Question

So I’m working on some MongoDB protocol stuff. All integers are signed little-endian. Using Ruby’s standard Array#pack method, I can convert from an integer to the binary string I want just fine:

positive_one = Array(1).pack('V')   #=> '\x01\x00\x00\x00'
negative_one = Array(-1).pack('V')  #=> '\xFF\xFF\xFF\xFF'

However, going the other way, the String#unpack method has the ‘V’ format documented as specifically returning unsigned integers:

positive_one.unpack('V').first #=> 1
negative_one.unpack('V').first #=> 4294967295

There’s no formatter for signed little-endian byte order. I’m sure I could play games with bit-shifting, or write my own byte-mangling method that doesn’t use array packing, but I’m wondering if anyone else has run into this and found a simple solution. Thanks very much.

Answer 1

After unpacking with "V", you can apply the following conversion

class Integer
  def to_signed_32bit
    if self & 0x8000_0000 == 0x8000_0000
      self - 0x1_0000_0000  
    else
      self
    end
  end
end

You’ll need to change the magic constants 0x1_0000_0000 (which is 2**32) and 0x8000_0000 (2**31) if you’re dealing with other sizes of integers.