java string hashCode() ruby implementation

java_hash_code.rb

TWO_31 = 2 ** 31
TWO_32 = 2 ** 32

def java_hash_code(str)
  size = str.size
  hash = 0
  str.chars.each_with_index do |ch, i|
    hash += ch.ord * (31 ** (size-(i+1)))
    hash = hash % TWO_32 - TWO_31
  end
  hash
end

This is not correct. Try, just converting , for instance. It should return 44, but it doesn't