Implemented Shell sort as Array extension

Sort.playground/Pages/Shell.xcplaygroundpage/Contents.swift

@@ -0,0 +1,34 @@
+//: [Previous](@previous)
+
+import Foundation
+
+// Shell sort is an improved version of insertion sort. The original list is broken into smaller sublists and then individually sorted using insertion sort.
+
+extension Array where Element: Comparable {
+
+    public mutating func shellSorted(order sign: (Element, Element) -> Bool) {
+        var subcount = self.count / 2
+
+        while subcount > 0 {
+            for i in 0..<self.count {
+                let temp = self[i]
+                var j = i
+
+                while j >= subcount && sign(self[j - subcount], temp) {
+                    self[j] = self[j - subcount]
+                    j -= subcount
+                }
+                self[j] = temp
+            }
+            subcount /= 2
+        }
+
+    }
+}
+
+//: Usage
+
+var data = [4, 1, 5, 6, 1, 2, 9, 8, 5, 4, 2, 7, 6]
+data.shellSorted(order: >)
+
+//: [Next](@next)

Sort.playground/contents.xcplayground

@@ -6,5 +6,6 @@
         <page name='Quick-Hoare-Scheme'/>
         <page name='Quick-Lomuto-Scheme'/>
         <page name='Bubble'/>
+        <page name='Shell'/>
     </pages>
 </playground>