Make Checksummer faster and more correct

The new implementation is faster because of two reasons:

- It more aggressively avoids calculating checksums of already-calculated items
- It effectfully updates a hash rather than using Immutable::Set

It is also more correct because it replaces use of <recur> with a numerical reference to a previously-seen object.

Each newly checksummed object will have a number suffix, e.g.
`Array#5<…>`. Each previously seen object will have an @-reference, e.g.
`Array#9<@4,@5,>`.

This will invalidate existing stored checksums in Nanoc sites, but it is unavoidable and worth the tradeoff.

nanoc-core/lib/nanoc/core/checksummer.rb

@@ -45,6 +45,13 @@ def calc(obj, digest_class = CompactDigest)
           digest.to_s
         end
 
+        # TODO: remove (older, slower implementation)
+        def calc_v1(obj, digest_class = CompactDigest)
+          digest = digest_class.new
+          update_v1(obj, digest)
+          digest.to_s
+        end
+
         def calc_for_content_of(obj)
           obj.content_checksum_data || obj.checksum_data || Nanoc::Core::Checksummer.calc(obj.content)
         end
@@ -61,14 +68,32 @@ def define_behavior(klass, behavior)
 
         private
 
-        def update(obj, digest, visited = Immutable::Set.new)
+        def update(obj, digest, visited = {})
+          num = visited[obj]
+          if num
+            # If there already is an entry for this object, refer to it by its number.
+            digest.update("@#{num}")
+          else
+            # This object isn’t known yet. Assign it a new number.
+            num = visited.length
+            visited[obj] = num
+
+            digest.update(obj.class.to_s)
+            digest.update("##{num}<")
+            behavior_for(obj).update(obj, digest) { |o| update(o, digest, visited) }
+            digest.update('>')
+          end
+        end
+
+        # TODO: remove (older, slower implementation)
+        def update_v1(obj, digest, visited = Immutable::Set.new)
           digest.update(obj.class.to_s)
 
           if visited.include?(obj)
             digest.update('<recur>')
           else
             digest.update('<')
-            behavior_for(obj).update(obj, digest) { |o| update(o, digest, visited.add(obj)) }
+            behavior_for(obj).update(obj, digest) { |o| update_v1(o, digest, visited.add(obj)) }
             digest.update('>')
           end
         end

nanoc-core/spec/nanoc/core/action_sequence_spec.rb

@@ -147,9 +147,9 @@
     example do
       expect(subject).to eql(
         [
-          [:filter, :erb, 'PeWUm2PtXYtqeHJdTqnY7kkwAow='],
+          [:filter, :erb, 'B1gmzMdP+iEDgTz7SylLoB6yLNw='],
           [:snapshot, [:bar], true, ['/foo.md']],
-          [:layout, '/default.erb', '97LAe1pYTLKczxBsu+x4MmvqdkU='],
+          [:layout, '/default.erb', 'QQW0vu/3fP4Ihc5xhQKuPer3xUc='],
         ],
       )
     end