Interpose

This function returns the given items, with element inserted between every count items. The implementation below makes sure that the elements are only interposed and not appended at the end.

func interpose<T>(items: [T], element: T, count: Int = 1) -> [T] {

    typealias Acc = (ac: [T], cur: Int, cnt: Int)

    return items.reduce((ac: [], cur: 0, cnt: 1), { (a: Acc, o: T) -> Acc in 

        switch a {

        // the last item will not have any interposition

        case let (ac, cur, _) where (cur+1) == items.count: return (ac + [o], 0, 0)

        // interpose

        case let (ac, cur, c) where c == count:

           return (ac + [o, element], cur + 1, 1)

        // next

        case let (ac, cur, c):

           return (ac + [o], cur + 1, c + 1)

        }

    }).ac

}

print(interpose(items: [1, 2, 3, 4, 5], element: 9))

// : [1, 9, 2, 9, 3, 9, 4, 9, 5]

print(interpose(items: [1, 2, 3, 4, 5], element: 9, count: 2))

// : [1, 2, 9, 3, 4, 9, 5]