module Digest::UUID

def self.uuid_from_hash(hash_class, uuid_namespace, name)

See RFC 4122 for details of UUID at: http://www.ietf.org/rfc/rfc4122.txt

uuid_from_hash always generates the same UUID for a given name and namespace combination.
Using Digest::MD5 generates version 3 UUIDs; Digest::SHA1 generates version 5 UUIDs.

Generates a v5 non-random UUID (Universally Unique IDentifier). 
def self.uuid_from_hash(hash_class, uuid_namespace, name)
  if hash_class == Digest::MD5
    version = 3
  elsif hash_class == Digest::SHA1
    version = 5
  else
    raise ArgumentError, "Expected Digest::SHA1 or Digest::MD5, got #{hash_class.name}."
  end
  hash = hash_class.new
  hash.update(uuid_namespace)
  hash.update(name)
  ary = hash.digest.unpack('NnnnnN')
  ary[2] = (ary[2] & 0x0FFF) | (version << 12)
  ary[3] = (ary[3] & 0x3FFF) | 0x8000
  "%08x-%04x-%04x-%04x-%04x%08x" % ary
end

def self.uuid_v3(uuid_namespace, name)

Convenience method for uuid_from_hash using Digest::MD5. 
def self.uuid_v3(uuid_namespace, name)
  self.uuid_from_hash(Digest::MD5, uuid_namespace, name)
end

def self.uuid_v4

Convenience method for SecureRandom.uuid. 
def self.uuid_v4
  SecureRandom.uuid
end

def self.uuid_v5(uuid_namespace, name)

Convenience method for uuid_from_hash using Digest::SHA1. 
def self.uuid_v5(uuid_namespace, name)
  self.uuid_from_hash(Digest::SHA1, uuid_namespace, name)
end