A common Ruby idiom for variadic methods is to flatten the argument
list so that it can be called with either variadic arguments or an
Array.
e.g. from the sqlite3 gem:
```ruby
def bind_params(*bind_vars)
bind_vars.flatten.each do |var|
# ...
```
However, as soon as `flatten` encounter another array, it has to
protect against recursion, which requires allocating an expensive
identity Hash.
So this pattern has a relatively high cost when called with
a single array argument (e.g. `bind_params [1, 2, 3]`).
We can specialize for that common case without noticeably
impacting other usages of `Array#flatten`:
| |compare-ruby|built-ruby|
|:-----------------------------|-----------:|---------:|
|small_flat_ary.flatten | 7.547M| 7.974M|
| | -| 1.06x|
|small_flat_ary.flatten! | 6.031M| 6.105M|
| | -| 1.01x|
|large_flat_ary.flatten | 465.896k| 485.861k|
| | -| 1.04x|
|large_flat_ary.flatten! | 455.872k| 478.103k|
| | -| 1.05x|
|small_pairs_ary.flatten | 1.327M| 1.407M|
| | -| 1.06x|
|small_pairs_ary.flatten! | 1.165M| 1.153M|
| | 1.01x| -|
|large_pairs_ary.flatten | 96.612k| 96.976k|
| | -| 1.00x|
|large_pairs_ary.flatten! | 94.108k| 97.289k|
| | -| 1.03x|
|mostly_flat_ary.flatten | 399.648k| 417.327k|
| | -| 1.04x|
|mostly_flat_ary.flatten! | 378.315k| 395.946k|
| | -| 1.05x|
|small_nested_ary.flatten | 2.513M| 7.788M|
| | -| 3.10x|
|small_nested_ary.flatten! | 2.024M| 5.914M|
| | -| 2.92x|
|large_nested_ary.flatten | 341.635k| 485.319k|
| | -| 1.42x|
|large_nested_ary.flatten! | 329.837k| 475.014k|
| | -| 1.44x|
|small_nested_ary.flatten(1) | 9.497M| 61.350M|
| | -| 6.46x|
|small_nested_ary.flatten!(1) | 5.136M| 17.668M|
| | -| 3.44x|
|large_nested_ary.flatten(1) | 1.575M| 60.241M|
| | -| 38.26x|
|large_nested_ary.flatten!(1) | 1.369M| 16.835M|
| | -| 12.30x|
Jean Boussier