class Hash
  # Slice a hash to include only the given keys. This is useful for
  # limiting an options hash to valid keys before passing to a method:
  #
  #   def search(criteria = {})
  #     assert_valid_keys(:mass, :velocity, :time)
  #   end
  #
  #   search(options.slice(:mass, :velocity, :time))
  #
  # If you have an array of keys you want to limit to, you should splat them:
  #
  #   valid_keys = [:mass, :velocity, :time]
  #   search(options.slice(*valid_keys))
  def slice(*keys)
    keys = keys.map! { |key| convert_key(key) } if respond_to?(:convert_key)
    hash = self.class.new
    keys.each { |k| hash[k] = self[k] if has_key?(k) }
    hash
  end

  # Replaces the hash with only the given keys.
  # Returns a hash contained the removed key/value pairs
  #   {:a => 1, :b => 2, :c => 3, :d => 4}.slice!(:a, :b) # => {:c => 3, :d => 4}
  def slice!(*keys)
    keys = keys.map! { |key| convert_key(key) } if respond_to?(:convert_key)
    omit = slice(*self.keys - keys)
    hash = slice(*keys)
    replace(hash)
    omit
  end

  def extract!(*keys)
    result = {}
    keys.each {|key| result[key] = delete(key) }
    result
  end
end