Commits on Jun 16, 2015

1. Major refactoring and performance improvements. …

   Overview of changes:
   
   * Instruction set has been redesigned to be smaller, mostly by
     collapsing empty-width matches into one instruction type.
     In addition to moving instruction-matching out of the matching
     engine, this makes matching engine code much simpler.
   * Rewrote input handling to use an inline representation of
     `Option<char>` and clearer position handling with the `Input` trait.
   * Added a new bounded backtracking matching engine that is invoked for
     small regexes/inputs. It's about twice as fast as the full NFA
     matching engine.
   * Implemented caching for both the NFA and backtracking engines.
     This avoids costly allocations on subsequent uses of the regex.
   * Overhauled prefix handling at both discovery and matching.
     Namely, sets of prefix literals can now be extracted from regexes.
     Depending on what the prefixes look like, an Aho-Corasick DFA
     is built from them.
     (This adds a dependency on the `aho-corasick` crate.)
   * When appropriate, use `memchr` to jump around in the input when
     there is a single common byte prefix.
     (This adds a dependency on the `memchr` crate.)
   * Bring the `regex!` macro up to date. Unfortunately, it still
     implements the full NFA matching engine and doesn't yet have
     access to the new prefix DFA handling. Thus, its performance
     has gotten *worse* than the dynamic implementation in most
     cases. The docs have been updated to reflect this change.
   
   Surprisingly, all of this required exactly one new application of
   `unsafe`, which is isolated in the `memchr` crate. (Aho-Corasick has no
   `unsafe` either!)
   
   There should be *no* breaking changes in this commit. The only public
   facing change is the addition of a method to the `Replacer` trait, but
   it comes with a default implementation so that existing implementors
   won't break. (Its purpose is to serve as a hint as to whether or not
   replacement strings need to be expanded. This is crucial to speeding
   up simple replacements.)
   
   Closes #21.

   

   BurntSushi committed Jun 16, 2015
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