This makes the case of searching for a dictionary of a very large number
of literals much much faster. (~10x or so.) In particular, we achieve this
by short-circuiting the construction of a full regex when we know we have
a simple alternation of literals. Building the regex for a large dictionary
(>100,000 literals) turns out to be quite slow, even if it internally will
dispatch to Aho-Corasick.
Even that isn't quite enough. It turns out that even *parsing* such a regex
is quite slow. So when the -F/--fixed-strings flag is set, we short
circuit regex parsing completely and jump straight to Aho-Corasick.
We aren't quite as fast as GNU grep here, but it's much closer (less than
2x slower).
In general, this is somewhat of a hack. In particular, it seems plausible
that this optimization could be implemented entirely in the regex engine.
Unfortunately, the regex engine's internals are just not amenable to this
at all, so it would require a larger refactoring effort. For now, it's
good enough to add this fairly simple hack at a higher level.
Unfortunately, if you don't pass -F/--fixed-strings, then ripgrep will
be slower, because of the aforementioned missing optimization. Moreover,
passing flags like `-i` or `-S` will cause ripgrep to abandon this
optimization and fall back to something potentially much slower. Again,
this fix really needs to happen inside the regex engine, although we
might be able to special case -i when the input literals are pure ASCII
via Aho-Corasick's `ascii_case_insensitive`.
Fixes#497, Fixes#838
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
