groupBy

//MARK: - ??=
infix operator ??= {
	associativity right
	precedence 90
	assignment
}
func ??= <Wrapped> (inout optional: Wrapped?, @autoclosure defaultValue: () throws -> Wrapped?) rethrows {
	optional = try optional ?? defaultValue()
}

extension SequenceType {
	public func indexBy <IndexValue: Hashable> (indexingValue: (Generator.Element) -> IndexValue) -> [IndexValue: Generator.Element] {
		var indexed: [IndexValue: Generator.Element] = [:]
		for element in self {
			let key = indexingValue(element)
			indexed[key] = element
		}
		return indexed
	}
	public func groupBy <GroupingValue: Hashable> (groupingValue: (Generator.Element) -> GroupingValue) -> [GroupingValue: [Generator.Element]] {
		var grouped: [GroupingValue: [Generator.Element]] = [:]
		for element in self {
			let key = groupingValue(element)
			grouped[key] ??= []
			grouped[key]?.append(element)
		}
		return grouped
	}
}