RotatedCollection.swift

struct RotatedCollection<Base: Collection> {
    let base: Base
    let index: Base.Index
    
    init(base: Base, shiftingToStart index: Base.Index) {
        self.base = base
        self.index = index
    }
}

extension RotatedCollection: Collection {
    struct Index {
        let base: Base.Index
        let isWrapped: Bool
    }
    
    var startIndex: Index {
        Index(base: index, isWrapped: false)
    }
    
    var endIndex: Index {
        Index(base: index, isWrapped: true)
    }
    
    subscript(index: Index) -> Base.SubSequence.Element {
        base[index.base]
    }
    
    func index(after index: Index) -> Index {
        let next = base.index(after: index.base)
        if !index.isWrapped && next == base.endIndex {
            return Index(base: base.startIndex, isWrapped: true)
        } else {
            return Index(base: next, isWrapped: index.isWrapped)
        }
    }
}

extension RotatedCollection.Index: Comparable {
    static func == (lhs: RotatedCollection.Index, rhs: RotatedCollection.Index) -> Bool {
        lhs.isWrapped == rhs.isWrapped && lhs.base == rhs.base
    }
    
    static func < (lhs: RotatedCollection.Index, rhs: RotatedCollection.Index) -> Bool {
        if lhs.isWrapped == rhs.isWrapped {
            return lhs.base < rhs.base
        } else {
            return rhs.isWrapped
        }
    }
}

extension Collection {
    func rotated(shiftingToStart index: Index) -> RotatedCollection<Self> {
        RotatedCollection(base: self, shiftingToStart: index)
    }
}