path_finder.rb

require 'pathname'

class PathFinder
  SUFFIXES = ['',
              '.rb',
              *%w(DLEXT DLEXT2).map { |key|
                val = RbConfig::CONFIG[key]
                next unless val and not val.empty?
                ".#{val}"
              }
  ].compact.uniq

  using Module.new {
    refine Gem::Specification do
      def find_path(file, suffixes)
        raw_require_paths.find do |path|
          base = File.join(gems_dir, full_name, path.untaint, file).untaint

          suffixes.each do |suf|
            return base + suf if File.file?(base + suf)
          end
        end

        if have_extensions?
          base = File.join(extension_dir, file)
          suffixes.find { |suf| File.file?(base + suf) }
        else
          nil
        end
      end
    end
  }

  def self.find(feature)
    new.path_finder(feature)
  end

  def search_from_load_path(feature)
    pathname = Pathname.new(feature)
    ext = File.extname(feature)

    suffixes = ([ext] + SUFFIXES).uniq

    if pathname.absolute?
      suffixes.each do |suf|
        path = pathname.sub_ext(suf).to_path
        return path if File.file?(path)
      end
    else
      $LOAD_PATH.each do |load_path|
        base = Pathname.new(load_path) + feature

        suffixes.each do |suf|
          path = base.sub_ext(suf).to_path
          return path if File.file?(path)
        end
      end
    end

    nil
  end

  def search_from_gems(feature)
    spec = Gem::Specification.find_by_path(feature)
    spec&.add_self_to_load_path
    spec&.find_path(feature, Gem.suffixes)
  end

  def path_finder(feature)
    search_from_load_path(feature) || search_from_gems(feature)
  end
end

pp PathFinder.find('ripper')
pp PathFinder.find('ruby_parser')