--- a/libtommath/bn_mp_sub_d.c	Tue May 26 23:27:26 2020 +0800
+++ b/libtommath/bn_mp_sub_d.c	Tue May 26 17:36:47 2020 +0200
@@ -1,27 +1,19 @@
 #include "tommath_private.h"
 #ifdef BN_MP_SUB_D_C
-/* LibTomMath, multiple-precision integer library -- Tom St Denis
- *
- * LibTomMath is a library that provides multiple-precision
- * integer arithmetic as well as number theoretic functionality.
- *
- * The library was designed directly after the MPI library by
- * Michael Fromberger but has been written from scratch with
- * additional optimizations in place.
- *
- * SPDX-License-Identifier: Unlicense
- */
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
 
 /* single digit subtraction */
-int mp_sub_d(const mp_int *a, mp_digit b, mp_int *c)
+mp_err mp_sub_d(const mp_int *a, mp_digit b, mp_int *c)
 {
-   mp_digit *tmpa, *tmpc, mu;
-   int       res, ix, oldused;
+   mp_digit *tmpa, *tmpc;
+   mp_err    err;
+   int       ix, oldused;
 
    /* grow c as required */
    if (c->alloc < (a->used + 1)) {
-      if ((res = mp_grow(c, a->used + 1)) != MP_OKAY) {
-         return res;
+      if ((err = mp_grow(c, a->used + 1)) != MP_OKAY) {
+         return err;
       }
    }
 
@@ -31,13 +23,13 @@
    if (a->sign == MP_NEG) {
       mp_int a_ = *a;
       a_.sign = MP_ZPOS;
-      res     = mp_add_d(&a_, b, c);
+      err     = mp_add_d(&a_, b, c);
       c->sign = MP_NEG;
 
       /* clamp */
       mp_clamp(c);
 
-      return res;
+      return err;
    }
 
    /* setup regs */
@@ -58,33 +50,25 @@
       c->sign = MP_NEG;
       c->used = 1;
    } else {
+      mp_digit mu = b;
+
       /* positive/size */
       c->sign = MP_ZPOS;
       c->used = a->used;
 
-      /* subtract first digit */
-      *tmpc    = *tmpa++ - b;
-      mu       = *tmpc >> ((sizeof(mp_digit) * (size_t)CHAR_BIT) - 1u);
-      *tmpc++ &= MP_MASK;
-
-      /* handle rest of the digits */
-      for (ix = 1; ix < a->used; ix++) {
+      /* subtract digits, mu is carry */
+      for (ix = 0; ix < a->used; ix++) {
          *tmpc    = *tmpa++ - mu;
-         mu       = *tmpc >> ((sizeof(mp_digit) * (size_t)CHAR_BIT) - 1u);
+         mu       = *tmpc >> (MP_SIZEOF_BITS(mp_digit) - 1u);
          *tmpc++ &= MP_MASK;
       }
    }
 
    /* zero excess digits */
-   while (ix++ < oldused) {
-      *tmpc++ = 0;
-   }
+   MP_ZERO_DIGITS(tmpc, oldused - ix);
+
    mp_clamp(c);
    return MP_OKAY;
 }
 
 #endif
-
-/* ref:         HEAD -> master, tag: v1.1.0 */
-/* git commit:  08549ad6bc8b0cede0b357a9c341c5c6473a9c55 */
-/* commit time: 2019-01-28 20:32:32 +0100 */