# File lib/linecache.rb, line 365
  def sha1(filename)
    filename = unmap_file(filename)
    return nil unless @@file_cache.member?(filename)
    return @@file_cache[filename].sha1.hexdigest if
      @@file_cache[filename].sha1
    sha1 = Digest::SHA1.new
    @@file_cache[filename].lines[:plain].each do |line|
      sha1 << line + "\n"
    end
    @@file_cache[filename].sha1 = sha1
    sha1.hexdigest
  end