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LineReader Build Status


LineReader is a byte-delimiter-focused buffered reader for Rust, meant as a faster, less error-prone alternative to BufRead::read_until.

It provides three main functions:

next_line() -> Option<io::Result<&[u8]>>

Returns None on end-of-file, or an io::Result-wrapped byte slice of the next line from the reader.

Line length is limited to the size of the internal buffer - longer lines will be spread across multiple reads.

In contrast with read_until, detecting end-of-file is more natural with the use of Option; line length is naturally limited to some sensible value without the use of by_ref().take(limit); copying is minimised by returning borrowed slices; you'll never forget to call buf.clear().

next_batch() -> Option<io::Result<&[u8]>>

Behaves identically to next_line(), except it returns a slice of all the complete lines in the buffer.

for_each() -> io::Result<()>

Calls a closure on each line of the input, while the closure returns Ok(true) and no IO errors are detected. Such errors terminate iteration and are returned from the function.


extern crate linereader;
use linereader::LineReader;

let mut file = File::open(myfile).expect("open");

// Defaults to a 64 KiB buffer and b'\n' delimiter; change with one of:
//  * LineReader::with_capacity(usize);
//  * LineReader::with_delimiter(u8);
//  * LineReader::with_delimiter_and_capacity(u8, usize)
let mut reader = LineReader::new(file);

while let Some(line) = reader.next_line() {
    let line = line.expect("read error");
    // line is a &[u8] owned by reader.


LineReader contains no unsafe code, but it does hand out manually-calculated slice positions from an internal buffer, and the only enforcement Rust performs is to ensure they remain within that buffer.

There is no incomplete line detection, and as documented, lines that extend beyond the configured buffer can be spread across multiple "lines", with only the lack of a terminating delimiter as a hint. Care should be taken that this doesn't cause undesired behaviour in code using the library.

Methods such as get_mut() offer direct access to the wrapped reader, and their use without also calling reset() could result in undesired behaviour if the reader's state is changed.


bstr: as of 0.2.8 the for_byte_line and for_byte_line_with_terminator BufRead extension trait functions should perform similarly, without LineReader's line length limitations.


Tests performed using 'Dickens_Charles_Pickwick_Papers.xml', concatinated to itself 480 times. The resulting file is 976 MB and 10.3 million lines long.

Westmere Xeon 2.1GHz, FreeBSD/ZFS.

Method Time Lines/sec Bandwidth
read() 0.25s 41429738/s 3907.62 MB/s
LR::next_batch() 0.27s 38258946/s 3608.55 MB/s
LR::next_line() 1.51s 6874006/s 648.35 MB/s
read_until() 1.94s 5327387/s 502.47 MB/s
read_line() 2.54s 4081562/s 384.97 MB/s
lines() 3.23s 3199491/s 301.77 MB/s


A fast Rust line reader




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