SimpleCov Test Coverage for Files Changed in a Branch

check_changed_coverage.rb

output = `cat #{ENV["CIRCLE_ARTIFACTS"] || "."}/coverage/index.html | grep Changed -A 2 | grep "[0-9\.]*%"`
percentage_match = output.match(/([0-9\.]+)%/)

raise "Unable to determine test coverage change" unless percentage_match

RED = "\033[0;31m"
BOLD = "\033[1m"
NO_COLOR = "\033[0m"

percentage = percentage_match[0].to_f
if percentage < 90
  warn "\n"
  warn "#{RED}#{BOLD}⚠️  📉  INSUFFICIENT TEST COVERAGE#{NO_COLOR} 📉  ⚠️"
  warn "\n"
  warn "New and changed files need a test coverage of >= 90% -- Simplecov only detected #{RED}#{BOLD}#{percentage}#{NO_COLOR} 😱"
  warn "\n"
  warn "Changed files:"
  warn " * #{`git diff --name-only origin/master`.split("\n").join("\n * ")}"
  warn "\n"
  exit 1
end

puts "Test coverage for changed files: #{percentage}"

circle.yml

test:
  post:
    - ruby scripts/check_changed_coverage.rb

spec_helper.rb

SimpleCov.start "rails" do
  changed_files = `git diff --name-only origin/master`.split("\n")
  add_group "Changed" do |source_file|
    changed_files.detect do |filename|
      source_file.filename.ends_with?(filename)
    end
  end
end

if ENV["CIRCLE_ARTIFACTS"]
  dir = File.join(ENV["CIRCLE_ARTIFACTS"], "coverage")
  SimpleCov.coverage_dir(dir)
end

Thank you. Had to make some modifications, but it works as expected.