global

def Digest(name)

# => LoadError: library not found for class Digest::Foo -- digest/foo
Digest(:Foo)

# => Digest::SHA256
Digest(:SHA256)

# => Digest::MD5
Digest("MD5")

require 'digest'

when on-demand loading is involved.
Returns a Digest subclass by +name+ in a thread-safe manner even

Digest(name) -> digest_subclass
call-seq: 
def Digest(name)
  const = name.to_sym
  Digest::REQUIRE_MUTEX.synchronize {
    # Ignore autoload's because it is void when we have #const_missing
    Digest.const_missing(const)
  }
rescue LoadError
  # Constants do not necessarily rely on digest/*.
  if Digest.const_defined?(const)
    Digest.const_get(const)
  else
    raise
  end
end