all tasks

Task/Sorting-algorithms-Quicksort/Go/sorting-algorithms-quicksort.go

@@ -0,0 +1,93 @@
+package main
+
+import "fmt"
+
+func main() {
+    list := []int{31, 41, 59, 26, 53, 58, 97, 93, 23, 84}
+    fmt.Println("unsorted:", list)
+
+    quicksort(list)
+    fmt.Println("sorted!  ", list)
+}
+
+func quicksort(a []int) {
+    var pex func(int, int)
+    pex = func(lower, upper int) {
+        for {
+            switch upper - lower {
+            case -1, 0: // 0 or 1 item in segment.  nothing to do here!
+                return
+            case 1: // 2 items in segment
+                // < operator respects strict weak order
+                if a[upper] < a[lower] {
+                    // a quick exchange and we're done.
+                    a[upper], a[lower] = a[lower], a[upper]
+                }
+                return
+            // Hoare suggests optimized sort-3 or sort-4 algorithms here,
+            // but does not provide an algorithm.
+            }
+
+            // Hoare stresses picking a bound in a way to avoid worst case
+            // behavior, but offers no suggestions other than picking a
+            // random element.  A function call to get a random number is
+            // relatively expensive, so the method used here is to simply
+            // choose the middle element.  This at least avoids worst case
+            // behavior for the obvious common case of an already sorted list.
+            bx := (upper + lower) / 2
+            b := a[bx]  // b = Hoare's "bound" (aka "pivot")
+            lp := lower // lp = Hoare's "lower pointer"
+            up := upper // up = Hoare's "upper pointer"
+        outer:
+            for {
+                // use < operator to respect strict weak order
+                for lp < upper && !(b < a[lp]) {
+                    lp++
+                }
+                for {
+                    if lp > up {
+                        // "pointers crossed!"
+                        break outer
+                    }
+                    // < operator for strict weak order
+                    if a[up] < b {
+                        break // inner
+                    }
+                    up--
+                }
+                // exchange
+                a[lp], a[up] = a[up], a[lp]
+                lp++
+                up--
+            }
+            // segment boundary is between up and lp, but lp-up might be
+            // 1 or 2, so just call segment boundary between lp-1 and lp.
+            if bx < lp {
+                // bound was in lower segment
+                if bx < lp-1 {
+                    // exchange bx with lp-1
+                    a[bx], a[lp-1] = a[lp-1], b
+                }
+                up = lp - 2
+            } else {
+                // bound was in upper segment
+                if bx > lp {
+                    // exchange
+                    a[bx], a[lp] = a[lp], b
+                }
+                up = lp - 1
+                lp++
+            }
+            // "postpone the larger of the two segments" = recurse on
+            // the smaller segment, then iterate on the remaining one.
+            if up-lower < upper-lp {
+                pex(lower, up)
+                lower = lp
+            } else {
+                pex(lp, upper)
+                upper = up
+            }
+        }
+    }
+    pex(0, len(a)-1)
+}