1 files changed, 79 insertions, 0 deletions
diff --git a/lib/gitlab/gfm/reference_rewriter.rb b/lib/gitlab/gfm/reference_rewriter.rb
new file mode 100644
index 00000000000..a1c6ee7bd69
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+++ b/lib/gitlab/gfm/reference_rewriter.rb
@@ -0,0 +1,79 @@
+module Gitlab
+  module Gfm
+    ##
+    # Class that unfolds local references in text.
+    #
+    # The initializer takes text in Markdown and project this text is valid
+    # in context of.
+    #
+    # `unfold` method tries to find all local references and unfold each of
+    # those local references to cross reference format, assuming that the
+    # argument passed to this method is a project that references will be
+    # viewed from (see `Referable#to_reference method).
+    #
+    # Examples:
+    #
+    # 'Hello, this issue is related to #123 and
+    #  other issues labeled with ~"label"', will be converted to:
+    #
+    # 'Hello, this issue is related to gitlab-org/gitlab-ce#123 and
+    #  other issue labeled with gitlab-org/gitlab-ce~"label"'.
+    #
+    # It does respect markdown lexical rules, so text in code block will not be
+    # replaced, see another example:
+    #
+    # 'Merge request for issue #1234, see also link:
+    #  http://gitlab.com/some/link/#1234, and code `puts #1234`' =>
+    #
+    # 'Merge request for issue gitlab-org/gitlab-ce#1234, se also link:
+    #  http://gitlab.com/some/link/#1234, and code `puts #1234`'
+    #
+    class ReferenceRewriter
+      def initialize(text, source_project, current_user)
+        @text = text
+        @source_project = source_project
+        @current_user = current_user
+        @original_html = markdown(text)
+      end
+
+      def rewrite(target_project)
+        pattern = Gitlab::ReferenceExtractor.references_pattern
+
+        @text.gsub(pattern) do |reference|
+          unfold_reference(reference, Regexp.last_match, target_project)
+        end
+      end
+
+      private
+
+      def unfold_reference(reference, match, target_project)
+        before = @text[0...match.begin(0)]
+        after = @text[match.end(0)..-1]
+
+        referable = find_referable(reference)
+        return reference unless referable
+
+        cross_reference = referable.to_reference(target_project)
+        return reference if reference == cross_reference
+
+        new_text = before + cross_reference + after
+        substitution_valid?(new_text) ? cross_reference : reference
+      end
+
+      def find_referable(reference)
+        extractor = Gitlab::ReferenceExtractor.new(@source_project,
+                                                   @current_user)
+        extractor.analyze(reference)
+        extractor.all.first
+      end
+
+      def substitution_valid?(substituted)
+        @original_html == markdown(substituted)
+      end
+
+      def markdown(text)
+        Banzai.render(text, project: @source_project, no_original_data: true)
+      end
+    end
+  end
+end