multipart_range_response.rb

class MultipartRangesWriter
  ALPHABET = ('0'..'9').to_a + ('a'..'z').to_a + ('A'..'Z').to_a

  def initialize
    # RFC1521 says that a boundary "must be no longer than 70 characters, not counting the two leading hyphens".
    # Modulo-based random is biased but it doesn't matter much for us
    @boundary = SecureRandom.bytes(64).unpack("C*").map {|b| ALPHABET[b % ALPHABET.length] }.join
  end

  class NullWriter < Struct.new(:offset)
    def write(str)
      advance(str.bytesize)
    end

    def advance(by)
      self.offset += by
    end
  end

  def content_type_header_value
    "multipart/byte-ranges; boundary=#{@boundary}"
  end

  # Compute the full length of the MIME message with all the parts,
  # headers and whatnot
  def full_message_length(*http_ranges)
    writer = NullWriter.new(0)
    http_ranges.each.with_index do |range, i|
      write_boundary_and_headers(writer, i, range.begin, range.end)
      writer.advance(range.end - range.begin + 1)
    end
    writer.offset
  end

  def write_boundary_and_headers(socket, part_index, http_range)
    range_size = http_range.end - http_range.begin + 1
    if part_index > 0
      socket.write("\r\n")
    end
    socket.write("--#{@boundary}\r\n")
    socket.write("Content-Type: binary/octet-stream\r\n")
    socket.write("Content-Range: bytes #{http_range.begin}-#{http_range.end}/#{range_size}\r\n")
    socket.write("\r\n")
  end
end