Fix bug in bignum single-digit addition.

Sigh; this is a continuation of the same topic that was
addressed in November 14, 2016: "Fix bug in bignum addition".
Commit SHA: c30a96120f53b960db56bc05a7ce6310bb2528f5.

Though I fixed s_mp_add, s_mp_add_d has the same problem.  Two
digit-sized quantities are added together in the digit-sized
type, and so the CARRYOUT(w) from the result is always zero.

This bug causes the following consequences (using behavior
under 32 bit as an example):

  1. Wrong arithmetic:

      (succ (pred (expt 2 32)) -> 0

  2. Wrong conversion from decimal:

      4294967296 -> 0
      4294967297 -> 1
      4294967298 -> 2
      4294967299 -> 3

     As well as all values that begin with the above digit
     sequences.

  4. Wrong conversion from floating-point.

      (toint 4294967296.0) -> 0

* mpi/mpi.c (s_mp_add_d): Add missing cast to the initial
addition that adds the incoming digit d to the least
significant limb of the bignum, so that the addition is done
in the wider mp_word type, allowing CARRYOUT(w) to calculate a
nonzero k value.

1 files changed, 1 insertions, 1 deletions

diff --git a/mpi/mpi.c b/mpi/mpi.c
index 472011d4..198a5a86 100644
--- a/mpi/mpi.c
+++ b/mpi/mpi.c
@@ -3300,7 +3300,7 @@ mp_err s_mp_add_d(mp_int *mp, mp_digit d) /* unsigned digit addition */
   mp_size ix = 1, used = USED(mp);
   mp_digit *dp = DIGITS(mp);
 
-  w = dp[0] + d;
+  w = convert(mp_word, dp[0]) + d;
   dp[0] = ACCUM(w);
   k = CARRYOUT(w);