How to stable sort an array in swift?

Question

I've been using the sort() function but it mixes up the relative order.

This is how my code looks.

recipes.sort { $0.skill.value <= $1.skill.value }

Swift API says that:

The sorting algorithm is not stable. A nonstable sort may change the relative order of elements that compare equal.

How can I change this so that the relative order stays the same as before?

Answer 1

The implementation below just work like the sorted method in the standard library, without additional limit.

extension RandomAccessCollection {

    /// return a sorted collection
    /// this use a stable sort algorithm
    ///
    /// - Parameter areInIncreasingOrder: return nil when two element are equal
    /// - Returns: the sorted collection
    public func stableSorted(by areInIncreasingOrder: (Element, Element) throws -> Bool) rethrows -> [Element] {

        let sorted = try enumerated().sorted { (one, another) -> Bool in
            if try areInIncreasingOrder(one.element, another.element) {
                return true
            } else {
                return one.offset < another.offset
            }
        }
        return sorted.map { $0.element }
    }
}

A stable sort needs to preserve the original order. So we give every element a weight of order besides its value, the index, then the original sort method will just work, as there will never be 2 equal elements.