This appears to be another transcription bug from copying this code from
the prefix literal detection from inside the regex crate. Namely, when
it comes to inner literals, we only want to treat counted repetition as
two separate cases: the case when the minimum match is 0 and the case
when the minimum match is more than 0. In the former case, we treat
`e{0,n}` as `e*` and in the latter we treat `e{m,n}` where `m >= 1` as
just `e`.
We could definitely do better here. e.g., This means regexes like
`(foo){10}` will only have `foo` extracted as a literal, where searching
for the full literal would likely be faster.
The actual bug here was that we were not implementing this logic
correctly. Namely, we weren't always "cutting" the literals in the
second case to prevent them from being expanded.
Fixes#1319, Closes#1367
When the -w/--word-regexp was used, ripgrep would in many cases fail to
apply literal optimizations. This occurs specifically when the regex
given by the user is an alternation of literals with no common prefixes
or suffixes, e.g.,
rg -w 'foo|bar|baz|quux'
In this case, the inner literal detector fails. Normally, this would
result in literal prefixes being detected by the regex engine. But
because of the -w/--word-regexp flag, the actual regex that we run ends
up looking like this:
(^|\W)(foo|bar|baz|quux)($|\W)
which of course defeats any prefix or suffix literal optimizations in
the regex crate's somewhat naive extractor. (A better extractor could
still do literal optimizations in the above case.)
So this commit fixes this by falling back to prefix or suffix literals
when they're available instead of prematurely giving up and assuming the
regex engine will do the rest.
This fixes an interesting performance bug where the inner literal
extractor would sometimes choose a sub-optimal literal. For example,
consider the regex:
\x20+Sherlock Holmes\x20+
(The `\x20` is the ASCII code for a space character, which we use here
to just make it clearer. It otherwise does not matter.)
Previously, this would see the initial \x20 and then stop collecting
literals after the `+` repetition operator. This was because the inner
literal detector was adapter from the prefix literal detector, which had
to stop here. Namely, while \x20S would be a valid prefix (for example),
\x20\x20S would also be a valid prefix. As would \x20\x20\x20S and so
on. So the prefix detector would have to stop at the repetition
operator. Otherwise, only searching for \x20S could potentially scan
farther then the starting position of the next match.
However, for inner literals, this calculus no longer makes sense. We can
freely search for, e.g., \x20S without missing matches that start with
\x20\x20S precisely because we know this is an inner literal which may
not correspond to the start of a match.
With this fix, the literal that is now detected is
\x20Sherlock Holmes\x20
Which is much better. We achieve this by no longer "cutting" literals
after seeing a `+` repetition operator. Instead, we permit literals to
continue to be extended.
The reason why this is important is because using \x20 as the literal to
search for is generally bad juju since it is so common. In fact, we
should probably add more logic here to either avoid such things or give
up entirely on the inner literal optimization if it detected a literal
that we think is very common. But we punt on such things here.
When looking for an inner literal to speed up searches, if only a prefix
is found, then we generally give up doing inner literal optimizations since
the regex engine will generally handle it for us. Unfortunately, this
decision was being made *before* we wrap the regex in (^|\W)...($|\W) when
using the -w/--word-regexp flag, which would then defeat the literal
optimizations inside the regex engine.
We fix this with a bit of a hack that says, "if we're doing a word regexp,
then give me back any literal you find, even if it's a prefix."
It seems the inner literal detector fails spectacularly in cases of
concatenations that involve groups. The issue here is that if the prefix
of a group inside a concatenation can match the empty string, then any
literals generated to that point in the concatenation need to be cut
such that they are never extended. The detector isn't really built to
handle this case, so we just act conservative cut literals whenever we
see a sub-group. This may make some regexes slower, but the inner
literal detector already misses plenty of cases.
Literal detection (including in the regex engine) is a key component
that needs to be completely rethought at some point.
Fixes#1064
libripgrep is not any one library, but rather, a collection of libraries
that roughly separate the following key distinct phases in a grep
implementation:
1. Pattern matching (e.g., by a regex engine).
2. Searching a file using a pattern matcher.
3. Printing results.
Ultimately, both (1) and (3) are defined by de-coupled interfaces, of
which there may be multiple implementations. Namely, (1) is satisfied by
the `Matcher` trait in the `grep-matcher` crate and (3) is satisfied by
the `Sink` trait in the `grep2` crate. The searcher (2) ties everything
together and finds results using a matcher and reports those results
using a `Sink` implementation.
Closes#162
