReader` had no /// internal buffer. /// /// Seeking always discards the internal buffer, even if the seek position /// would otherwise fall within it. This guarantees that calling /// `.into_inner()` immediately after a seek yields the underlying reader /// at the same position. /// /// To seek without discarding the internal buffer, use [`RevBufReader::seek_relative`]. /// /// See [`std::io::Seek`] for more details. /// /// Note: In the edge case where you're seeking with `SeekFrom::Current(n)` /// where `n` minus the internal buffer length overflows an `i64`, two /// seeks will be performed instead of one. If the second seek returns /// `Err`, the underlying reader will be left at the same position it would /// have if you called `seek` with `SeekFrom::Current(0)`. /// /// [`RevBufReader::seek_relative`]: struct.RevBufReader.html#method.seek_relative /// [`std::io::Seek`]: trait.Seek.html fn seek(&mut self, pos: SeekFrom) -> io::Result<u64> { let result: u64; if let SeekFrom::Current(n) = pos { let remainder = (self.cap - self.pos) as i64; // It should be safe to assume that remainder fits within an i64 as the alternative // means we managed to allocate 8 exbibytes and that's absurd. // But it's not out of the realm of possibility for some weird underlying reader to // support seeking by i64::min_value() so we need to handle underflow when subtracting // remainder. if let Some(offset) = n.checked_sub(remainder) { result = self.inner.seek(SeekFrom::Current(offset))?; } else { // Seek backwards by our remainder, and then by the offset self.inner.seek(SeekFrom::Current(-remainder))?; self.discard_buffer(); result = self.inner.seek(SeekFrom::Current(n))?; } } else { // Seeking with Start/End doesn't care about our buffer length. result = self.inner.seek(pos)?; } self.discard_buffer(); Ok(result) } } #[cfg(test)] mod tests { use super::RevBufReader; use std::io::prelude::*; use std::io::{self, SeekFrom}; #[test] fn test_buffered_reader() { let inner: &[u8] = &[5, 6, 7, 0, 1, 2, 3, 4]; let mut reader = RevBufReader::with_capacity(2, io::Cursor::new(inner)); let mut buf = [0, 0, 0]; let nread = reader.read(&mut buf); assert_eq!(nread.unwrap(), 3); assert_eq!(buf, [2, 3, 4]); assert_eq!(reader.buffer(), []); let mut buf = [0, 0]; let nread = reader.read(&mut buf); assert_eq!(nread.unwrap(), 2); assert_eq!(buf, [0, 1]); assert_eq!(reader.buffer(), []); let mut buf = [0]; let nread = reader.read(&mut buf); assert_eq!(nread.unwrap(), 1); assert_eq!(buf, [7]); assert_eq!(reader.buffer(), [6]); let mut buf = [0, 0, 0]; let nread = reader.read(&mut buf); assert_eq!(nread.unwrap(), 1); assert_eq!(buf, [6, 0, 0]); assert_eq!(reader.buffer(), []); let nread = reader.read(&mut buf); assert_eq!(nread.unwrap(), 1); assert_eq!(buf, [5, 0, 0]); assert_eq!(reader.buffer(), []); assert_eq!(reader.read(&mut buf).unwrap(), 0); } #[test] fn test_buffered_reader_seek() { let inner: &[u8] = &[5, 6, 7, 0, 1, 2, 3, 4]; let mut reader = RevBufReader::with_capacity(2, io::Cursor::new(inner)); assert_eq!(reader.seek(SeekFrom::End(-3)).ok(), Some(5)); assert_eq!(reader.fill_buf().ok(), Some(&[0, 1][..])); assert_eq!(reader.seek(SeekFrom::Current(0)).ok(), Some(5)); assert_eq!(reader.fill_buf().ok(), Some(&[0, 1][..])); assert_eq!(reader.seek(SeekFrom::Current(-1)).ok(), Some(4)); assert_eq!(reader.fill_buf().ok(), Some(&[7, 0][..])); reader.consume(1); assert_eq!(reader.seek(SeekFrom::Current(2)).ok(), Some(5)); } #[test] fn test_buffered_reader_seek_relative() { let inner: &[u8] = &[5, 6, 7, 0, 1, 2, 3, 4]; let mut reader = RevBufReader::with_capacity(2, io::Cursor::new(inner)); assert!(reader.seek_relative(-3).is_ok()); assert_eq!(reader.fill_buf().ok(), Some(&[0, 1][..])); assert!(reader.seek_relative(0).is_ok()); assert_eq!(reader.fill_buf().ok(), Some(&[0, 1][..])); assert!(reader.seek_relative(-1).is_ok()); assert_eq!(reader.fill_buf().ok(), Some(&[0][..])); assert!(reader.seek_relative(1).is_ok()); assert_eq!(reader.fill_buf().ok(), Some(&[0, 1][..])); assert!(reader.seek_relative(-2).is_ok()); assert_eq!(reader.fill_buf().ok(), Some(&[6, 7][..])); } #[test] fn test_buffered_reader_invalidated_after_read() { let inner: &[u8] = &[5, 6, 7, 0, 1, 2, 3, 4]; let mut reader = RevBufReader::with_capacity(3, io::Cursor::new(inner)); assert_eq!(reader.fill_buf().ok(), Some(&[2, 3, 4][..])); reader.consume(3); let mut buffer = [0, 0, 0, 0, 0]; assert_eq!(reader.read(&mut buffer).ok(), Some(5)); assert_eq!(buffer, [5, 6, 7, 0, 1]); assert!(reader.seek_relative(2).is_ok()); let mut buffer = [0, 0]; assert_eq!(reader.read(&mut buffer).ok(), Some(2)); assert_eq!(buffer, [5, 6]); } #[test] fn test_buffered_reader_invalidated_after_seek() { let inner: &[u8] = &[5, 6, 7, 0, 1, 2, 3, 4]; let mut reader = RevBufReader::with_capacity(3, io::Cursor::new(inner)); assert_eq!(reader.fill_buf().ok(), Some(&[2, 3, 4][..])); reader.consume(3); assert!(reader.seek(SeekFrom::Current(-5)).is_ok()); assert!(reader.seek_relative(2).is_ok()); let mut buffer = [0, 0]; assert_eq!(reader.read(&mut buffer).ok(), Some(2)); assert_eq!(buffer, [5, 6]); } #[test] fn test_buffered_reader_seek_underflow() { // gimmick reader that yields its position modulo 256 for each byte struct PositionReader { pos: u64, } impl Read for PositionReader { fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> { let len = buf.len(); for x in buf { *x = self.pos as u8; self.pos = self.pos.wrapping_add(1); } Ok(len) } } impl Seek for PositionReader { fn seek(&mut self, pos: SeekFrom) -> io::Result<u64> { match pos { SeekFrom::Start(n) => { self.pos = n; } SeekFrom::Current(n) => { self.pos = self.pos.wrapping_add(n as u64); } SeekFrom::End(n) => { self.pos = n as u64; } } Ok(self.pos) } } let mut reader = RevBufReader::with_capacity(5, PositionReader { pos: 0 }); assert_eq!(reader.fill_buf().ok(), Some(&[251, 252, 253, 254, 255][..])); assert_eq!(reader.seek(SeekFrom::Start(5)).ok(), Some(5)); assert_eq!(reader.fill_buf().ok().map(|s| s.len()), Some(5)); // the following seek will require two underlying seeks let expected = 9_223_372_036_854_775_813; assert_eq!( reader.seek(SeekFrom::Current(i64::min_value())).ok(), Some(expected) ); assert_eq!(reader.fill_buf().ok().map(|s| s.len()), Some(5)); // seeking to 0 should empty the buffer. assert_eq!(reader.seek(SeekFrom::Current(0)).ok(), Some(expected)); assert_eq!(reader.get_ref().pos, expected); } #[test] fn test_read_until() { let inner: &[u8] = &[0, 1, 2, 1, 0]; let mut reader = RevBufReader::with_capacity(2, io::Cursor::new(inner)); let mut v = Vec::new(); reader.read_until(1, &mut v).unwrap(); assert_eq!(v, [0]); v.truncate(0); reader.read_until(1, &mut v).unwrap(); assert_eq!(v, [2, 1]); v.truncate(0); reader.read_until(0, &mut v).unwrap(); assert_eq!(v, [1]); v.truncate(0); reader.read_until(8, &mut v).unwrap(); assert_eq!(v, [0]); v.truncate(0); reader.read_until(9, &mut v).unwrap(); assert_eq!(v, []); } #[test] fn test_read_line() { let in_buf: &[u8] = b"a\nb\nc"; let mut reader = RevBufReader::with_capacity(2, io::Cursor::new(in_buf)); let mut s = String::new(); reader.read_line(&mut s).unwrap(); assert_eq!(s, "c"); s.truncate(0); reader.read_line(&mut s).unwrap(); assert_eq!(s, "b\n"); s.truncate(0); reader.read_line(&mut s).unwrap(); assert_eq!(s, "a\n"); s.truncate(0); reader.read_line(&mut s).unwrap(); assert_eq!(s, ""); } #[test] fn test_lines() { let in_buf: &[u8] = b"a\nb\nc"; let reader = RevBufReader::with_capacity(2, io::Cursor::new(in_buf)); let mut it = reader.lines(); assert_eq!(it.next().unwrap().unwrap(), "c".to_string()); assert_eq!(it.next().unwrap().unwrap(), "b".to_string()); assert_eq!(it.next().unwrap().unwrap(), "a".to_string()); assert!(it.next().is_none()); } }