Issue #16119 has been updated by Hanmac (Hans Mackowiak).
i see in your patch that you not only check for `Array` but for `to_ary` too, which is nice
@shevegen :
instead of `[input]` i would use `Array(input)` that doesn't create an extra array if input is already one
----------------------------------------
Feature #16119: Optimize Array#flatten and flatten! for already flattened arrays
https://bugs.ruby-lang.org/issues/16119#change-80933
* Author: dylants (Dylan Thacker-Smith)
* Status: Open
* Priority: Normal
* Assignee:
* Target version:
----------------------------------------
## Problem
When doing an object profile from stackprof, I noticed object allocations being made from `Array#flatten!` which was unlike other in-place methods like `Array#uniq!` and `Array#compact!`. In this case, I wanted to optimize for the array already being flattened and found that `ary.flatten! if ary.any?(Array)` was significantly faster. The code confirmed my suspicion that `ary.flatten!` was almost equivalent to `ary.replace(ary.flatten)` in implementation. The object allocations I noticed were from a temporary result array, a stack array to handle nesting and a hash table to prevent infinite recursion, all of which can be avoided in the simple case of an already flattened array.
## Solution
Iterate over the array to find the first nested array. If no nested array is found, then `flatten!` just returns `nil` without creating additional objects and `flatten` returns a shared copy of the original array. If a nested array is found, then it creates and initializes the temporary objects to resume with the existing algorithm for flattening the array.
## Benchmark
```ruby
require 'benchmark'
N = 100000
def report(x, name)
x.report(name) do
N.times do
yield
end
end
end
arrays = {
small_flat_ary: 5.times.to_a,
large_flat_ary: 100.times.to_a,
small_pairs_ary: [[1, 2]] * 5,
large_pairs_ary: [[1, 2]] * 100,
}
Benchmark.bmbm do |x|
arrays.each do |name, ary|
report(x, "#{name}.flatten!") do
ary.flatten!
end
report(x, "#{name}.flatten") do
ary.flatten
end
end
end
```
results on the latest master (`ruby 2.7.0dev (2019-08-22T14:10:55Z master fd20b32130) [x86_64-darwin18]`)
```
user system total real
small_flat_ary.flatten! 0.082001 0.000294 0.082295 ( 0.082685)
small_flat_ary.flatten 0.078655 0.000211 0.078866 ( 0.079176)
large_flat_ary.flatten! 0.552551 0.001643 0.554194 ( 0.556166)
large_flat_ary.flatten 0.551520 0.001327 0.552847 ( 0.554091)
small_pairs_ary.flatten! 0.100073 0.000302 0.100375 ( 0.100687)
small_pairs_ary.flatten 0.109440 0.000232 0.109672 ( 0.109850)
large_pairs_ary.flatten! 1.021200 0.001650 1.022850 ( 1.024227)
large_pairs_ary.flatten 1.049046 0.002938 1.051984 ( 1.055228)
```
results with the attached patch
```
user system total real
small_flat_ary.flatten! 0.034868 0.000150 0.035018 ( 0.035180)
small_flat_ary.flatten 0.040504 0.000148 0.040652 ( 0.040914)
large_flat_ary.flatten! 0.458273 0.000786 0.459059 ( 0.460005)
large_flat_ary.flatten 0.453846 0.000833 0.454679 ( 0.455324)
small_pairs_ary.flatten! 0.055211 0.000205 0.055416 ( 0.055673)
small_pairs_ary.flatten 0.060157 0.000226 0.060383 ( 0.060540)
large_pairs_ary.flatten! 0.901698 0.001650 0.903348 ( 0.905246)
large_pairs_ary.flatten 0.917180 0.001370 0.918550 ( 0.920008)
```
---Files--------------------------------
0001-Optimize-Array-flatten-and-flatten-for-already-fl.diff.txt (2.79 KB)
--
https://bugs.ruby-lang.org/
Unsubscribe: <mailto:ruby-core-request / ruby-lang.org?subject=unsubscribe>
<http://lists.ruby-lang.org/cgi-bin/mailman/options/ruby-core>