This change is broken up in to two parts:
1. Using a Hash to track if a node is already in a NodeSet
2. Only calling take_ownership when an owner is set
== Using a Hash
Previously various methods such as NodeSet#push and NodeSet#unshift
would call Array#include? (on the internal "nodes" Array) to see if a
node is already present in the set. This is quite problematic
performance wise, especially for large NodeSets. In fact, for the
attached benchmark the vast majority of the time was spent in
Array#include? calls.
Because a NodeSet demands ordering of nodes and must be able to access
them by index (something Set can't do without relying on Enumerable), a
Hash is used to separately keep track of what nodes are in a NodeSet.
This means that checking the presence of a node is simply a matter of
checking a Hash key's presence.
== Calling take_ownership
The if-check for the "owner" variable has been moved out of the
"take_ownership" method and into the methods that call "take_ownership".
This ensures the method isn't called in the first place if no owner is
present, at the cost of slightly more code repetition. The same applies
to the "remove_ownership" method.
== Conclusion
The combined result is a speedup of about 50x when running the attached
concurrent_time_bench.rb benchmark.
This benchmark is simple enough that the overhead of evaluation is not
far greater than parsing. This makes it suitable for benchmarking the
performance increase of caching XPath ASTs.
Yorick Peterse