* fns.c (Frandom): Fix rare bug where the result isn't random.

2 files changed, 13 insertions, 2 deletions

diff --git a/src/ChangeLog b/src/ChangeLog
index 06e4c3291b5..f7f4a5f03b2 100644
--- a/src/ChangeLog
+++ b/src/ChangeLog
@@ -1,5 +1,7 @@
 2014-03-19  Paul Eggert  <eggert@cs.ucla.edu>
 
+        * fns.c (Frandom): Fix rare bug where the result isn't random.
+
         Fix porting inconsistency about rounding to even.
         * floatfns.c (emacs_rint) [!HAVE_RINT]: Round to even.
         This way, the unusual !HAVE_RINT case acts like the usual
diff --git a/src/fns.c b/src/fns.c
index 7b3d41d5374..499e4b490a6 100644
--- a/src/fns.c
+++ b/src/fns.c
@@ -79,8 +79,17 @@ See Info node `(elisp)Random Numbers' for more details.  */)
     seed_random (SSDATA (limit), SBYTES (limit));
 
   val = get_random ();
-  if (NATNUMP (limit) && XFASTINT (limit) != 0)
-    val %= XFASTINT (limit);
+  if (INTEGERP (limit) && 0 < XINT (limit))
+    while (true)
+      {
+        /* Return the remainder, except reject the rare case where
+           get_random returns a number so close to INTMASK that the
+           remainder isn't random.  */
+        EMACS_INT remainder = val % XINT (limit);
+        if (val - remainder <= INTMASK - XINT (limit) + 1)
+          return make_number (remainder);
+        val = get_random ();
+      }
   return make_number (val);
 }