Implement lua rounding style for __mod operator

1 files changed, 6 insertions, 0 deletions

diff --git a/src/openssl.c b/src/openssl.c
index 9dbcda7..69115ce 100644
--- a/src/openssl.c
+++ b/src/openssl.c
@@ -1809,6 +1809,12 @@ static int bn__mod(lua_State *L) {
         if (!BN_mod(r, a, b, getctx(L)))
                 return auxL_error(L, auxL_EOPENSSL, "bignum:__mod");
 
+        /* lua has different rounding behaviour for mod than C */
+        if (!BN_is_zero(r) && (BN_is_negative(a) ^ BN_is_negative(b))) {
+                if (!BN_add(r, r, b))
+                        return auxL_error(L, auxL_EOPENSSL, "bignum:__mod");
+        }
+
         return 1;
 } /* bn__mod() */