Accumulates value starting with the first element and applying operation from left to right to current accumulator value and each element.
inline fun <S, T : S, K> Grouping<T, K>.reduce((source)
operation: (key: K, accumulator: S, element: T) -> S
): Map<K, S>
Platform and version requirements: Kotlin 1.1
Groups elements from the Grouping source by key and applies the reducing operation to the elements of each group sequentially starting from the second element of the group, passing the previously accumulated value and the current element as arguments, and stores the results in a new map. An initial value of accumulator is the first element of the group.
Return a Map associating the key of each group with the result of accumulating the group elements.