Query method is immutable reference

[tmpfs]

Closes #16.

[tmpfs]

You were right @marcus-pousette, we got another small perf improvement 👍

[tmpfs]

@marcus-pousette, I had to revert the changes I made to the Filter type to accommodate owned values which is required for when filters need to perform transformations (such as to_lowercase()). Using a std::borrow::Cow we can allow for no-op filters that just do Cow::from(s) and also transformations that incur allocations such as Cow::Owned(s.to_lowercase()). Some of those performance gains in add_document() have been lost as we have to use String instead of &str but it does allow for no-op filters to have minimal overhead.

[marcus-pousette]

Left some comments, that might be worth considering.

but the main problem as that I think the count_documents fn has to respect removed set

Good find and solution regarding the Filter I thought about this when reviewing your last PR but I thought there would be a explicit lifetime trick you could do in order to circumvent this, but I realize now it is not possible yet to associate lifetimes with borrows in a fn.

[marcus-pousette]

Suggested change

	let mut term_counts: HashMap<String, Vec<usize>> = HashMap::new();
	let mut all_terms: Vec<String> = Vec::new();
	let mut term_counts: HashMap<Cow<str>, Vec<usize>> = HashMap::new();
	let mut all_terms: Vec<Cow<str>> = Vec::new();

[marcus-pousette]

I think this looks good now. Have you done all the changes you wanted to do? @tmpfs

[tmpfs]

Sure @marcus-pousette, I think this is good enough for this PR, ping me when you push your Cow fix, interested to see how that shapes up!