endsInKeyword check slow on large Javascript

[mdetrano]

I noticed something in the endsInKeyword function that I think is causing a large performance hit when processing large amounts of Javascript.

The check does a preg_match on the full output in order to see if it is ending with certain keywords. However, as that output grows this check takes longer and longer to run.

Would it not be better to limit the amount of output matched to something reasonable? For instance, I changed:

$testOutput = $this->output . $this->a;

to

$testOutput = substr($this->output . $this->a, -50);

and processing time for my experiment dropped from over a minute to a second or two. No change in the Javascript produced (though I am not entirely sure what condition this function is used for).

[tedivm]

This is a really good catch. It's worth noting that the keyword list is here-

JShrink/src/JShrink/Minifier.php

Line 113 in 35a83e0

protected static $keywords = ["delete", "do", "for", "in", "instanceof", "return", "typeof", "yield"];

The largest keyword is instanceof.

The regex for that turns into [^\w]instanceof[ ]?$. It's looking for a single character in front of the keyword, and then an optional space before the end of the output. That's a maximum of largest keyword + 2, or in this case 10, characters needed for the check.

If you want to make the PR, assuming tests pass, I'll happily merge it.

[mdetrano]

Added pull request with fix based on max keyword length

#141