Improve quicksort to avoid worst case performance.

The current implementation will have quadratic behaviour for already
sorted arrays because it picks the last element as pivot. In an sorted
array this splits the array repeatedly into the biggest value and all
other values.

The implementation in this commit uses the *median of three* as pivot.

`janet -e "(sort (range 10000))"` to reproduce quadratic behaviour.

src/boot/boot.janet

@@ -790,35 +790,38 @@
 ###
 ###
 
-(defn- sort-part
-  [a lo hi by]
-  (def pivot (in a hi))
-  (var i lo)
-  (forv j lo hi
-    (def aj (in a j))
-    (when (by aj pivot)
-      (def ai (in a i))
-      (set (a i) aj)
-      (set (a j) ai)
-      (++ i)))
-  (set (a hi) (in a i))
-  (set (a i) pivot)
-  i)
+(defn- median-of-three [a b c]
+  (if (not= (> a b) (> a  c))
+    a
+    (if (not= (> b a) (> b c)) b c)))
 
-(defn- sort-help
-  [a lo hi by]
-  (when (> hi lo)
-    (def piv (sort-part a lo hi by))
-    (sort-help a lo (- piv 1) by)
-    (sort-help a (+ piv 1) hi by))
+(defn sort-help [a lo hi by]
+  (when (< lo hi)
+    (def pivot
+      (median-of-three (in a hi) (in a lo)
+                       (in a (math/floor (/ (+ lo hi) 2)))))
+    (var left lo)
+    (var right hi)
+    (while true
+      (while (by (in a left) pivot) (++ left))
+      (while (by pivot (in a right)) (-- right))
+      (when (<= left right)
+        (def tmp (in a left))
+        (set (a left) (in a right))
+        (set (a right) tmp)
+        (++ left)
+        (-- right))
+      (if (>= left right) (break)))
+    (sort-help a lo right by)
+    (sort-help a left hi by))
   a)
-
+    
 (defn sort
   "Sort an array in-place. Uses quick-sort and is not a stable sort."
   [a &opt by]
   (sort-help a 0 (- (length a) 1) (or by <)))
 
-(undef sort-part)
+(undef median-of-three)
 (undef sort-help)
 
 (defn sort-by