gistfile1.ru

class Pack
  def initialize(path)
    @f = File.open(path, 'r')
  end
end

class Index
  def initialize(path)
    File.open(path, 'r') do |f|
      @f = f
      # Check the magic number
      raise unless f.read(4) == "\377tOc"
      raise unless f.read(4).unpack("N") == [2]

      # First read the fanout table
      read_fanout

      # Read in the table of SHA1 hashes
      read_hashes

      # Skip the CRC23 table
      skip_crc23

      # Read the offset for the hashes
      read_offsets

      # Read the file trailer
      read_trailer
    end
  ensure
    @f = nil
  end

  def inspect
    # @fanout.inspect
    str = ""
    str << "OBJECTS\n"
    str << "=======\n"
    @hashes.each_with_index do |sha, i|
      str << "#{sha} (#{@offsets[i]})\n"
    end
    str << "\n"
    str << "TRAILER\n"
    str << "=======\n"
    str << "  #{@pack_checksum}\n"
    str << "  #{@indx_checksum}\n"
    str
  end

  def read_fanout
    @fanout = (1..256).map { @f.read(4).unpack("N") }.flatten
  end

  def read_hashes
    @hashes = (1..num_objects).map { read_sha }
  end

  def skip_crc23
    @f.seek(crc23_len, IO::SEEK_CUR)
  end

  def read_offsets
    @offsets = (1..num_objects).map { @f.read(4).unpack("N") }.flatten
  end

  def read_trailer
    @pack_checksum = read_sha
    @indx_checksum = read_sha
  end

  def read_sha
    (1..5).map { @f.read(4).unpack("N").first.to_s(16) }.join
  end

  def crc23_len
    4 * num_objects
  end

  def num_objects
    @fanout.last
  end

end