crystal pack and unpack

crystal_pack.cr

class String
  def unpack(format : String) : Array
    io = IO::Memory.new(self)
    slice = Bytes.new(self.size)
    io.read(slice)
    [slice.hexstring]
  end
end

class Array
  def pack(format : String) : String
    size = self[0].size
    hex = self[0]
    raise Exception.new("No match size") unless size % 2 == 0
    slice = Slice(UInt8).new(size / 2)
    0.step(to: size - 1, by: 2) do |i|
      high_nibble = hex.to_unsafe[i].unsafe_chr.to_u8?(16)
      low_nibble = hex.to_unsafe[i + 1].unsafe_chr.to_u8?(16)
      raise Exception.new("No match nibble(high and low)") unless high_nibble && low_nibble
      slice[i / 2] = (high_nibble << 4) | low_nibble
    end
    io = IO::Memory.new(size / 2)
    io.write(slice)
    io.to_s
  end
end