Add Enumerable#index_with.

In the app I'm working on I've wished that index_by had a buddy that would
assign the hash value instead of the key multiple times.

Enter index_with. Useful when building a hash from a static list of
symbols. Before you'd do:

```ruby
POST_ATTRIBUTES.map { |attr_name| [ attr_name, public_send(attr_name) ] }.to_h
```

But now that's a little clearer and faster with:

````ruby
POST_ATTRIBUTES.index_with { |attr_name| public_send(attr_name) }
```

It's also useful when you have an enumerable that should be converted to a hash,
but you don't want to muddle the code up with the overhead that it takes to create
that hash. So before, that's:

```ruby
WEEKDAYS.each_with_object(Hash.new) do |day, intervals|
  intervals[day] = [ Interval.all_day ]
end
```

And now it's just:

```ruby
WEEKDAYS.index_with([ Interval.all_day ])
```

It's also nice to quickly get a hash with either nil, [], or {} as the value.

1 files changed, 25 insertions, 1 deletions

diff --git a/activesupport/lib/active_support/core_ext/enumerable.rb b/activesupport/lib/active_support/core_ext/enumerable.rb
index edde4f46b9..7713c52cb1 100644
--- a/activesupport/lib/active_support/core_ext/enumerable.rb
+++ b/activesupport/lib/active_support/core_ext/enumerable.rb
@@ -1,6 +1,9 @@
 # frozen_string_literal: true
 
 module Enumerable
+  INDEX_WITH_DEFAULT = Object.new
+  private_constant :INDEX_WITH_DEFAULT
+
   # Enumerable#sum was added in Ruby 2.4, but it only works with Numeric elements
   # when we omit an identity.
 
@@ -37,10 +40,11 @@ module Enumerable
     end
   end
 
-  # Convert an enumerable to a hash.
+  # Convert an enumerable to a hash keying it by the block return value.
   #
   #   people.index_by(&:login)
   #   # => { "nextangle" => <Person ...>, "chade-" => <Person ...>, ...}
+  #
   #   people.index_by { |person| "#{person.first_name} #{person.last_name}" }
   #   # => { "Chade- Fowlersburg-e" => <Person ...>, "David Heinemeier Hansson" => <Person ...>, ...}
   def index_by
@@ -53,6 +57,26 @@ module Enumerable
     end
   end
 
+  # Convert an enumerable to a hash keying it with the enumerable items and with the values returned in the block.
+  #
+  #   post = Post.new(title: "hey there", body: "what's up?")
+  #
+  #   %i( title body ).index_with { |attr_name| post.public_send(attr_name) }
+  #   # => { title: "hey there", body: "what's up?" }
+  def index_with(default = INDEX_WITH_DEFAULT)
+    if block_given?
+      result = {}
+      each { |elem| result[elem] = yield(elem) }
+      result
+    elsif default != INDEX_WITH_DEFAULT
+      result = {}
+      each { |elem| result[elem] = default }
+      result
+    else
+      to_enum(:index_with) { size if respond_to?(:size) }
+    end
+  end
+
   # Returns +true+ if the enumerable has more than 1 element. Functionally
   # equivalent to <tt>enum.to_a.size > 1</tt>. Can be called with a block too,
   # much like any?, so <tt>people.many? { |p| p.age > 26 }</tt> returns +true+