Store the list length in sort.

Computing length each time is very unnecessary.

1 files changed, 12 insertions, 8 deletions

diff --git a/csc/sort.csc b/csc/sort.csc
index 62b1c9d..6be1e16 100644
--- a/csc/sort.csc
+++ b/csc/sort.csc
@@ -7,13 +7,13 @@
   (begin
 
 
-    (define (sort cmp xs)
-      (let ((len (length xs)))
-        (if (<= len 1)
-            xs
-          (let-values (((half-a half-b) (split-at (truncate-quotient len 2) xs)))
-            (let ((sorted-half-a (sort cmp half-a))
-                  (sorted-half-b (sort cmp half-b)))
+    (define (sort-len cmp xs len)
+      (if (<= len 1)
+        xs
+        (let ((n/2 (truncate-quotient len 2)))
+          (let-values (((half-a half-b) (split-at n/2 xs)))
+            (let ((sorted-half-a (sort-len cmp half-a n/2))
+                  (sorted-half-b (sort-len cmp half-b (- len n/2))))
               (let loop ((a sorted-half-a)
                          (b sorted-half-b)
                          (acc '()))
@@ -21,4 +21,8 @@
                       ((null? b) (revappend acc a))
                       ((cmp (car b) (car a))
                        (loop a (cdr b) (cons (car b) acc)))
-                      (else (loop (cdr a) b (cons (car a) acc))))))))))))
+                      (else (loop (cdr a) b (cons (car a) acc))))))))))
+
+
+    (define (sort cmp xs)
+      (sort-len cmp xs (length xs)))))