This commit introduces a new `Journey::Ast` class that wraps the root
node of an ast. The purpose of this class is to reduce the number of
walks through the ast by taking a single pass through each node and
caching values for later use.
To avoid retaining additional memory, we clear out these ast objects
after eager loading.
Benefits
---
* Performance improvements (see benchmarks below)
* Keeps various ast manipulations together in a single class, rather
than scattered throughout
* Adding some names to things will hopefully make this code a little
easier to follow for future readers
Benchmarks
---
We benchmarked loading a real routes file with > 3500 routes.
master was at a9336a67b0 when we ran these. Note that these benchmarks
also include a few small changes that we didn't include in this commit,
but that we will follow up with after this gets merged - these
additional changes do not change the benchmarks significantly.
Time:
```
master - 0.798 ± 0.024 (500 runs)
this branch - 0.695 ± 0.029 (500 runs)
```
Allocations:
```
master - 980149 total allocated objects
this branch - 931357 total allocated objects
```
Stackprof:
Seeing `ActionDispatch::Journey::Visitors::Each#visit` more frequently
on the stack is what led us down this path in the first place. These
changes seem to have done the trick.
```
master:
TOTAL (pct) SAMPLES (pct) FRAME
52 (0.5%) 52 (0.5%) ActionDispatch::Journey::Nodes::Node#symbol?
58 (0.5%) 45 (0.4%) ActionDispatch::Journey::Scanner#scan
45 (0.4%) 45 (0.4%) ActionDispatch::Journey::Nodes::Cat#type
43 (0.4%) 43 (0.4%) ActionDispatch::Journey::Visitors::FunctionalVisitor#terminal
303 (2.7%) 43 (0.4%) ActionDispatch::Journey::Visitors::Each#visit
69 (0.6%) 40 (0.4%) ActionDispatch::Routing::Mapper::Scope#each
this commit:
TOTAL (pct) SAMPLES (pct) FRAME
82 (0.6%) 42 (0.3%) ActionDispatch::Journey::Scanner#next_token
31 (0.2%) 31 (0.2%) ActionDispatch::Journey::Nodes::Node#symbol?
30 (0.2%) 30 (0.2%) ActionDispatch::Journey::Nodes::Node#initialize
```
See also the benchmark script in https://github.com/rails/rails/pull/39935#issuecomment-887791294
Co-authored-by: Eric Milford <ericmilford@gmail.com>
`Pattern#from_sting` was introduced in bb207ea7 to support creating
patterns from strings in tests (removing a conditional in the initialize
method that had been there only for the sake of those tests).
`Pattern#build` was introduced in 947ebe9a, and was similarly used only
in tests.
My understanding is that [`Mapping#path`][path] is the only place where
we initialize a `Path::Pattern` in library code.
Since these two methods appear to be used only in tests, this
commit moves them out of the class and into a test helper.
My reasoning for doing this is that I am doing some performance work
that may involve a modification to how `Path::Pattern`s get initialized,
and I will have more confidence in that work if I know for sure that
these methods are test helpers that can be modified freely.
[path]: https://github.com/rails/rails/blob/master/actionpack/lib/action_dispatch/routing/mapper.rb#L192-L194
Daniel Colson