Pairs#move_cursor is slow down iter

[huacnlee]

Hi @tomtau,

I am very sorry about that. 😥

I have found Pairs#iter performance issue when we merged #754.

Because when we iter pairs, each iter next will called Position#line_col once to move cursor. That will slow down the iter .

Especially that, in sometimes we just need get a little pair's line,col, not all of pairs. So to do this is not value of money.

By my test:

fn bench_pairs_iter(c: &mut Criterion) {
    let data = include_str!("data.json");

    fn iter_all_pairs(pairs: pest::iterators::Pairs<autocorrect::Rule>) {
        for pair in pairs {
            iter_all_pairs(pair.into_inner());
        }
    }

    c.bench_function("nested iter", |b| {
        let pairs = autocorrect::JsonParser::parse(autocorrect::Rule::item, &data).unwrap();

        b.iter(move || iter_all_pairs(pairs.clone()));
    });

    c.bench_function("flatten iter", |b| {
        let pairs = autocorrect::JsonParser::parse(autocorrect::Rule::item, &data).unwrap();

        b.iter(move || {
            for _pair in pairs.clone().flatten().into_iter() {
                // do nothing
            }
        });
    });
}

nested iter                             time:   [258.27 µs 260.05 µs 262.64 µs]
nested iter (fast-line-col)             time:   [14.943 µs 14.963 µs 14.993 µs]
flatten iter                            time:   [2.0367 µs 2.1104 µs 2.2144 µs]

In a large files (or have a lot of pairs, For instance: enabled WHITESPACE, each spaces will a pair item), iterate all pairs may take a long time than before.

So we need decide to how to do, remove it or find some solution to fix that.

[tomtau]

How about feature-guarding it? It'll be disabled by default -- if someone needs it, they can enable that feature?

[huacnlee]

Or change API like peekable or flatten, to make a special iterator?

pairs.locatable().next()

This will make a LocatablePairs, then prepare line, col only work in it.

[tomtau]

@huacnlee yes, that's perhaps simpler in the sense that the combination of possible feature flags doesn't increase more. Would you like to open a PR with this fix?

[huacnlee]

I have trying to find other way to solve it.

[huacnlee]

@tomtau To see my new PR, I find a new way to solve it.

#785