regex/literals: re-enable Tuned Boyer-Moore

We've added tests and carefully scrutinized it. Let's try this again.

Author: BurntSushi

src/literals.rs

@@ -8,6 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+use std::cmp;
 use std::mem;
 
 use aho_corasick::{Automaton, AcAutomaton, FullAcAutomaton};
@@ -682,13 +683,7 @@ impl BoyerMooreSearch {
     /// to beat the asm deep magic that is memchr. Unfortunately,
     /// I had trouble proving a useful turnover point. Hopefully,
     /// we can find one in the future.
-    fn should_use(_pattern: &[u8]) -> bool {
-        // TBM is disabled until the bm_backstop_boundary unit test can pass
-        // and we're more confident that the implementation is correct.
-        //
-        // See: https://github.com/rust-lang/regex/issues/446
-        false
-        /*
+    fn should_use(pattern: &[u8]) -> bool {
         // The minimum pattern length required to use TBM.
         const MIN_LEN: usize = 9;
         // The minimum frequency rank (lower is rarer) that every byte in the
@@ -716,7 +711,6 @@ impl BoyerMooreSearch {
         pattern.len() > MIN_LEN
             // all the bytes must be more common than the cutoff.
             && pattern.iter().all(|c| freq_rank(*c) >= cutoff)
-        */
     }
 
     /// Check to see if there is a match at the given position