Tweak sort: use insertion sort for small arrays

src/boot/boot.janet

@@ -795,34 +795,45 @@
     a
     (if (not= (> b a) (> b c)) b c)))
 
-(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))
+(defn- insertion-sort [a lo hi by]
+  (for i (+ lo 1) (+ hi 1)
+    (def temp (in a i))
+    (var j (- i 1))
+    (while (and (>= j lo) (by temp (in a j)))
+      (set (a (+ j 1)) (in a j))
+      (-- j))
+
+    (set (a (+ j 1)) temp))
   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 <)))
+  (default by <)
+  (def stack @[[0 (- (length a) 1)]])
+  (while (not (empty? stack))
+    (def [lo hi] (array/pop stack))
+    (when (< lo hi)
+      (when (< (- hi lo) 32) (insertion-sort a lo hi by) (break))
+      (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)))
+      (array/push stack [lo right])
+      (array/push stack [left hi])))
+  a)
 
 (undef median-of-three)
-(undef sort-help)
+(undef insertion-sort)
 
 (defn sort-by
   `Returns a new sorted array that compares elements by invoking