perform OSSCheck in Dangerfile

.gitignore

@@ -48,6 +48,7 @@ SwiftLint.pkg
 SwiftLintFramework.framework.zip
 benchmark_*
 portable_swiftlint.zip
+osscheck/
 
 # SPM
 .build

Dangerfile

@@ -1,5 +1,5 @@
 # Warn when there is a big PR
-warn("Big PR") if git.lines_of_code > 500
+warn('Big PR') if git.lines_of_code > 500
 
 # Sometimes its a README fix, or something like that - which isn't relevant for
 # including in a CHANGELOG for example
@@ -7,7 +7,7 @@ has_app_changes = !git.modified_files.grep(/Source/).empty?
 has_test_changes = !git.modified_files.grep(/Tests/).empty?
 
 # Add a CHANGELOG entry for app changes
-if !git.modified_files.include?("CHANGELOG.md") && has_app_changes
+if !git.modified_files.include?('CHANGELOG.md') && has_app_changes
   fail("Please include a CHANGELOG entry to credit yourself! \nYou can find it at [CHANGELOG.md](https://github.com/realm/SwiftLint/blob/master/CHANGELOG.md).")
     markdown <<-MARKDOWN
 Here's an example of your CHANGELOG entry:
@@ -22,5 +22,52 @@ end
 
 # Non-trivial amounts of app changes without tests
 if git.lines_of_code > 50 && has_app_changes && !has_test_changes
-  warn "This PR may need tests."
+  warn 'This PR may need tests.'
+end
+
+# Run OSSCheck if there were app changes
+if has_app_changes
+  @repos = ['realm/realm-cocoa', 'jpsim/SourceKitten']
+
+  def generate_reports(clone, branch)
+    Dir.chdir('osscheck') do
+      @repos.each do |repo|
+        `git clone "https://github.com/#{repo}" --depth 1` if clone
+        repo_name = repo.partition('/').last
+        Dir.chdir(repo_name) do
+          File.open("../#{branch}_reports/#{repo_name}.txt", 'w') do |file|
+            file.puts `../../.build/debug/swiftlint`
+          end
+        end
+      end
+    end
+  end
+
+  # Prep
+  ['osscheck/branch_reports', 'osscheck/master_reports'].each do |dir|
+    FileUtils.mkdir_p(dir)
+  end
+  # Build branch
+  `swift build`
+  # Generate branch reports
+  generate_reports(true, 'branch')
+  # Build master
+  `git checkout master`
+  `git pull`
+  `git submodule update --init --recursive`
+  `swift build`
+  # Generate master reports
+  generate_reports(false, 'master')
+  @repos.each do |repo|
+    repo_name = repo.partition('/').last
+    branch = File.read("osscheck/branch_reports/#{repo_name}.txt").split(/\n+/).reject { |c| c.empty? }
+    master = File.read("osscheck/master_reports/#{repo_name}.txt").split(/\n+/).reject { |c| c.empty? }
+    (master - branch).each do |fixed|
+      message "This PR fixed a violation in #{repo_name}: #{fixed}"
+    end
+    (branch - master).each do |violation|
+      warn "This PR introduced a violation in #{repo_name}: #{violation}"
+    end
+  end
+  FileUtils.rm_rf('osscheck')
 end