tweaks by @ericasadun

Sequence+Stride.swift

/// A strided non-contiguous sequence of elements that incorporates
/// every nth element of a base sequence.
public struct StridedSequence<BaseSequence: Sequence> : Sequence, IteratorProtocol {
    
    public mutating func next() -> BaseSequence.Element? {
        defer {
            for _ in 0 ..< _strideLength - 1 {
                let _ = _iterator.next()
            }
        }
        return _iterator.next()    }
    
    /// Access only through `Sequence.striding(by:)`
    internal init(_ sequence: BaseSequence, stride strideLength: Int) {
        _iterator = sequence.makeIterator()
        _strideLength = strideLength
    }
    
    internal var _iterator: BaseSequence.Iterator
    internal var _strideLength: Int
}

extension Sequence {
    /// Returns a strided iterator of sequence elements. The
    /// stride length is set to incorporate every nth element.
    ///
    /// - Parameter strideLength: the distance for each stride
    /// - Returns: A strided sequence of values
    func striding(by strideLength: Int) -> StridedSequence<Self> {
        return StridedSequence(self, stride: strideLength)
    }
}

/// A strided non-contiguous sequence of elements that incorporates
/// every nth element of a base sequence.
public struct StridedSequence<BaseSequence: Sequence> : Sequence, IteratorProtocol {
    
    public mutating func next() -> BaseSequence.Element? {
        defer {
            for _ in 0 ..< _strideLength - 1 {
                let _ = _iterator.next()
            }
        }
        return _iterator.next()    }
    
    /// Access only through `Sequence.striding(by:)`
    internal init(_ sequence: BaseSequence, stride strideLength: Int) {
        _iterator = sequence.makeIterator()
        _strideLength = strideLength
    }
    
    internal var _iterator: BaseSequence.Iterator
    internal var _strideLength: Int
}

extension Sequence {
    /// Returns a strided iterator of sequence elements. The
    /// stride length is set to incorporate every nth element.
    ///
    /// - Parameter strideLength: the distance for each stride
    /// - Returns: A strided sequence of values
    func striding(by strideLength: Int) -> StridedSequence<Self> {
        return StridedSequence(self, stride: strideLength)
    }
}