add limit_and_inverted method

app/graphql/connections/cursor_connection.rb

@@ -51,6 +51,17 @@ module Connections
     #
     # last: 2, order: desc => first 2
     # -> d1, d2
+    def limit_and_inverted(first: nil, last: nil, after: nil, before: nil, order: nil)
+      limit = [first, last, max_page_size].compact.min + 1
+      inverted = last.present? || before.present?
+
+      if order == :desc && after.nil? && before.nil?
+        inverted = !inverted
+      end
+
+      [limit, inverted]
+    end
+
     def load_nodes
       @nodes ||= begin
         ensure_valid_params

spec/graphql/connections/cursor_connection_spec.rb

@@ -0,0 +1,60 @@
+RSpec.describe Connections::CursorConnection do
+  describe '.limit_and_inverted' do
+    let(:max_page_size) { 100 }
+
+    subject do
+      cursor = Connections::CursorConnection.new(Dossier)
+      allow(cursor).to receive(:max_page_size).and_return(max_page_size)
+      limit, inverted = cursor.send(:limit_and_inverted, **args)
+      { limit:,  inverted: }
+    end
+
+    context 'without explicit args' do
+      let(:args) { { } }
+
+      it { is_expected.to eq(limit: max_page_size + 1, inverted: false) }
+    end
+
+    context 'when asked for 2 first elements' do
+      let(:args) { { first: 2 } }
+
+      it { is_expected.to eq(limit: 3, inverted: false) }
+    end
+
+    context 'when asked for 2 first elements, in order desc' do
+      let(:args) { { first: 2, order: :desc} }
+
+      it { is_expected.to eq(limit: 3, inverted: true) }
+    end
+
+    context 'when exceeding the max_page_size' do
+      let(:args) { { first: max_page_size + 1 } }
+
+      it { is_expected.to eq(limit: max_page_size + 1, inverted: false) }
+    end
+
+    context 'when asked for 2 last elements' do
+      let(:args) { { last: 2 } }
+
+      it { is_expected.to eq(limit: 3, inverted: true) }
+    end
+
+    context 'when asked for 2 last elements, in order desc' do
+      let(:args) { { last: 2, order: :desc} }
+
+      it { is_expected.to eq(limit: 3, inverted: false) }
+    end
+ 
+    context '' do
+      let(:args) { { after: :after, first: 2 } }
+
+      it { is_expected.to eq(limit: 3, inverted: false) }
+    end
+
+    context '' do
+      let(:args) { { before: :before, first: 2 } }
+
+      it { is_expected.to eq(limit: 3, inverted: true) }
+    end
+  end
+end