How to implement trim for Vec?

Question

Rust provides a trim method for strings: str.trim() removing leading and trailing whitespace. I want to have a method that does the same for bytestrings. It should take a Vec<u8> and remove leading and trailing whitespace (space, 0x20 and htab, 0x09).

Writing a trim_left() is easy, you can just use an iterator with skip_while(): Rust Playground

fn main() {
    let a: &[u8] = b"     fo o ";
    let b: Vec<u8> = a.iter().map(|x| x.clone()).skip_while(|x| x == &0x20 || x == &0x09).collect();
    println!("{:?}", b);
}

But to trim the right characters I would need to look ahead if no other letter is in the list after whitespace was found.

Answer 1

Here's an implementation that returns a slice, rather than a new Vec<u8>, as str::trim() does. It's also implemented on [u8], since that's more general than Vec<u8> (you can obtain a slice from a vector cheaply, but creating a vector from a slice is more costly, since it involves a heap allocation and a copy).

trait SliceExt {
    fn trim(&self) -> &Self;
}

impl SliceExt for [u8] {
    fn trim(&self) -> &[u8] {
        fn is_whitespace(c: &u8) -> bool {
            *c == b'\t' || *c == b' '
        }

        fn is_not_whitespace(c: &u8) -> bool {
            !is_whitespace(c)
        }

        if let Some(first) = self.iter().position(is_not_whitespace) {
            if let Some(last) = self.iter().rposition(is_not_whitespace) {
                &self[first..last + 1]
            } else {
                unreachable!();
            }
        } else {
            &[]
        }
    }
}

fn main() {
    let a = b"     fo o ";
    let b = a.trim();
    println!("{:?}", b);
}

If you really need a Vec<u8> after the trim(), you can just call into() on the slice to turn it into a Vec<u8>.

fn main() {
    let a = b"     fo o ";
    let b: Vec<u8> = a.trim().into();
    println!("{:?}", b);
}

Answer 2

All we have to do is find the index of the first non-whitespace character, one time counting forward from the start, and another time counting backwards from the end.

fn is_not_whitespace(e: &u8) -> bool {
    *e != 0x20 && *e != 0x09
}

fn main() {
    let a: &[u8] = b"     fo o ";

    // find the index of first non-whitespace char
    let begin = a.iter()
        .position(is_not_whitespace);

    // find the index of the last non-whitespace char
    let end = a.iter()
        .rev()
        .position(is_not_whitespace)
        .map(|j| a.len() - j);

    // build it
    let vec = begin.and_then(|i| end.map(|j| a[i..j].iter().collect()))
        .unwrap_or(Vec::new());

    println!("{:?}", vec);
}