carray: bugfix: allow negative indexing in ref operation.

* ffi.c (carray_ref): If the index is negative,
displace it by the length of the array. (Then if
it is still negative, the function will throw.)

1 files changed, 3 insertions, 0 deletions

diff --git a/ffi.c b/ffi.c
index 82a28b5b..67ced1f0 100644
--- a/ffi.c
+++ b/ffi.c
@@ -4888,6 +4888,9 @@ val carray_ref(val carray, val idx)
   struct carray *scry = carray_struct_checked(carray);
   cnum ix = c_num(idx);
 
+  if (ix < 0)
+    ix += scry->nelem;
+
   if (ix < 0 || (scry->nelem >= 0 && ix >= scry->nelem)) {
     uw_throwf(error_s, lit("~a: ~s: index ~s out of bounds"),
               self, carray, idx, nao);