update PatchCop to support spanning errors

We want to keep an error if it _includes_ any lines in a given patch. We also
want only the first such line, since it likely includes more than one (this
singularism was implicit before, because each line was only passed through once)

Also factor out some private methods to simplify the original flat_map logic.

lib/pronto/rubocop/patch_cop.rb

@@ -13,12 +13,9 @@ def initialize(patch, runner)
       def messages
         return [] unless valid?
 
-        offenses.flat_map do |offense|
-          patch
-            .added_lines
-            .select { |line| line.new_lineno == offense.line }
-            .map { |line| OffenseLine.new(self, offense, line).message }
-        end
+        offenses
+          .map { |offense| first_relevant_message(patch, offense) }
+          .compact
       end
 
       def processed_source
@@ -67,6 +64,11 @@ def offenses
           .reject(&:disabled?)
       end
 
+      def offense_includes?(offense, line_number)
+        offense_range = (offense.location.first_line..offense.location.last_line)
+        offense_range.include?(line_number)
+      end
+
       def team
         @team ||=
           if ::RuboCop::Cop::Team.respond_to?(:mobilize)
@@ -76,6 +78,19 @@ def team
             ::RuboCop::Cop::Team.new(registry, rubocop_config)
           end
       end
+
+      def first_relevant_line(patch, offense)
+        patch.added_lines.detect do |line|
+          offense_includes?(offense, line.new_lineno)
+        end
+      end
+
+      def first_relevant_message(patch, offense)
+        offending_line = first_relevant_line(patch, offense)
+        return nil unless offending_line
+
+        OffenseLine.new(self, offense, offending_line).message
+      end
     end
   end
 end

spec/pronto/rubocop/patch_cop_spec.rb

@@ -12,7 +12,8 @@
   let(:ast) { double :ast, each_node: nil }
   let(:processed_source) { double :processed_source, ast: ast }
   let(:team) { double :team, inspect_file: [offense] }
-  let(:offense) { double :offense, disabled?: false, line: 42 }
+  let(:offense_location) { double :location, first_line: 42, last_line: 43 }
+  let(:offense) { double :offense, disabled?: false, location: offense_location }
   let(:offense_line) { double :offense_line, message: 'Err' }
 
   before do
@@ -31,5 +32,13 @@
     it do
       expect(patch_cop.messages).to eq(['Err'])
     end
+
+    context 'when there is an error including the patch, but not starting inside it' do
+      let(:offense_location) { double :location, first_line: 40, last_line: 43 }
+
+      it do
+        expect(patch_cop.messages).to eq(['Err'])
+      end
+    end
   end
 end