exchange pagination class to inside json_api scope

lib/active_model/serializer/adapter/json_api/pagination_links.rb

@@ -0,0 +1,52 @@
+module ActiveModel
+  class Serializer
+    class Adapter
+      class JsonApi < Adapter
+        class PaginationLinks
+          FIRST_PAGE = 1
+
+          attr_reader :collection
+
+          def initialize(collection)
+            raise_unless_any_gem_installed
+            @collection = collection
+          end
+
+          def page_links
+            build_links
+          end
+
+          private
+
+          def build_links
+            pages_from.each_with_object({}) do |(key, value), hash|
+              hash[key] = "?page=#{value}&per_page=#{collection.size}"
+            end
+          end
+
+          def pages_from
+            return {} if collection.total_pages == FIRST_PAGE
+
+            {}.tap do |pages|
+              unless collection.current_page == FIRST_PAGE
+                pages[:first] = FIRST_PAGE
+                pages[:prev]  = collection.current_page - FIRST_PAGE
+              end
+
+              unless collection.current_page == collection.total_pages
+                pages[:next] = collection.current_page + FIRST_PAGE
+                pages[:last] = collection.total_pages
+              end
+            end
+          end
+
+          def raise_unless_any_gem_installed
+            return if defined?(WillPaginate) || defined?(Kaminari)
+            raise "AMS relies on either Kaminari or WillPaginate." +
+              "Please install either dependency by adding one of those to your Gemfile"
+          end
+        end
+      end
+    end
+  end
+end