RangeSliceCollection.swift

struct RangeSliceCollection<Base: Collection, Ranges: Collection>: Collection where Ranges.Element == Range<Base.Index> {
    var base: Base
    var ranges: Ranges
    
    var startIndex: Ranges.Index { ranges.startIndex }
    var endIndex: Ranges.Index { ranges.endIndex }
    
    func index(after i: Ranges.Index) -> Ranges.Index {
        ranges.index(after: i)
    }
    
    subscript(position: Ranges.Index) -> Base.SubSequence {
        let range = ranges[position]
        return base[range]
    }
}

extension RangeSliceCollection: BidirectionalCollection where Base: BidirectionalCollection, Ranges: BidirectionalCollection {
    func index(before i: Ranges.Index) -> Ranges.Index {
        ranges.index(before: i)
    }
}

extension RangeSliceCollection: RandomAccessCollection where Base: RandomAccessCollection, Ranges: RandomAccessCollection {
    func index(_ i: Ranges.Index, offsetBy distance: Int) -> Ranges.Index {
        ranges.index(i, offsetBy: distance)
    }
    
    func distance(from start: Ranges.Index, to end: Ranges.Index) -> Int {
        ranges.distance(from: start, to: end)
    }
}