Sometimes you want to parse as much UTF-8 as possible.

valid_utf8_prefix.rs

//! Parsing UTF-8 prefixes out of bytes

/*
   Copyright 2016 Alex hill

   Licensed under the Apache License, Version 2.0 (the "License");
   you may not use this file except in compliance with the License.
   You may obtain a copy of the License at

       http://www.apache.org/licenses/LICENSE-2.0

   Unless required by applicable law or agreed to in writing, software
   distributed under the License is distributed on an "AS IS" BASIS,
   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   See the License for the specific language governing permissions and
   limitations under the License.
*/

use std::str;

pub fn valid_prefix<'a>(b: &'a [u8]) -> Prefix<'a> {
    match str::from_utf8(b) {
        Ok(everything) => Prefix { valid: Some(everything), rest: None },
        Err(e) => {
            let up_to = e.valid_up_to();
            if up_to > 0 && up_to == b.len() {
                Prefix {
                    valid: Some(unsafe{str::from_utf8_unchecked(&b[.. up_to])}),
                    rest: None
                }
            }
            else if up_to > 0 {
                Prefix {
                    valid: Some(unsafe{str::from_utf8_unchecked(&b[.. up_to])}),
                    rest: Some(&b[up_to ..])
                }
            }
            else {
                Prefix {
                    valid: None,
                    rest: Some(b)
                }
            }
        }
    }
}

#[derive(Debug, Eq, PartialEq, Clone)]
pub struct Prefix<'a> {
    pub valid: Option<&'a str>,
    pub rest: Option<&'a [u8]>
}
    
#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn valid_utf8() {
        let input = "This string is valid UTF-8";
        assert_eq!(Prefix{valid: Some(input), rest: None},
                   valid_prefix(input.as_bytes()));
    }

    #[test]
    fn total_garbage() {
        let input: &[u8] = &[0x80, 0x80, 0x80];
        assert_eq!(Prefix{valid: None, rest: Some(input)},
                   valid_prefix(input));
    }

    #[test]
    fn some_prefix() {
        let input: &[u8] = b"spam\xff";
        assert_eq!(Prefix{valid: Some("spam"), rest: Some(b"\xff" as &[u8])},
                   valid_prefix(input));
    }
}