OffsetIndexCollection.swift

struct OffsetIndexCollection<Base: RandomAccessCollection>: RandomAccessCollection {
    var base: Base
    var distance: Int
    
    @inlinable @inline(__always)
    var startIndex: Base.Index {
        index(base.startIndex, offsetBy: distance)
    }
    
    @inlinable @inline(__always)
    var endIndex: Base.Index {
        index(base.endIndex, offsetBy: distance)
    }

    @inlinable @inline(__always)
    subscript(position: Base.Index) -> Base.Element {
        base[index(position, offsetBy: distance)]
    }
    
    @inlinable @inline(__always)
    func index(before i: Base.Index) -> Base.Index {
        base.index(before: i)
    }
    
    @inlinable @inline(__always)
    func index(after i: Base.Index) -> Base.Index {
        base.index(after: i)
    }
    
    @inlinable @inline(__always)
    func index(_ i: Base.Index, offsetBy distance: Int) -> Base.Index {
        base.index(i, offsetBy: distance)
    }
    
    @inlinable @inline(__always)
    func distance(from start: Base.Index, to end: Base.Index) -> Int {
        base.distance(from: start, to: end)
    }
}

extension OffsetIndexCollection: MutableCollection where Base: MutableCollection {
    @inlinable @inline(__always)
    subscript(position: Index) -> Base.Element {
        get { base[index(position, offsetBy: distance)] }
        set { base[index(position, offsetBy: distance)] = newValue }
    }
}

extension OffsetIndexCollection: RangeReplaceableCollection where Base: RangeReplaceableCollection {
    @inlinable @inline(__always)
    init(distance: Int) {
        self.init(base: .init(), distance: distance)
    }
    
    @inlinable @inline(__always)
    init() {
        fatalError("must call init(distance:)")
    }
    
    @inlinable @inline(__always)
    mutating func replaceSubrange(_ subrange: Range<Index>, with newElements: some Collection<Base.Element>) {
        base.replaceSubrange(index(subrange.lowerBound, offsetBy: distance)..<index(subrange.lowerBound, offsetBy: distance), with: newElements)
    }
}

extension RandomAccessCollection {
    func offsettingIndices(by distance: Int) -> OffsetIndexCollection<Self> {
        .init(base: self, distance: distance)
    }
}