1 files changed, 95 insertions, 0 deletions
diff --git a/lib/gitlab/danger/base_linter.rb b/lib/gitlab/danger/base_linter.rb
new file mode 100644
index 00000000000..df2e9e745aa
--- /dev/null
+++ b/lib/gitlab/danger/base_linter.rb
@@ -0,0 +1,95 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module Danger
+    class BaseLinter
+      MIN_SUBJECT_WORDS_COUNT = 3
+      MAX_LINE_LENGTH = 72
+      WIP_PREFIX = 'WIP: '
+
+      attr_reader :commit, :problems
+
+      def self.problems_mapping
+        {
+          subject_too_short: "The %s must contain at least #{MIN_SUBJECT_WORDS_COUNT} words",
+          subject_too_long: "The %s may not be longer than #{MAX_LINE_LENGTH} characters",
+          subject_starts_with_lowercase: "The %s must start with a capital letter",
+          subject_ends_with_a_period: "The %s must not end with a period"
+        }
+      end
+
+      def self.subject_description
+        'commit subject'
+      end
+
+      def initialize(commit)
+        @commit = commit
+        @problems = {}
+      end
+
+      def failed?
+        problems.any?
+      end
+
+      def add_problem(problem_key, *args)
+        @problems[problem_key] = sprintf(self.class.problems_mapping[problem_key], *args)
+      end
+
+      def lint_subject
+        if subject_too_short?
+          add_problem(:subject_too_short, self.class.subject_description)
+        end
+
+        if subject_too_long?
+          add_problem(:subject_too_long, self.class.subject_description)
+        end
+
+        if subject_starts_with_lowercase?
+          add_problem(:subject_starts_with_lowercase, self.class.subject_description)
+        end
+
+        if subject_ends_with_a_period?
+          add_problem(:subject_ends_with_a_period, self.class.subject_description)
+        end
+
+        self
+      end
+
+      private
+
+      def subject
+        message_parts[0].delete_prefix(WIP_PREFIX)
+      end
+
+      def subject_too_short?
+        subject.split(' ').length < MIN_SUBJECT_WORDS_COUNT
+      end
+
+      def subject_too_long?
+        line_too_long?(subject)
+      end
+
+      def line_too_long?(line)
+        line.length > MAX_LINE_LENGTH
+      end
+
+      def subject_starts_with_lowercase?
+        return false if ('A'..'Z').cover?(subject[0])
+
+        first_char = subject.sub(/\A(\[.+\]|\w+:)\s/, '')[0]
+        first_char_downcased = first_char.downcase
+        return true unless ('a'..'z').cover?(first_char_downcased)
+
+        first_char.downcase == first_char
+      end
+
+      def subject_ends_with_a_period?
+        subject.end_with?('.')
+      end
+
+      def message_parts
+        @message_parts ||= commit.message.split("\n", 3)
+      end
+    end
+  end
+end