fixed rotation of 0-length vectors

fixes #33

Author: backtracking

CHANGES.md

@@ -1,4 +1,6 @@
 
+  - fixed division by zero in rotations of 0-length vectors
+    (reported by Nikolaus Huber)
   - fixed integer overflows in bound tests in `fill`, `blit`, `sub`
     (fix by Nikolaus Huber)
 

bitv.ml

@@ -406,10 +406,10 @@ let rec rotatel v d =
     rotater v (-d)
   else
   let n = v.length in
-  let d = d mod n in
-  if d == 0 then
+  if d == 0 || n == 0 then
     copy v
   else begin
+    let d = d mod n in
     let r = create n false in
     unsafe_blit v.bits 0 r.bits d (n - d); (* shiftl *)
     unsafe_blit v.bits (n - d) r.bits 0 d; (* wraparound ms to ls *)
@@ -421,10 +421,10 @@ and rotater v d =
     rotatel v (-d)
   else
   let n = v.length in
-  let d = d mod n in
-  if d == 0 then
+  if d == 0 || n == 0 then
     copy v
   else begin
+    let d = d mod n in
     let r = create n false in
     unsafe_blit v.bits d r.bits 0 (n - d); (* shiftr *)
     unsafe_blit v.bits 0 r.bits (n - d) d; (* wraparound ls to ms *)

test.ml

@@ -90,7 +90,12 @@ let () =
   let v = of_string "110101110" in
   assert (equal (rotatel v 1) (of_string "101011101"));
   assert (equal (rotatel v (-1)) (of_string "011010111"));
-  assert (equal (rotater v 1) (of_string "011010111"))
+  assert (equal (rotater v 1) (of_string "011010111"));
+  (* 0-length rotation *)
+  let v = create 0 false in
+  assert (length (rotatel v 0) = 0);
+  assert (length (rotater v 0) = 0);
+  ()
 
 let test_rotate n =
   let v = init n (fun _ -> Random.bool ()) in