How to stably sort an array in swift?

Question

I used the sort () function, but it mixes the relative order.

This is what my code looks like.

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

Swift API says that:

The sorting algorithm is unstable. Unstable sorting can change the relative order of elements that are compared equal.

How can I change this so that the relative order remains the same?

Answer 1

        let sortedArray = (recipes as NSArray).sortedArray(options: .stable, usingComparator: { (lhs, rhs) -> ComparisonResult in
            let lhs = (lhs as! Recipe)
            let rhs = (rhs as! Recipe)
            if lhs.skill.value == rhs.skill.value {
                return ComparisonResult.orderedSame
            } else if lhs.skill.value < rhs.skill.value {
                return ComparisonResult.orderedAscending
            } else {
                return ComparisonResult.orderedDescending
            }
        })

Taken from here: https://medium.com/@cocotutch/a-swift-sorting-problem-e0ebfc4e46d4

Answer 2

The implementation below only works as a method sortedin the standard library without further restriction.

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: (Iterator.Element, Iterator.Element) -> Bool?) -> [Iterator.Element] {

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

. , , , , .

Answer 3

I appreciate the elegance of the answer from Lez . I adapted it to have the same signature as Sequence.sorted(by:):

extension Sequence {
  func stableSorted(
    by areInIncreasingOrder: (Element, Element) throws -> Bool)
    rethrows -> [Element]
  {
    return try enumerated()
      .sorted { a, b -> Bool in
        try areInIncreasingOrder(a.element, b.element) ||
          (a.offset < b.offset && !areInIncreasingOrder(b.element, a.element))
      }
      .map { $0.element }
  }
}

Answer 4

I use this wrapper

extension Array where Element: Comparable, Element: AnyObject {
    public func stableSorted() -> [Element] {
        let array = self as NSArray
        let result = array.sortedArray(options: .stable) { (left, right) -> ComparisonResult in
            let left = left as! Element
            let right = right as! Element
            if left < right {
                return ComparisonResult.orderedAscending
            }
            if left > right {
                return ComparisonResult.orderedDescending
            }
            return ComparisonResult.orderedSame
        }
        return result as! [Element]
    }
    public func stableReversed() -> [Element] {
        let array = self as NSArray
        let result = array.sortedArray(options: .stable) { (left, right) -> ComparisonResult in
            let left = left as! Element
            let right = right as! Element
            if left > right {
                return ComparisonResult.orderedAscending
            }
            if left < right {
                return ComparisonResult.orderedDescending
            }
            return ComparisonResult.orderedSame
        }
        return result as! [Element]
    }
}