multiplicity.swift

enum CollectionMultiplicity<C: Collection> {
    case none
    case one(first: C.Index)
    case many(first: C.Index, second: C.Index)
}

extension Collection {
    func indexedMultiplicity(where predicate: (Element) -> Bool) -> CollectionMultiplicity<Self> {
        guard let i = firstIndex(where: predicate) else {
            return .none
        }
        guard let j = suffix(from: index(after: i)).firstIndex(where: predicate) else {
            return .one(first: i)
        }
        return .many(first: i, second: j)
    }
}

enum SequenceMultiplicity<S: Sequence> {
    case none
    case one(first: S.Element)
    case many(first: S.Element, second: S.Element)
}

extension Sequence {
    func multiplicity(where predicate: (Element) -> Bool) -> SequenceMultiplicity<Self> {
        var it = makeIterator()
        while let x = it.next() {
            if predicate(x) {
                while let y = it.next() {
                    if predicate(y) {
                        return .many(first: x, second: y)
                    }
                }
                return .one(first: x)
            }
        }
        return .none
    }
}