strsplit.rs

#![warn(rust_2018_idioms)]

#[derive(Debug)]
pub struct StrSplit<'haystack, D> {
    remainder: Option<&'haystack str>,
    delimiter: D,
}

impl<'haystack, D> StrSplit<'haystack, D> {
    pub fn new(haystack: &'haystack str, delimiter: D) -> Self {
        Self {
            remainder: Some(haystack),
            delimiter,
        }
    }
}

pub trait Delimiter {
    fn find_next(&self, s: &str) -> Option<(usize, usize)>;
}

impl<'haystack, D> Iterator for StrSplit<'haystack, D>
where
    D: Delimiter,
{
    type Item = &'haystack str;
    fn next(&mut self) -> Option<Self::Item> {
        let remainder = self.remainder.as_mut()?;
        if let Some((delim_start, delim_end)) = self.delimiter.find_next(remainder) {
            let until_delimiter = &remainder[..delim_start];
            *remainder = &remainder[delim_end..];
            Some(until_delimiter)
        } else {
            self.remainder.take()
        }
    }
}

impl Delimiter for &str {
    fn find_next(&self, s: &str) -> Option<(usize, usize)> {
        s.find(self).map(|start| (start, start + self.len()))
    }
}

impl Delimiter for char {
    fn find_next(&self, s: &str) -> Option<(usize, usize)> {
        s.char_indices()
            .find(|(_, c)| c == self)
            .map(|(start, _)| (start, start + self.len_utf8()))
    }
}

pub fn until_char(s: &str, c: char) -> &'_ str {
    StrSplit::new(s, c)
        .next()
        .expect("StrSplit always gives at least one result")
}

#[test]
fn until_char_test() {
    assert_eq!(until_char("hello world", 'o'), "hell");
}

#[test]
fn it_works() {
    let haystack = "a b c d e";
    let letters: Vec<_> = StrSplit::new(haystack, " ").collect();
    assert_eq!(letters, vec!["a", "b", "c", "d", "e"]);
}

#[test]
fn tail() {
    let haystack = "a b c d ";
    let letters: Vec<_> = StrSplit::new(haystack, " ").collect();
    assert_eq!(letters, vec!["a", "b", "c", "d", ""]);
}

@MaxAsif Either of those are fine. They both work here because the type inside the Option in self.remainder is Copy, and so therefore you can pull it out in the initial if let Some and still be allowed to overwrite it further down. That wouldn't be the case if the type wasn't Copy (e.g., if it were a String). In that case you'd have to use the former (i.e., the commented-out code).