sorted_over.swift

extension Collection {
    func sorted<T: Comparable>(over: (Element) -> T) -> [Element] {
        var projections: [(T, Index)] = indices.map({ (over(self[$0]), $0) })
        projections.sort(by: { $0.0 < $1.0 })
        return projections.map({ self[$0.1] })
    }
}