Advent Day 1

Advent1.swift

public struct RotatingIterator<Base: IteratorProtocol>: Sequence, IteratorProtocol {
    private var skippedElements: [Base.Element] = []
    private var base: Base
    
    internal init(_ base: Base, count: Int) {
        self.base = base
        for _ in 0..<count {
            guard let element = self.base.next() else { break }
            self.skippedElements.append(element)
        }
    }
    
    public mutating func next() -> Base.Element? {
        if let element = base.next() { return element }
        // FIXME: Swift compiler bug prevents this from working.
        // return skippedElements.popFirst()
        return skippedElements.isEmpty ? nil : skippedElements.removeFirst()
    }
}

extension Sequence {
    public func rotating(by count: Int) -> RotatingIterator<Iterator> {
        return RotatingIterator(makeIterator(), count: count)
    }
}

func captcha1<S: Sequence>(_ input: S) -> S.Iterator.Element where S.Iterator.Element: Numeric {
    return zip(input, input.rotating(by: 1)).lazy.filter({ $0.0 == $0.1 }).map({ $0.0 }).reduce(0, +)
}

func captcha2<Element>(_ input: [Element]) -> Element where Element: Numeric {
    precondition(input.count % 2 == 0)
    return zip(input, input.rotating(by: input.count / 2)).lazy.filter({ $0.0 == $0.1 }).map({ $0.0 }).reduce(0, +)
}