module Enumerable

  # Calculates a sum from the elements. Examples:
  #
  #  payments.sum { |p| p.price * p.tax_rate }
  #  payments.sum(&:price)
  #
  # The latter is a shortcut for:
  #
  #  payments.inject { |sum, p| sum + p.price }
  #
  # It can also calculate the sum without the use of a block.
  #
  #  [5, 15, 10].sum # => 30
  #  ["foo", "bar"].sum # => "foobar"
  #  [[1, 2], [3, 1, 5]].sum => [1, 2, 3, 1, 5]
  #
  # The default sum of an empty list is zero. You can override this default:
  #
  #  [].sum(Payment.new(0)) { |i| i.amount } # => Payment.new(0)
  #
  def sum(identity = 0, &block)
    if block_given?
      map(&block).sum(identity)
    else
      inject { |sum, element| sum + element } || identity
    end
  end

  # Iterates over a collection, passing the current element *and* the
  # +memo+ to the block. Handy for building up hashes or
  # reducing collections down to one object. Examples:
  #
  #   %w(foo bar).each_with_object({}) { |str, hsh| hsh[str] = str.upcase } #=> {'foo' => 'FOO', 'bar' => 'BAR'}
  #
  # *Note* that you can't use immutable objects like numbers, true or false as
  # the memo. You would think the following returns 120, but since the memo is
  # never changed, it does not.
  #
  #   (1..5).each_with_object(1) { |value, memo| memo *= value } # => 1
  #
  def each_with_object(memo, &block)
    returning memo do |m|
      each do |element|
        block.call(element, m)
      end
    end
  end unless [].respond_to?(:each_with_object)

  # Convert an enumerable to a hash. Examples:
  #
  #   people.index_by(&:login)
  #     => { "nextangle" => <Person ...>, "chade-" => <Person ...>, ...}
  #   people.index_by { |person| "#{person.first_name} #{person.last_name}" }
  #     => { "Chade- Fowlersburg-e" => <Person ...>, "David Heinemeier Hansson" => <Person ...>, ...}
  #
  def index_by
    inject({}) do |accum, elem|
      accum[yield(elem)] = elem
      accum
    end
  end

  # Returns true if the collection has more than 1 element. Functionally equivalent to collection.size > 1.
  # Works with a block too ala any?, so people.many? { |p| p.age > 26 } # => returns true if more than 1 person is over 26.
  def many?(&block)
    size = block_given? ? select(&block).size : self.size
    size > 1
  end

  # Returns true if none of the elements match the given block.
  #
  #   success = responses.none? {|r| r.status / 100 == 5 }
  #
  # This is a builtin method in Ruby 1.8.7 and later.
  def none?(&block)
    !any?(&block)
  end unless [].respond_to?(:none?)
end
