Is there a quick and easy way to create a checksum from Ruby's basic data structures?

Question

I have a data structure (Hash) that looks something like this:

{
    foo: "Test string",
    bar: [475934759, 5619827847]
}

I'm trying to create a checksum from that Hash to check for equality in the future. I tried using the hash method of the Hash, which resulted in a satisfyingly nice-looking hash, but it turns out that the same Hash will produce a different hash after the interpreter has been restarted.

I really just want to be able to create a ~128 bit checksum from a Hash, String or Array instance.

Is this possible?

Answer 1

You could calculate your own hash based on the object's Marshal dump or JSON representation.

This calculates the MD5 hash of a Marshal dump:

require 'digest/md5'

hash = {
  foo: "Test string",
  bar: [475934759, 5619827847]
}

Marshal::dump(hash)
#=> "\x04\b{\a:\bfooI\"\x10Test string\x06:\x06ET:\bbar[\ai\x04'0^\x1Cl+\b\x87\xC4\xF7N\x01\x00"

Digest::MD5.hexdigest(Marshal::dump(hash))
#=> "1b6308abdd8f5f6290e2825a078a1a02"

Update

You can implement your own strategy, although I would not recommend to change core functionality:

class Hash
  def _dump(depth)
    # this doesn't cause a recursion because sort returns an array
    Marshal::dump(self.sort, depth)
  end

  def self._load(marshaled_hash)
    Hash[Marshal::load(marshaled_hash)]
  end
end

Marshal::dump({foo:1, bar:2})
#=> "\x04\bu:\tHash\e\x04\b[\a[\a:\bbari\a[\a:\bfooi\x06"

Marshal::dump({bar:2, foo:1})
#=> "\x04\bu:\tHash\e\x04\b[\a[\a:\bbari\a[\a:\bfooi\x06"

Marshal::load(Marshal::dump({foo:1, bar:2}))
#=> {:bar=>2, :foo=>1}