A multimap implementation.

Multimap.swift

struct Multimap<Key: Hashable, Value: Hashable> {

    fileprivate struct _Element: Hashable {
        let key: Key
        let value: Value
    }

    private var elements: Set<_Element> = []

    mutating func add(_ value: Value, for key: Key) {
        let element = _Element(key: key, value: value)
        elements.insert(element)
    }
}

extension Multimap: Sequence {

    func makeIterator() -> Iterator {
        Iterator(iterator: elements.makeIterator())
    }

    struct Iterator: IteratorProtocol {

        fileprivate var iterator: Set<_Element>.Iterator

        mutating func next() -> (key: Key, value: Value)? {
            guard let element = iterator.next() else { return nil }
            return (key: element.key, value: element.value)
        }
    }
}

var multimap = Multimap<Int, String>()
multimap.add("Daniel", for: 0)
multimap.add("Mike", for: 0)
multimap.add("Foo", for: 2)
multimap.add("Mike", for: 0)

print(Array(multimap))