Instead of using two vectors, we can hold both current and next
threads in a single buffer, with stacks growing on each end.
Benchmarking shows this to be slightly faster, and should use less memory.
Remove ExecState to store threads inside the ThreadedRegexVM so that
memory buffers can be reused between executions. Extract an ExecConfig
struct with all the data thats execution specific to avoid storing
it needlessly inside the ThreadedRegexVM.
To allow more general look arounds out of the actual search range,
pass a second range (the actual subject). This allows us to remove
various flags such as PrevAvailable or NotBeginOfSubject, which are
now easy to check from the subject range.
Fixes#1902
No need to have two separate regexes to handle forward and backward
matching, just passing RegexCompileFlags::Backward will add support
for backward matching to the regex. For backward only regex, pass
RegexCompileFlags::NoForward as well to disable generation of
forward matching code.
The previous method, which was a bit faster in the general use case,
can hit some cases where we get quadratic behaviour and very slow
matching.
By using an instruction, we can guarantee our complexity of O(N*M)
as we will never have more than N threads (N being the instruction
count) and we run the threads once per codepoint in the subject
string.
That slows down the general case slightly, but ensure we dont have
pathological cases.
This new version is much faster than the previous instruction based
search because it does not use a plain `.*` searcher, but a specific,
smarter instruction specialized for finding the next start if we are
in the correct conditions.
Identify each step with a counter, and check if the instruction
was already processed this step. This makes the matching faster,
by removing the need to maintain a vector of instructions executed
this step.
On big regex, reseting all those flags on all instructions for each
character can become the dominant operation. Track that actual
instructions index processed (the scheduled are already tracked in
the next_threads vector), and only reset these.
Regex can be compiled for backward matching instead of forward matching
and the ThreadedRegexVM is able to iterate in reverse on the subject
string to find the last match instead of the first.
