codeowners.rb

require "pathspec/gitignorespec"

class Codeowners
  def initialize(codeowners)
    @captures, @index = codeowners.each_line(chomp: true).map(&:strip).reject { |line| line.empty? || line.start_with?("#") }.map { |line| line.split(/\s+/, 2) }.map do |glob, owners|
      pattern = ::GitIgnoreSpec.new(glob).instance_variable_get(:@regex)

      ["(#{pattern})", owners.split(/\s+/)]
    end.reverse.transpose

    @pattern = Regexp.compile(@captures.join("|"))
  end

  def [](path)
    @pattern.match(path) do |matchdata|
      idx = matchdata.captures.find_index { |capture| !capture.nil? }
      @index[idx]
    end
  end

  def for(paths)
    paths.filter_map { |path| self[path] }.flatten.to_set
  end
end