module MultiJson::ConvertibleHashKeys

def prepare_hash(hash, &key_modifier)
  return hash unless block_given?
  case hash
  when Array
    hash.map do |value|
      prepare_hash(value, &key_modifier)
    end
  when Hash
    hash.inject({}) do |result, (key, value)|
      new_key   = key_modifier.call(key)
      new_value = prepare_hash(value, &key_modifier)
      result.merge! new_key => new_value
    end
  when String, Numeric, true, false, nil
    hash
  else
    if hash.respond_to?(:to_json)
      hash
    elsif hash.respond_to?(:to_s)
      hash.to_s
    else
      hash
    end
  end
end

def stringify_keys(hash)
  prepare_hash(hash) do |key|
    key.respond_to?(:to_s) ? key.to_s : key
  end
end

def symbolize_keys(hash)
  prepare_hash(hash) do |key|
    key.respond_to?(:to_sym) ? key.to_sym : key
  end
end