Multiqueue data structure

A multiqueue is to a queue as a multisite is to a set. This generic structure might be useful to some programmer somewhere and I haven't seen it in textbooks, so here is an example in Swift.

// This implementation uses O(N) memory, O(1) enqueue and O(N) dequeue.

// Other implementations with faster dequeue are possible by either

// sacrificing enqueue speed or using more memory.

struct MultiQueue<T> {

    private var elementsAndQuantities: [(element: T, quantity: Int)] = []

    // Quantity must be greater than zero

    mutating func enqueue(_ value: T, quantity: Int) {

        elementsAndQuantities.append((value, quantity))

    }

    

    mutating func dequeue() -> T? {

        guard !elementsAndQuantities.isEmpty else {

            return nil

        }

        let retval = elementsAndQuantities[0].element

        if elementsAndQuantities[0].quantity == 1 {

            elementsAndQuantities.removeFirst()

        } else {

            elementsAndQuantities[0].quantity -= 1

        }

        return retval

    }

}