rspec helper for reporting missing test coverage for changed lines according to git diff with selected branch and simplecov json resultset

diff_coverage_reporter.rb

class DiffCoverageReporter
  attr_reader :coverage_file, :default_branch
  def initialize(file_path: "coverage/.resultset.json", default_branch: "master")
    @coverage_file = Rails.root.join(file_path)
    @default_branch = default_branch
  end

  def coverage_report
    JSON.parse(coverage_file.read).dig("RSpec", "coverage")
  end

  def changed_files
    `git diff --name-only origin/#{default_branch}`.split("\n").select do |file|
      file.match?(%r{app/})
    end
  end

  def changed_line_numbers
    @changed_line_numbers ||= changed_files.each_with_object({}) do |file, file_numbers|
      diff = `git diff origin/#{default_branch} -U0 -- #{file}`
      changes = diff.split("\n").each_with_object(Set.new) do |line, numbers|
        if /^@@ -\d+,\d+ \+(\d+),\d+ @@$/.match?(line)
          result = line.match(/@@ -(\d+),(\d+) \+(\d+),(\d+) @@/).captures.map(&:to_i)
          removed = (result[0]..(result[0] + result[1]))
          added = (result[2]..(result[2] + result[3]))
          numbers.merge(removed.to_a)
          numbers.merge(added.to_a)
        end
      end
      next if changes.empty?
      file_numbers[file] = changes.map { [_1, true] }.to_h
    end
  end

  # NOTE: require_relative "spec/support/diff_coverage_reporter"; DiffCoverageReporter.diff_missing_coverage
  def diff_missing_coverage
    @diff_coverage_report ||= coverage_report.map do |file, obj|
      path = file.gsub(/^#{Rails.root}\//, "")
      diff = `git diff origin/#{default_branch} -U0 -- #{file}`
      changes = diff.split("\n").each_with_object(Set.new) do |line, numbers|
        if /^@@ -\d+,\d+ \+(\d+),\d+ @@$/.match?(line)
          result = line.match(/@@ -(\d+),(\d+) \+(\d+),(\d+) @@/).captures.map(&:to_i)
          removed = (result[0]..(result[0] + result[1]))
          added = (result[2]..(result[2] + result[3]))
          numbers.merge(removed.to_a)
          numbers.merge(added.to_a)
        end
      end.map { [_1, true] }.to_h
      [
        path,
        obj["lines"].each_with_index.each_with_object([]) do |(number, index), groups|
          line_coverage = number.nil? || number.positive?
          line_changed = changed_line_numbers.dig(path, index)
          changed_and_not_covered = !line_coverage && line_changed
          next unless changed_and_not_covered

          prev_group = groups.last
          view_index = index + 1
          if prev_group && prev_group.last == view_index - 1
            prev_group[1] = view_index
          else
            groups << [view_index, view_index]
          end
        end
      ]
    end.reject { |_, groups| groups.empty? }
  end

  def all_missing_coverage
    coverage_report.map do |file, obj|
      path = file.gsub(/^#{Rails.root}\//, "")
      [
        path,
        obj["lines"].each_with_index.each_with_object([]) do |(number, index), groups|
          line_coverage = number.nil? || number.positive?
          next if line_coverage

          prev_group = groups.last
          view_index = index + 1
          if prev_group && prev_group.last == view_index - 1
            prev_group[1] = view_index
          else
            groups << [view_index, view_index]
          end
        end
      ]
    end.reject { |_, groups| groups.empty? }
  end

  def self.instance
    @@instance ||= new
  end

  # NOTE: require_relative "spec/support/diff_coverage_reporter"; DiffCoverageReporter.print_all_missing_coverage
  def self.print_all_missing_coverage
    instance.all_missing_coverage.each do |path, groups|
      groups.each do |start_line, end_line|
        lines = start_line == end_line ? start_line : "#{start_line}-#{end_line}"
        puts "#{path}:#{lines}"
      end
    end
    true
  end

  # NOTE: require_relative "spec/support/diff_coverage_reporter"; DiffCoverageReporter.print_diff_missing_coverage
  def self.print_diff_missing_coverage
    instance.diff_missing_coverage.each do |path, groups|
      groups.each do |start_line, end_line|
        lines = start_line == end_line ? start_line : "#{start_line}-#{end_line}"
        puts "#{path}:#{lines}"
      end
    end
    true
  end

  # NOTE: require_relative "spec/support/diff_coverage_reporter"; DiffCoverageReporter.export_diff_missing_coverage
  def self.export_diff_missing_coverage
    result = instance.diff_missing_coverage
    return if result.empty?

    File.write(Rails.root.join("coverage/.diff_missing_coverage.json"), result.to_json)
    true
  end
end

RSpec.configure do |config|
  config.after(:suite) do
    unless defined?(SimpleCov)
      puts "SimpleCov is not enabled, skipping coverage reporting"
      next
    end
    next if config.instance_variable_get(:@files_or_directories_to_run).reject { _1.match?(/^spec$/) }.present?

    puts "Generating diff coverage report"

    DiffCoverageReporter.print_diff_missing_coverage
    DiffCoverageReporter.export_diff_missing_coverage
  end
end