Iterate a sequence with access to the previous/next elements if they exist

NeighborSequence.swift

/// A collection that returns the current element as well as the previous and next elements when available
public struct NeighborSequence<Base: Sequence>: Sequence {
    public typealias Element = (previous: Base.Element?, current: Base.Element, next: Base.Element?)
    
    private let base: Base
    
    fileprivate init(_ base: Base) {
        self.base = base
    }
    
    public func makeIterator() -> AnyIterator<Element> {
        var iterator = base.makeIterator()
        
        var previous: Base.Element?
        var currentValue = iterator.next()
        
        return .init {
            guard let current = currentValue else { return nil }
            defer { previous = current }
            
            let next = iterator.next()
            currentValue = next
            
            return (previous, current, next)
        }
    }
}

extension Sequence {
    /// Return `Self` as a `NeighborSequence`
    ///
    /// Each element in a `NeighborSequence` returns the element at the specified
    /// index as well as the previous and next elements when possible.
    ///
    /// This allows you to see either side of a given element.
    public var neighbors: NeighborSequence<Self> {
        return .init(self)
    }
}