Move digest path calculation out of loop

On every iteration of generating a cache for a collection a “digest path” is calculated even though it’s exactly the same for every element.

This PR exposes a method `digest_path_from_virtual` that returns back a “digest_path”. This can in turn be passed back into `cache_fragment_name`. This not only does less work, but it also (you guessed it) uses  less memory.

before: Total allocated: 762539 bytes (7035 objects)
after: Total allocated: 743590 bytes (6621 objects) 


(762539 - 743590)/ 762539.0 # => 2.4% faster ⚡️⚡️

1 files changed, 17 insertions, 9 deletions

diff --git a/actionview/lib/action_view/helpers/cache_helper.rb b/actionview/lib/action_view/helpers/cache_helper.rb
index 15d187a9ec..b1a14250c3 100644
--- a/actionview/lib/action_view/helpers/cache_helper.rb
+++ b/actionview/lib/action_view/helpers/cache_helper.rb
@@ -208,27 +208,35 @@ module ActionView
       #
       # The digest will be generated using +virtual_path:+ if it is provided.
       #
-      def cache_fragment_name(name = {}, skip_digest: nil, virtual_path: nil)
+      def cache_fragment_name(name = {}, skip_digest: nil, virtual_path: nil, digest_path: nil)
         if skip_digest
           name
         else
-          fragment_name_with_digest(name, virtual_path)
+          fragment_name_with_digest(name, virtual_path, digest_path)
+        end
+      end
+
+      def digest_path_from_virtual(virtual_path) # :nodoc:
+        digest = Digestor.digest(name: virtual_path, finder: lookup_context, dependencies: view_cache_dependencies)
+
+        if digest.present?
+          "#{virtual_path}:#{digest}"
+        else
+          virtual_path
         end
       end
 
     private
 
-      def fragment_name_with_digest(name, virtual_path)
+      def fragment_name_with_digest(name, virtual_path, digest_path)
         virtual_path ||= @virtual_path
 
-        if virtual_path
+        if virtual_path || digest_path
           name = controller.url_for(name).split("://").last if name.is_a?(Hash)
 
-          if digest = Digestor.digest(name: virtual_path, finder: lookup_context, dependencies: view_cache_dependencies).presence
-            [ "#{virtual_path}:#{digest}", name ]
-          else
-            [ virtual_path, name ]
-          end
+          digest_path ||= digest_path_from_virtual(virtual_path)
+
+          [ digest_path, name ]
         else
           name
         end