Properly support matching ignoring order

[jyp]

I'd like to replace helm by avy in my configuration, but I cannot live with proper support for matching parts of the input in any order.

As indicated in #296, ivy--regex-ignore-order behaves incorrectly. It was concluded that doing matching by building a regexp matching all the permutations would be prohibitively slow. I'd like to offer the following counter arguments:

1. If the user is willing to pay this cost, why not allow it?
2. Notwithstanding the above point, the conclusion was reached on the assumption that the cost of matching a regexp is proportional to the length of the regexp. However, a proper regexp matcher should not behave like this. I'm guessing that emacs' native matcher is 'bad', but one could use an efficient matcher. (There exists a 'good' elisp implementation: http://www.emacswiki.org/emacs/TaggedNFA)
3. Even using a slow matcher, one can implement matching-ignoring-order by filtering the candidates with each of the sub-expressions, in series. In fact, ivy--filter has already code to support this behaviour today:

                         (dolist (re re-list)
                           (setq res
                                 (ignore-errors
                                   (funcall
                                    (if (cdr re)
                                        #'cl-remove-if-not
                                      #'cl-remove-if)
                                    (let ((re-str (car re)))
                                      (lambda (x) (string-match re-str x)))
                                    res))))

unfortunately, several other places in the code assume that the regexp is a string. That is, if ivy--regex-ignore-order was to return a list, errors would be thrown.

Would it be reasonable to support list of regexps consistently?

[Konubinix]

Hi, I like you analysis of the problem and suggested solution. May be you could propose a pull request with one solution.

[jyp]

Thanks for the feedback. I'll see what I can do. Yet, the first step will be to obtain a copyright assigment from my employer, so don't hold your breath :)

[abo-abo]

Thanks, please test.

[Konubinix]

I am testing it today. For now, it works like a charm. Thank you :-)