msgpack_util.rb

require 'msgpack'
require 'json'

def usage
  puts <<EOT
usage: ruby msgpack_util.rb <option> <input-file-path> [<output-file-path>]
option:
  -d, --decode: Decode msgpack formatted data into JSON formatted data
  -e, --encode: Encode JSON formatted data into msgpack formatted data
EOT
  exit 1
end

def decode(input, output)
  open_output(output) do |o|
    u = MessagePack::Unpacker.new(File.open(input))
    u.each do |obj|
      o.puts obj.to_json
    end
  end
end

def encode(input, output)
  open_output(output) do |o|
    File.new(input).each_line do |line|
      j = JSON.parse(line)
      o.print j.to_msgpack
    end
  end
end

def open_output(output)
  o = output ? File.new(output, 'w') : $stdout
  yield o
ensure
  o.close if o && o.kind_of?(File)
end

usage if ARGV.size < 2
input_file_path = ARGV[1]
output_file_path = ARGV[2]

case ARGV[0]
when '-d', '--decode'
  decode(input_file_path, output_file_path)
when '-e', '--encode'
  encode(input_file_path, output_file_path)
else
  usage
end