Add ordered_match() function

[leni536]

I added an ordered_match() function that can be used to compare targets to all possible matches for a given subject. This relationship is encoded in the ctre::partial_ordering type which can be equal, less, greater or unordered. This can be useful for efficient searching in ordered collections of strings.

Usage:

constexpr auto re = "abc[1-3]"_ctre;
constexpr auto ord = ctre::partial_ordering(re.ordered_match("aaa"));
static_assert(ord == ctre::partial_ordering::less);

I only made a handful of manual tests and I didn't find any target, subject pair where it returns an unexpected result. It sure needs lots of testing though.

[hanickadot]

What should be ordering of subject "B" against regex "A|C"?

[leni536]

unordered

This pull request relied on my original refactoring of atoms_characters.hpp. I will resolve this when you merge the other one.

[hanickadot]

So for ordered_match I need to check all branches? If I understand it correctly...
if it match the subject => equal
if not and all fails are after (encoded as numbers) a not matched char => less than
if not and all fails are before (encoded as numbers) a not matched char => greater than
if it`s neither of above => unordered

??

[leni536]

I rebased the branch on the other refactoring method, now it merges without conflict.

This is how I understand how it works with an example: Suppose we have the target "[a-c]+". By default all three of less, greater or equal are potential return values. By evaluating the stack we eliminate possible return values when it's appropriate. For example if the first character of the subject is 'b' then we can eliminate both greater and less at that point since there are potential matches starting with both a and c. Then when the next character is for example 'x' then the result is unordered since we eliminated both less and greater, and now equal.

I think my approach in some rare cases can return unordered when it could potentially return less or greater. I don't think it's a correctness issue, unordered only means that it's not a match. However when we return less or greater then it's guaranteed that the subject compare less or greater to all possible matches.

Edit:
Also the only case where my approach increases complexity compared to match() is handling possessive repeats.