Selection sort in Swift implemented as an extension on `MutableCollectionType`

SelectionSort.swift

// Swift 2.1

extension MutableCollectionType where Self.Generator.Element : Comparable {
    
    /// Return a new sorted collection of the elements in `source` 
    /// using [Selection Sort](https://en.wikipedia.org/wiki/Selection_sort).
    ///
    /// Complexity O(n^2).
    @warn_unused_result(mutable_variant="selectionSortInPlace")
    public func selectionSort() -> Self {
        
        var output = self
        
        for primaryIndex in self.indices {
            
            var minimum = primaryIndex
            
            for secondaryIndex in primaryIndex.successor() ..< output.endIndex {
                
                if output[minimum] > output[secondaryIndex] {
                    minimum = secondaryIndex
                }
                
            }
            
            if primaryIndex != minimum {
                swap(&output[primaryIndex], &output[minimum])
            }
            
        }
        
        return output
    }
    
    /// Sort `self` in-place using [Selection Sort](https://en.wikipedia.org/wiki/Selection_sort).
    /// 
    /// Complexity O(n^2).
    public mutating func selectionSortInPlace() {
        
        for primaryIndex in self.indices {
            
            var minimum = primaryIndex
            
            for secondaryIndex in primaryIndex.successor() ..< self.endIndex {
                if self[minimum] > self[secondaryIndex] {
                    minimum = secondaryIndex
                }
            }
            
            if primaryIndex != minimum {
                swap(&self[primaryIndex], &self[minimum])
            }
            
        }
    }
}


func testSelectionSort() {

    var integers = [2, 10, 5, 3, 8, 4, 7, 9, 6, 1]
    
    // Test that `selectionSort()` returns a sorted array.
    assert(integers.selectionSort() == [1, 2, 3, 4, 5, 6, 7, 8, 9, 10])
    
    // Test that `integers` has not changed.
    assert(integers == [2, 10, 5, 3, 8, 4, 7, 9, 6, 1])

    // mutate `integers` to become sorted.
    integers.selectionSortInPlace()
    
    // Test that `integers` is now sorted.
    assert(integers == [1, 2, 3, 4, 5, 6, 7, 8, 9, 10])

    print("All tests pass")
    
}

testSelectionSort()