AdjacentTripletsIterator.swift

private struct AdjacentTripletsIterator<Base: Collection>: IteratorProtocol {
    typealias Element = (Base.Element, Base.Element, Base.Element)
    
    private var baseIt: Base.Iterator
    private var nextResult: Element?
    
    init(base: Base) {
        self.baseIt = base.makeIterator()
        if let elem1 = baseIt.next(), let elem2 = baseIt.next(), let elem3 = baseIt.next() {
            nextResult = (elem1, elem2, elem3)
        }
    }
    
    mutating func next() -> Element? {
        guard let result = nextResult else {
            return nil
        }
        if let nextElem = baseIt.next() {
            nextResult = (result.1, result.2, nextElem)
        } else {
            nextResult = nil
        }
        return result
    }
}


extension Collection {
    func adjacentTriplets() -> some Collection<(Element, Element, Element)> {
        AdjacentTripletsIterator(base: self)
    }
}