Chisel - Decoders

Overview

This repository contains a very simple, lean implementation of a decoder that will consume u8 bytes from a given Read implementation, and decode into the Rust internal char type using UTF-8 . This is an offshoot lib from an ongoing toy parser project, and is used as the first stage of the scanning/lexing phase of the parser in order avoid unnecessary allocations during the u8 sequence -> char conversion.

Note that the implementation is pretty fast and loose, and under the covers utilises some bit-twiddlin' in conjunction with the unsafe transmute function to do the conversions. No string allocations are used during conversion. There is minimal checking (other than bit-masking) of the inbound bytes - it is not intended to be a full-blown UTF8 validation library, although improved/feature-flagged validation may be added at a later date.

Usage

Usage is very simple, provided you have something that implements Read in order to source some bytes:

Create from a slice

Just wrap your array in a reader, and then plug it into a new instance of Utf8Decoder:

    let buffer: &[u8] = &[0x10, 0x12, 0x23, 0x12];
    let mut reader = BufReader::new(buffer);
    let _decoder = Utf8Decoder::new(&mut reader);

Create from a file

Just crack open your file, wrap in a Read instance and then plug into a new instance of CharStream:

    let path = PathBuf::from("somefile.txt");
    let f = File::open(path);
    let mut reader = BufReader::new(f.unwrap());
    let _decoder = Utf8Decoder::new(&mut reader);

Consuming chars

You can either pull out new chars from the reader wrapped inside a Result type:

    loop {
        let result = decoder.decode_next();
        if result.is_err() {
            break;
        }
    }

Alternatively, you can just use the Utf8Decoder as an Iterator:

    let stream = Utf8Decoder::new(&mut reader);
    for c in stream {
        match c {
            Some(c) => ...
            None => ...
        }       
    }