Change the bench programs to atoms of size 2

Trying it out confirms #31, and the better introspectivity of
FilteredRE2 explains why: turns out the data set has a pretty
small number of atoms of length 2 with high discriminatory power.

Lowering length to 2 increases the number of atoms from 1630 to just
1865 (+235, +14.4%) which explains why memory use is unaffected or
even goes down (some regexes which match none of the samples are
likely not even tried anymore) but performances
increase *dramatically* (48s -> 27s for re2, 38s -> 24s for regex).
This makes sense as devices are also where #31 got extreme bang for
its buck.

It's a bit sad seeing re2 catch up so much with our hard work, but it
makes sense if we assume `regex` has a more optimised regex matching
at the cost of memory: with better discrimination we drastically
decrease the amount of regex matching, which benefits the package with
the slower regex matching.

Although to be fair the re2 bench could also be slower due to the use
of an `re2::Set` instead of an aho-corasick automaton. In fact that's
pretty likely.

However effect seems non-existent to slightly negative for UA and OS:

- At 3-atoms, UAs have 849 atoms for 362 regex, and both re2 and regex
  run in about 10s (9.70~9.90 real), interestingly the RSS and memory
  footprint of regex are a lot lower there (25MB to 32~33 footprint).
- At 2-atoms, UAs have 874 atoms for 362 regex, and both re2 and regex
  run a bit slower, around 10.50 for re2 and 10.40 for regex, memory
  use is the same.
- OS is basically inbetween, going from 3-atoms to 2-atoms the number
  of atoms increases a small hair from 353 to 359 (for 201 regexes),
  the re2 performances remain stable (8.15~8.40) while regex seems to
  decrease a hair (from 7.10~7.20 to 7.60~7.70).

Note that this is all over 100 runs parsing 75158 user agents. But
that hints that maybe different configurations for the ua and device
parsers would make sense...

Fixes #30

Author: masklinn

regex-filtered/examples/bench_regex.rs

@@ -27,7 +27,9 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
         .lines()
         .collect::<Result<Vec<String>, _>>()?;
 
-    let f = regex_filtered::Builder::new().push_all(&regexes)?.build()?;
+    let f = regex_filtered::Builder::new_atom_len(2)
+        .push_all(&regexes)?
+        .build()?;
     eprintln!(
         "{} regexes in {}s",
         regexes.len(),

regex-filtered/re2/bench.cpp

@@ -36,7 +36,7 @@ int main(const int argc, const char* argv[]) {
   std::ifstream regexes_f(argv[1]);
 
   re2::RE2::Options opt;
-  re2::FilteredRE2 f(3);
+  re2::FilteredRE2 f(2);
   int id;
 
   std::string line;