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//! Buffering wrappers for I/O traits use crate::io::{BufRead, Error, ErrorKind, Read, Result, Seek, SeekFrom, Write}; use core::{cmp, fmt}; /// The `BufReader<R, S>` struct adds buffering to any reader. /// /// It can be excessively inefficient to work directly with a [`Read`] instance. /// For example, every call to [`read`][`TcpStream::read`] on [`TcpStream`] /// results in a system call. A `BufReader<R, S>` performs large, infrequent reads on /// the underlying [`Read`] and maintains an in-memory buffer of the results. /// /// `BufReader<R, S>` can improve the speed of programs that make *small* and /// *repeated* read calls to the same file or network socket. It does not /// help when reading very large amounts at once, or reading just one or a few /// times. It also provides no advantage when reading from a source that is /// already in memory, like a [`Vec`]`<u8>`. /// /// When the `BufReader<R, S>` is dropped, the contents of its buffer will be /// discarded. Creating multiple instances of a `BufReader<R, S>` on the same /// stream can cause data loss. Reading from the underlying reader after /// unwrapping the `BufReader<R, S>` with [`BufReader::into_inner`] can also cause /// data loss. /// /// [`TcpStream::read`]: Read::read /// [`TcpStream`]: crate::net::TcpStream /// /// # Examples /// /// ```no_run /// use std::prelude::*; /// use core2::io::BufReader; /// use std::fs::File; /// /// fn main() -> core::result::Result<()> { /// let f = File::open("log.txt")?; /// let mut reader = BufReader::new(f); /// /// let mut line = String::new(); /// let len = reader.read_line(&mut line)?; /// println!("First line is {} bytes long", len); /// Ok(()) /// } /// ``` pub struct BufReader<R, const S: usize> { inner: R, buf: [u8; S], pos: usize, cap: usize, } impl<R: Read, const S: usize> BufReader<R, S> { /// Creates a new `BufReader<R, S>` with a default buffer capacity. The default is currently 8 KB, /// but may change in the future. /// /// # Examples /// /// ```no_run /// use core2::io::BufReader; /// use std::fs::File; /// /// fn main() -> core::result::Result<()> { /// let f = File::open("log.txt")?; /// let reader = BufReader::new(f); /// Ok(()) /// } /// ``` pub fn new(inner: R) -> BufReader<R, S> { BufReader { inner, buf: [0; S], pos: 0, cap: 0, } } } impl<R, const S: usize> BufReader<R, S> { /// Gets a reference to the underlying reader. /// /// It is inadvisable to directly read from the underlying reader. /// /// # Examples /// /// ```no_run /// use core2::io::BufReader; /// use std::fs::File; /// /// fn main() -> core::result::Result<()> { /// let f1 = File::open("log.txt")?; /// let reader = BufReader::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 core2::io::BufReader; /// use std::fs::File; /// /// fn main() -> core::result::Result<()> { /// let f1 = File::open("log.txt")?; /// let mut reader = BufReader::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. /// /// [`fill_buf`]: BufRead::fill_buf /// /// # Examples /// /// ```no_run /// use std::{BufReader, BufRead}; /// use std::fs::File; /// /// fn main() -> core::result::Result<()> { /// let f = File::open("log.txt")?; /// let mut reader = BufReader::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[self.pos..self.cap] } /// Returns the number of bytes the internal buffer can hold at once. /// /// # Examples /// /// ```no_run /// use std::{BufReader, BufRead}; /// use std::fs::File; /// /// fn main() -> core::result::Result<()> { /// let f = File::open("log.txt")?; /// let mut reader = BufReader::new(f); /// /// let capacity = reader.capacity(); /// let buffer = reader.fill_buf()?; /// assert!(buffer.len() <= capacity); /// Ok(()) /// } /// ``` pub fn capacity(&self) -> usize { S } /// Unwraps this `BufReader<R, S>`, returning the underlying reader. /// /// Note that any leftover data in the internal buffer is lost. Therefore, /// a following read from the underlying reader may lead to data loss. /// /// # Examples /// /// ```no_run /// use core2::io::BufReader; /// use std::fs::File; /// /// fn main() -> core::result::Result<()> { /// let f1 = File::open("log.txt")?; /// let reader = BufReader::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: Read, const S: usize> Read for BufReader<R, S> { fn read(&mut self, buf: &mut [u8]) -> 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 == self.cap && buf.len() >= S { self.discard_buffer(); return self.inner.read(buf); } let nread = { let mut rem = self.fill_buf()?; rem.read(buf)? }; self.consume(nread); Ok(nread) } } impl<R: Read, const S: usize> BufRead for BufReader<R, S> { fn fill_buf(&mut self) -> Result<&[u8]> { // If we've reached the end of our internal buffer then we need to fetch // some more data from the underlying reader. // Branch using `>=` instead of the more correct `==` // to tell the compiler that the pos..cap slice is always valid. if self.pos >= self.cap { debug_assert!(self.pos == self.cap); self.cap = self.inner.read(&mut self.buf)?; self.pos = 0; } Ok(&self.buf[self.pos..self.cap]) } fn consume(&mut self, amt: usize) { self.pos = cmp::min(self.pos + amt, self.cap); } } impl<R, const S: usize> fmt::Debug for BufReader<R, S> where R: fmt::Debug, { fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result { fmt.debug_struct("BufReader") .field("reader", &self.inner) .field("buffer", &format_args!("{}/{}", self.cap - self.pos, S)) .finish() } } impl<R: Seek, const S: usize> Seek for BufReader<R, S> { /// 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 `BufReader<R, S>` had no /// internal buffer. /// /// Seeking always discards the internal buffer, even if the seek position /// would otherwise fall within it. This guarantees that calling /// [`BufReader::into_inner()`] immediately after a seek yields the underlying reader /// at the same position. /// /// To seek without discarding the internal buffer, use [`BufReader::seek_relative`]. /// /// See [`std::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)`. /// /// [`std::Seek`]: Seek fn seek(&mut self, pos: SeekFrom) -> 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 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) } } /// Wraps a writer and buffers its output. /// /// It can be excessively inefficient to work directly with something that /// implements [`Write`]. For example, every call to /// [`write`][`TcpStream::write`] on [`TcpStream`] results in a system call. A /// `BufWriter<W>` keeps an in-memory buffer of data and writes it to an underlying /// writer in large, infrequent batches. /// /// `BufWriter<W>` can improve the speed of programs that make *small* and /// *repeated* write calls to the same file or network socket. It does not /// help when writing very large amounts at once, or writing just one or a few /// times. It also provides no advantage when writing to a destination that is /// in memory, like a [`Vec`]<u8>`. /// /// It is critical to call [`flush`] before `BufWriter<W>` is dropped. Though /// dropping will attempt to flush the contents of the buffer, any errors /// that happen in the process of dropping will be ignored. Calling [`flush`] /// ensures that the buffer is empty and thus dropping will not even attempt /// file operations. /// /// # Examples /// /// Let's write the numbers one through ten to a [`TcpStream`]: /// /// ```no_run /// use std::prelude::*; /// use std::net::TcpStream; /// /// let mut stream = TcpStream::connect("127.0.0.1:34254").unwrap(); /// /// for i in 0..10 { /// stream.write(&[i+1]).unwrap(); /// } /// ``` /// /// Because we're not buffering, we write each one in turn, incurring the /// overhead of a system call per byte written. We can fix this with a /// `BufWriter<W>`: /// /// ```no_run /// use std::prelude::*; /// use core2::io::BufWriter; /// use std::net::TcpStream; /// /// let mut stream = BufWriter::new(TcpStream::connect("127.0.0.1:34254").unwrap()); /// /// for i in 0..10 { /// stream.write(&[i+1]).unwrap(); /// } /// stream.flush().unwrap(); /// ``` /// /// By wrapping the stream with a `BufWriter<W>`, these ten writes are all grouped /// together by the buffer and will all be written out in one system call when /// the `stream` is flushed. /// /// [`TcpStream::write`]: Write::write /// [`TcpStream`]: crate::net::TcpStream /// [`flush`]: Write::flush pub struct BufWriter<W: Write, const S: usize> { inner: Option<W>, buf: [u8; S], len: usize, // #30888: If the inner writer panics in a call to write, we don't want to // write the buffered data a second time in BufWriter's destructor. This // flag tells the Drop impl if it should skip the flush. panicked: bool, } /// An error returned by [`BufWriter::into_inner`] which combines an error that /// happened while writing out the buffer, and the buffered writer object /// which may be used to recover from the condition. /// /// # Examples /// /// ```no_run /// use core2::io::BufWriter; /// use std::net::TcpStream; /// /// let mut stream = BufWriter::new(TcpStream::connect("127.0.0.1:34254").unwrap()); /// /// // do stuff with the stream /// /// // we want to get our `TcpStream` back, so let's try: /// /// let stream = match stream.into_inner() { /// Ok(s) => s, /// Err(e) => { /// // Here, e is an IntoInnerError /// panic!("An error occurred"); /// } /// }; /// ``` #[derive(Debug)] pub struct IntoInnerError<W>(W, Error); impl<W, const S: usize> BufWriter<W, S> where W: Write, { /// Creates a new `BufWriter<W>` with a default buffer capacity. The default is currently 8 KB, /// but may change in the future. /// /// # Examples /// /// ```no_run /// use core2::io::BufWriter; /// use std::net::TcpStream; /// /// let mut buffer = BufWriter::new(TcpStream::connect("127.0.0.1:34254").unwrap()); /// ``` pub fn new(inner: W) -> BufWriter<W, S> { BufWriter { inner: Some(inner), buf: [0; S], len: 0, panicked: false, } } /// Send data in our local buffer into the inner writer, looping as /// necessary until either it's all been sent or an error occurs. /// /// Because all the data in the buffer has been reported to our owner as /// "successfully written" (by returning nonzero success values from /// `write`), any 0-length writes from `inner` must be reported as i/o /// errors from this method. fn flush_buf(&mut self) -> Result<()> { /// Helper struct to ensure the buffer is updated after all the writes /// are complete. It tracks the number of written bytes and drains them /// all from the front of the buffer when dropped. struct BufGuard<'a, const S: usize> { buffer: &'a mut [u8; S], written: usize, } impl<'a, const S: usize> BufGuard<'a, S> { fn new(buffer: &'a mut [u8; S]) -> Self { Self { buffer, written: 0 } } /// The unwritten part of the buffer fn remaining(&self) -> &[u8] { &self.buffer[self.written..] } /// Flag some bytes as removed from the front of the buffer fn consume(&mut self, amt: usize) { self.written += amt; } /// true if all of the bytes have been written fn done(&self) -> bool { self.written >= self.buffer.len() } } impl<const S: usize> Drop for BufGuard<'_, S> { fn drop(&mut self) { if self.written > 0 { let mut new_buf = [0; S]; new_buf.copy_from_slice(&self.buffer[self.written..]); *self.buffer = new_buf; } } } let mut guard = BufGuard::new(&mut self.buf); let inner = self.inner.as_mut().unwrap(); while !guard.done() { self.panicked = true; let r = inner.write(guard.remaining()); self.panicked = false; match r { Ok(0) => { return Err(Error::new( ErrorKind::WriteZero, "failed to write the buffered data", )); } Ok(n) => guard.consume(n), Err(ref e) if e.kind() == ErrorKind::Interrupted => {} Err(e) => return Err(e), } } Ok(()) } /// Buffer some data without flushing it, regardless of the size of the /// data. Writes as much as possible without exceeding capacity. Returns /// the number of bytes written. fn write_to_buf(&mut self, buf: &[u8]) -> usize { let available = S - self.len; let amt_to_buffer = available.min(buf.len()); (&mut self.buf[available..]).copy_from_slice(&buf[..amt_to_buffer]); self.len += amt_to_buffer; amt_to_buffer } /// Gets a reference to the underlying writer. /// /// # Examples /// /// ```no_run /// use core2::io::BufWriter; /// use std::net::TcpStream; /// /// let mut buffer = BufWriter::new(TcpStream::connect("127.0.0.1:34254").unwrap()); /// /// // we can use reference just like buffer /// let reference = buffer.get_ref(); /// ``` pub fn get_ref(&self) -> &W { self.inner.as_ref().unwrap() } /// Gets a mutable reference to the underlying writer. /// /// It is inadvisable to directly write to the underlying writer. /// /// # Examples /// /// ```no_run /// use core2::io::BufWriter; /// use std::net::TcpStream; /// /// let mut buffer = BufWriter::new(TcpStream::connect("127.0.0.1:34254").unwrap()); /// /// // we can use reference just like buffer /// let reference = buffer.get_mut(); /// ``` pub fn get_mut(&mut self) -> &mut W { self.inner.as_mut().unwrap() } /// Returns a reference to the internally buffered data. /// /// # Examples /// /// ```no_run /// use core2::io::BufWriter; /// use std::net::TcpStream; /// /// let buf_writer = BufWriter::new(TcpStream::connect("127.0.0.1:34254").unwrap()); /// /// // See how many bytes are currently buffered /// let bytes_buffered = buf_writer.buffer().len(); /// ``` pub fn buffer(&self) -> &[u8] { &self.buf } /// Returns the number of bytes the internal buffer can hold without flushing. /// /// # Examples /// /// ```no_run /// use core2::io::BufWriter; /// use std::net::TcpStream; /// /// let buf_writer = BufWriter::new(TcpStream::connect("127.0.0.1:34254").unwrap()); /// /// // Check the capacity of the inner buffer /// let capacity = buf_writer.capacity(); /// // Calculate how many bytes can be written without flushing /// let without_flush = capacity - buf_writer.buffer().len(); /// ``` pub fn capacity(&self) -> usize { S } /// Unwraps this `BufWriter<W>`, returning the underlying writer. /// /// The buffer is written out before returning the writer. /// /// # Errors /// /// An [`Err`] will be returned if an error occurs while flushing the buffer. /// /// # Examples /// /// ```no_run /// use core2::io::BufWriter; /// use std::net::TcpStream; /// /// let mut buffer = BufWriter::new(TcpStream::connect("127.0.0.1:34254").unwrap()); /// /// // unwrap the TcpStream and flush the buffer /// let stream = buffer.into_inner().unwrap(); /// ``` pub fn into_inner(mut self) -> core::result::Result<W, IntoInnerError<BufWriter<W, S>>> { match self.flush_buf() { Err(e) => Err(IntoInnerError(self, e)), Ok(()) => Ok(self.inner.take().unwrap()), } } } impl<W: Write, const S: usize> Write for BufWriter<W, S> { fn write(&mut self, buf: &[u8]) -> Result<usize> { if self.len + buf.len() > S { self.flush_buf()?; } // FIXME: Why no len > capacity? Why not buffer len == capacity? #72919 if buf.len() >= S { self.panicked = true; let r = self.get_mut().write(buf); self.panicked = false; r } else { self.buf.copy_from_slice(buf); Ok(buf.len()) } } fn write_all(&mut self, buf: &[u8]) -> Result<()> { // Normally, `write_all` just calls `write` in a loop. We can do better // by calling `self.get_mut().write_all()` directly, which avoids // round trips through the buffer in the event of a series of partial // writes in some circumstances. if self.len + buf.len() > S { self.flush_buf()?; } // FIXME: Why no len > capacity? Why not buffer len == capacity? #72919 if buf.len() >= S { self.panicked = true; let r = self.get_mut().write_all(buf); self.panicked = false; r } else { self.buf.copy_from_slice(buf); Ok(()) } } fn flush(&mut self) -> Result<()> { self.flush_buf().and_then(|()| self.get_mut().flush()) } } impl<W: Write, const S: usize> fmt::Debug for BufWriter<W, S> where W: fmt::Debug, { fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result { fmt.debug_struct("BufWriter") .field("writer", &self.inner.as_ref().unwrap()) .field("buffer", &format_args!("{}/{}", self.buf.len(), S)) .finish() } } impl<W: Write + Seek, const S: usize> Seek for BufWriter<W, S> { /// Seek to the offset, in bytes, in the underlying writer. /// /// Seeking always writes out the internal buffer before seeking. fn seek(&mut self, pos: SeekFrom) -> Result<u64> { self.flush_buf()?; self.get_mut().seek(pos) } } impl<W: Write, const S: usize> Drop for BufWriter<W, S> { fn drop(&mut self) { if self.inner.is_some() && !self.panicked { // dtors should not panic, so we ignore a failed flush let _r = self.flush_buf(); } } } impl<W> IntoInnerError<W> { /// Returns the error which caused the call to [`BufWriter::into_inner()`] /// to fail. /// /// This error was returned when attempting to write the internal buffer. /// /// # Examples /// /// ```no_run /// use core2::io::BufWriter; /// use std::net::TcpStream; /// /// let mut stream = BufWriter::new(TcpStream::connect("127.0.0.1:34254").unwrap()); /// /// // do stuff with the stream /// /// // we want to get our `TcpStream` back, so let's try: /// /// let stream = match stream.into_inner() { /// Ok(s) => s, /// Err(e) => { /// // Here, e is an IntoInnerError, let's log the inner error. /// // /// // We'll just 'log' to stdout for this example. /// println!("{}", e.error()); /// /// panic!("An unexpected error occurred."); /// } /// }; /// ``` pub fn error(&self) -> &Error { &self.1 } /// Returns the buffered writer instance which generated the error. /// /// The returned object can be used for error recovery, such as /// re-inspecting the buffer. /// /// # Examples /// /// ```no_run /// use core2::io::BufWriter; /// use std::net::TcpStream; /// /// let mut stream = BufWriter::new(TcpStream::connect("127.0.0.1:34254").unwrap()); /// /// // do stuff with the stream /// /// // we want to get our `TcpStream` back, so let's try: /// /// let stream = match stream.into_inner() { /// Ok(s) => s, /// Err(e) => { /// // Here, e is an IntoInnerError, let's re-examine the buffer: /// let buffer = e.into_inner(); /// /// // do stuff to try to recover /// /// // afterwards, let's just return the stream /// buffer.into_inner().unwrap() /// } /// }; /// ``` pub fn into_inner(self) -> W { self.0 } } impl<W> From<IntoInnerError<W>> for Error { fn from(iie: IntoInnerError<W>) -> Error { iie.1 } } impl<W> fmt::Display for IntoInnerError<W> { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { self.error().fmt(f) } } /// Private helper struct for implementing the line-buffered writing logic. /// This shim temporarily wraps a BufWriter, and uses its internals to /// implement a line-buffered writer (specifically by using the internal /// methods like write_to_buf and flush_buf). In this way, a more /// efficient abstraction can be created than one that only had access to /// `write` and `flush`, without needlessly duplicating a lot of the /// implementation details of BufWriter. This also allows existing /// `BufWriters` to be temporarily given line-buffering logic; this is what /// enables Stdout to be alternately in line-buffered or block-buffered mode. #[derive(Debug)] pub(super) struct LineWriterShim<'a, W: Write, const S: usize> { buffer: &'a mut BufWriter<W, S>, } impl<'a, W: Write, const S: usize> LineWriterShim<'a, W, S> { pub fn new(buffer: &'a mut BufWriter<W, S>) -> Self { Self { buffer } } /// Get a mutable reference to the inner writer (that is, the writer /// wrapped by the BufWriter). Be careful with this writer, as writes to /// it will bypass the buffer. fn inner_mut(&mut self) -> &mut W { self.buffer.get_mut() } /// Get the content currently buffered in self.buffer fn buffered(&self) -> &[u8] { self.buffer.buffer() } /// Flush the buffer iff the last byte is a newline (indicating that an /// earlier write only succeeded partially, and we want to retry flushing /// the buffered line before continuing with a subsequent write) fn flush_if_completed_line(&mut self) -> Result<()> { match self.buffered().last().copied() { Some(b'\n') => self.buffer.flush_buf(), _ => Ok(()), } } } impl<'a, W: Write, const S: usize> Write for LineWriterShim<'a, W, S> { /// Write some data into this BufReader with line buffering. This means /// that, if any newlines are present in the data, the data up to the last /// newline is sent directly to the underlying writer, and data after it /// is buffered. Returns the number of bytes written. /// /// This function operates on a "best effort basis"; in keeping with the /// convention of `Write::write`, it makes at most one attempt to write /// new data to the underlying writer. If that write only reports a partial /// success, the remaining data will be buffered. /// /// Because this function attempts to send completed lines to the underlying /// writer, it will also flush the existing buffer if it ends with a /// newline, even if the incoming data does not contain any newlines. fn write(&mut self, buf: &[u8]) -> Result<usize> { let newline_idx = match memchr::memrchr(b'\n', buf) { // If there are no new newlines (that is, if this write is less than // one line), just do a regular buffered write (which may flush if // we exceed the inner buffer's size) None => { self.flush_if_completed_line()?; return self.buffer.write(buf); } // Otherwise, arrange for the lines to be written directly to the // inner writer. Some(newline_idx) => newline_idx + 1, }; // Flush existing content to prepare for our write. We have to do this // before attempting to write `buf` in order to maintain consistency; // if we add `buf` to the buffer then try to flush it all at once, // we're obligated to return Ok(), which would mean suppressing any // errors that occur during flush. self.buffer.flush_buf()?; // This is what we're going to try to write directly to the inner // writer. The rest will be buffered, if nothing goes wrong. let lines = &buf[..newline_idx]; // Write `lines` directly to the inner writer. In keeping with the // `write` convention, make at most one attempt to add new (unbuffered) // data. Because this write doesn't touch the BufWriter state directly, // and the buffer is known to be empty, we don't need to worry about // self.buffer.panicked here. let flushed = self.inner_mut().write(lines)?; // If buffer returns Ok(0), propagate that to the caller without // doing additional buffering; otherwise we're just guaranteeing // an "ErrorKind::WriteZero" later. if flushed == 0 { return Ok(0); } // Now that the write has succeeded, buffer the rest (or as much of // the rest as possible). If there were any unwritten newlines, we // only buffer out to the last unwritten newline that fits in the // buffer; this helps prevent flushing partial lines on subsequent // calls to LineWriterShim::write. // Handle the cases in order of most-common to least-common, under // the presumption that most writes succeed in totality, and that most // writes are smaller than the buffer. // - Is this a partial line (ie, no newlines left in the unwritten tail) // - If not, does the data out to the last unwritten newline fit in // the buffer? // - If not, scan for the last newline that *does* fit in the buffer let tail = if flushed >= newline_idx { &buf[flushed..] } else if newline_idx - flushed <= self.buffer.capacity() { &buf[flushed..newline_idx] } else { let scan_area = &buf[flushed..]; let scan_area = &scan_area[..self.buffer.capacity()]; match memchr::memrchr(b'\n', scan_area) { Some(newline_idx) => &scan_area[..newline_idx + 1], None => scan_area, } }; let buffered = self.buffer.write_to_buf(tail); Ok(flushed + buffered) } fn flush(&mut self) -> Result<()> { self.buffer.flush() } /// Write some data into this BufReader with line buffering. This means /// that, if any newlines are present in the data, the data up to the last /// newline is sent directly to the underlying writer, and data after it /// is buffered. /// /// Because this function attempts to send completed lines to the underlying /// writer, it will also flush the existing buffer if it contains any /// newlines, even if the incoming data does not contain any newlines. fn write_all(&mut self, buf: &[u8]) -> Result<()> { match memchr::memrchr(b'\n', buf) { // If there are no new newlines (that is, if this write is less than // one line), just do a regular buffered write (which may flush if // we exceed the inner buffer's size) None => { self.flush_if_completed_line()?; self.buffer.write_all(buf) } Some(newline_idx) => { let (lines, tail) = buf.split_at(newline_idx + 1); if self.buffered().is_empty() { self.inner_mut().write_all(lines)?; } else { // If there is any buffered data, we add the incoming lines // to that buffer before flushing, which saves us at least // one write call. We can't really do this with `write`, // since we can't do this *and* not suppress errors *and* // report a consistent state to the caller in a return // value, but here in write_all it's fine. self.buffer.write_all(lines)?; self.buffer.flush_buf()?; } self.buffer.write_all(tail) } } } } /// Wraps a writer and buffers output to it, flushing whenever a newline /// (`0x0a`, `'\n'`) is detected. /// /// The [`BufWriter`] struct wraps a writer and buffers its output. /// But it only does this batched write when it goes out of scope, or when the /// internal buffer is full. Sometimes, you'd prefer to write each line as it's /// completed, rather than the entire buffer at once. Enter `LineWriter`. It /// does exactly that. /// /// Like [`BufWriter`], a `LineWriter`’s buffer will also be flushed when the /// `LineWriter` goes out of scope or when its internal buffer is full. /// /// If there's still a partial line in the buffer when the `LineWriter` is /// dropped, it will flush those contents. /// /// # Examples /// /// We can use `LineWriter` to write one line at a time, significantly /// reducing the number of actual writes to the file. /// /// ```no_run /// use std::fs::{self, File}; /// use std::prelude::*; /// use core2::io::LineWriter; /// /// fn main() -> core::result::Result<()> { /// let road_not_taken = b"I shall be telling this with a sigh /// Somewhere ages and ages hence: /// Two roads diverged in a wood, and I - /// I took the one less traveled by, /// And that has made all the difference."; /// /// let file = File::create("poem.txt")?; /// let mut file = LineWriter::new(file); /// /// file.write_all(b"I shall be telling this with a sigh")?; /// /// // No bytes are written until a newline is encountered (or /// // the internal buffer is filled). /// assert_eq!(fs::read_to_string("poem.txt")?, ""); /// file.write_all(b"\n")?; /// assert_eq!( /// fs::read_to_string("poem.txt")?, /// "I shall be telling this with a sigh\n", /// ); /// /// // Write the rest of the poem. /// file.write_all(b"Somewhere ages and ages hence: /// Two roads diverged in a wood, and I - /// I took the one less traveled by, /// And that has made all the difference.")?; /// /// // The last line of the poem doesn't end in a newline, so /// // we have to flush or drop the `LineWriter` to finish /// // writing. /// file.flush()?; /// /// // Confirm the whole poem was written. /// assert_eq!(fs::read("poem.txt")?, &road_not_taken[..]); /// Ok(()) /// } /// ``` pub struct LineWriter<W: Write, const S: usize> { inner: BufWriter<W, S>, } impl<W: Write, const S: usize> LineWriter<W, S> { /// Creates a new `LineWriter`. /// /// # Examples /// /// ```no_run /// use std::fs::File; /// use core2::io::LineWriter; /// /// fn main() -> core::result::Result<()> { /// let file = File::create("poem.txt")?; /// let file = LineWriter::new(file); /// Ok(()) /// } /// ``` pub fn new(inner: W) -> LineWriter<W, S> { LineWriter { inner: BufWriter::new(inner), } } /// Gets a reference to the underlying writer. /// /// # Examples /// /// ```no_run /// use std::fs::File; /// use core2::io::LineWriter; /// /// fn main() -> core::result::Result<()> { /// let file = File::create("poem.txt")?; /// let file = LineWriter::new(file); /// /// let reference = file.get_ref(); /// Ok(()) /// } /// ``` pub fn get_ref(&self) -> &W { self.inner.get_ref() } /// Gets a mutable reference to the underlying writer. /// /// Caution must be taken when calling methods on the mutable reference /// returned as extra writes could corrupt the output stream. /// /// # Examples /// /// ```no_run /// use std::fs::File; /// use core2::io::LineWriter; /// /// fn main() -> core::result::Result<()> { /// let file = File::create("poem.txt")?; /// let mut file = LineWriter::new(file); /// /// // we can use reference just like file /// let reference = file.get_mut(); /// Ok(()) /// } /// ``` pub fn get_mut(&mut self) -> &mut W { self.inner.get_mut() } /// Unwraps this `LineWriter`, returning the underlying writer. /// /// The internal buffer is written out before returning the writer. /// /// # Errors /// /// An [`Err`] will be returned if an error occurs while flushing the buffer. /// /// # Examples /// /// ```no_run /// use std::fs::File; /// use core2::io::LineWriter; /// /// fn main() -> core::result::Result<()> { /// let file = File::create("poem.txt")?; /// /// let writer: LineWriter<File> = LineWriter::new(file); /// /// let file: File = writer.into_inner()?; /// Ok(()) /// } /// ``` pub fn into_inner(self) -> core::result::Result<W, IntoInnerError<LineWriter<W, S>>> { self.inner .into_inner() .map_err(|IntoInnerError(buf, e)| IntoInnerError(LineWriter { inner: buf }, e)) } } impl<W: Write, const S: usize> Write for LineWriter<W, S> { fn write(&mut self, buf: &[u8]) -> Result<usize> { LineWriterShim::new(&mut self.inner).write(buf) } fn flush(&mut self) -> Result<()> { self.inner.flush() } fn write_all(&mut self, buf: &[u8]) -> Result<()> { LineWriterShim::new(&mut self.inner).write_all(buf) } fn write_fmt(&mut self, fmt: fmt::Arguments<'_>) -> Result<()> { LineWriterShim::new(&mut self.inner).write_fmt(fmt) } } impl<W: Write, const S: usize> fmt::Debug for LineWriter<W, S> where W: fmt::Debug, { fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result { fmt.debug_struct("LineWriter") .field("writer", &self.inner.inner) .field("buffer", &format_args!("{}/{}", self.inner.len, S)) .finish() } }