[ruby-core:76745] [Ruby trunk Feature#12648] `Enumerable#sort_by` with descending option

Issue #12648 has been updated by Greg L.


Taking a step back, we are using arrays simply because that is a object that allows sorting via multiple criteria (if criteria a is equal, test with criteria b, etc).

It is also my understanding that sort_by creates arrays of [enum_item, sort_value], and we are also using an array for sort_value.

I'm not much for writing (or reading) c, but it would seem that a new method could use whatever structures were most efficient, regardless of the fact that arrays are used for the parameter and the block return.

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Feature #12648: `Enumerable#sort_by` with descending option
https://bugs.ruby-lang.org/issues/12648#change-59966

* Author: Tsuyoshi Sawada
* Status: Open
* Priority: Normal
* Assignee: 
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I would like to pass an optional argument to `Enumerable#sort_by` or `Enumerable#sort_by!` to allow descending sort. When the sort key is singular, this could be done by passing a single optinal boolean variable that represents ascending when `false` (default) and descending when `true`:

```ruby
[3, 1, 2].sort_by(&:itself)        # => [1, 2, 3]
[3, 1, 2].sort_by(false, &:itself) # => [1, 2, 3]
[3, 1, 2].sort_by(true, &:itself)  # => [3, 2, 1]
```

When there are multiple sort keys, corresponding numbers of arguments should be passed:

```ruby
[3, 1, 2, 0].sort_by{|e| [e % 2, e]}               # => [0, 2, 1, 3]
[3, 1, 2, 0].sort_by(false, false){|e| [e % 2, e]} # => [0, 2, 1, 3]
[3, 1, 2, 0].sort_by(false, true){|e| [e % 2, e]}  # => [2, 0, 3, 1]
[3, 1, 2, 0].sort_by(true, false){|e| [e % 2, e]}  # => [1, 3, 0, 2]
[3, 1, 2, 0].sort_by(true, true){|e| [e % 2, e]}   # => [3, 1, 2, 0]
```




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