Re: [PATCH v3 2/4] grep/pcre2: simplify boolean spaghetti

From: Johannes Sixt <j6t@kdbg.org>
Date: Jan 24, 2021, 10:45 UTC
Message-ID: <ac4f3b95-49f7-7a90-b7f7-9efbb57a7392@kdbg.org>
In-Reply-To: <xmqqim7m292j.fsf@gitster.c.googlers.com>

Am 24.01.21 um 06:33 schrieb Junio C Hamano:

Show 17 quoted lines

> Ævar Arnfjörð Bjarmason  <avarab@gmail.com> writes:
> 
>>     NOT(A && B) is Equivalent to (NOT(A) OR NOT(B))
> 
> At this level, however, the left one looks much simpler than the
> right one ;-)
> 
> 
>>  	if (!opt->ignore_locale && is_utf8_locale() && has_non_ascii(p->pattern) &&
>> -	    !(!opt->ignore_case && (p->fixed || p->is_fixed)))
>> +	    (opt->ignore_case || !(p->fixed || p->is_fixed)))
>>  		options |= PCRE2_UTF;
> 
> In the context of this expression, well, I guess the rewritten one
> is probably simpler but can we explain the whole condition in fewer
> than three lines?  With or without the rewrite, it still looks too
> complicated to me.

Make the condition

 	if (!opt->ignore_locale &&
	    is_utf8_locale() &&
	    has_non_ascii(p->pattern) &&
	    (opt->ignore_case ||
		(!p->fixed &&
		 !p->is_fixed)))
	{
  		options |= PCRE2_UTF;
	}

With the knowledge of the equivalence

    (A => B)  <=>  (NOT(A) OR B)

(A => B means "if A then B"), the condition makes a lot of sense when read aloud:

    if
       NOT ignore locale
       AND
       is UTF8
       AND
       has non-ASCII
       AND
         if
            NOT ignore case
         then if also
            NOT fixed
            AND
            NOT is fixed
    then
        ...

The codition amounts to extending a series of conjunctions with more conjuctions IF a condition is satisfied. That's quite sensible.

You have to swap the polarity of the first condition of || in your head, though, to achieve that meaning. That works with every OR condition, BTW.

-- Hannes