inline fun <S, T : S> MongoIterable<T>.reduceIndexed(crossinline operation: (index: Int, acc: S, T) -> S): S (source)
Accumulates value starting with the first element and applying operation from left to right to current accumulator value and each element with its index in the original collection.
operation - function that takes the index of an element, current accumulator value
and the element itself and calculates the next accumulator value.