Round bignums consistently with other integers

* src/bignum.c (mpz_bufsize): New function.
(bignum_bufsize): Use it.
(mpz_get_d_rounded): New function.
(bignum_to_double): Use it.
* src/bignum.c (bignum_to_double):
* src/data.c (bignum_arith_driver):
When converting bignums to double, round instead of
truncating, to be consistent with what happens with fixnums.
* test/src/floatfns-tests.el (bignum-to-float): Test rounding.

1 files changed, 6 insertions, 0 deletions

diff --git a/test/src/floatfns-tests.el b/test/src/floatfns-tests.el
index 14576b603c0..61b1c25743d 100644
--- a/test/src/floatfns-tests.el
+++ b/test/src/floatfns-tests.el
@@ -35,6 +35,12 @@
   (should-error (fround 0) :type 'wrong-type-argument))
 
 (ert-deftest bignum-to-float ()
+  ;; 122 because we want to go as big as possible to provoke a rounding error,
+  ;; but not too big: 2**122 < 10**37 < 2**123, and the C standard says
+  ;; 10**37 <= DBL_MAX so 2**122 cannot overflow as a double.
+  (let ((a (1- (ash 1 122))))
+    (should (or (eql a (1- (floor (float a))))
+                (eql a (floor (float a))))))
   (should (eql (float (+ most-positive-fixnum 1))
                (+ (float most-positive-fixnum) 1))))