sliding_window.swift

struct SlidingWindowView<C: RandomAccessCollection>: RandomAccessCollection {
    enum EndBehavior {
        case stop
        case truncate
    }

    let base: C
    let windowSize: Int
    let stepSize: Int
    let endBehavior: EndBehavior

    var startIndex: Int {
        return 0
    }

    var endIndex: Int {
        switch endBehavior {
        case .stop:
            return Swift.max(base.count - windowSize, -1) / stepSize + 1
        case .truncate:
            return (base.count + stepSize - 1) / stepSize
        }
    }

    subscript(_ pos: Int) -> C.SubSequence {
        get {
            let offset = pos * stepSize
            let start = base.index(base.startIndex, offsetBy: offset)
            let end: C.Index
            switch endBehavior {
            case .stop:
                end = base.index(start, offsetBy: windowSize)
            case .truncate:
                end = base.index(start, offsetBy: windowSize, limitedBy: base.endIndex) ?? base.endIndex
            }
            return base[start..<end]
        }
    }
}