--- a/libtommath/bn_mp_sub_d.c	Wed Feb 28 22:11:39 2018 +0800
+++ b/libtommath/bn_mp_sub_d.c	Thu Oct 15 19:55:15 2020 +0800
@@ -1,93 +1,74 @@
-#include <tommath_private.h>
+#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.
- *
- * The library is free for all purposes without any express
- * guarantee it works.
- *
- * Tom St Denis, [email protected], http://libtom.org
- */
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
 
 /* single digit subtraction */
-int
-mp_sub_d (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;
-     }
-  }
+   /* grow c as required */
+   if (c->alloc < (a->used + 1)) {
+      if ((err = mp_grow(c, a->used + 1)) != MP_OKAY) {
+         return err;
+      }
+   }
 
-  /* if a is negative just do an unsigned
-   * addition [with fudged signs]
-   */
-  if (a->sign == MP_NEG) {
-     a->sign = MP_ZPOS;
-     res     = mp_add_d(a, b, c);
-     a->sign = c->sign = MP_NEG;
+   /* if a is negative just do an unsigned
+    * addition [with fudged signs]
+    */
+   if (a->sign == MP_NEG) {
+      mp_int a_ = *a;
+      a_.sign = MP_ZPOS;
+      err     = mp_add_d(&a_, b, c);
+      c->sign = MP_NEG;
 
-     /* clamp */
-     mp_clamp(c);
+      /* clamp */
+      mp_clamp(c);
 
-     return res;
-  }
+      return err;
+   }
 
-  /* setup regs */
-  oldused = c->used;
-  tmpa    = a->dp;
-  tmpc    = c->dp;
+   /* setup regs */
+   oldused = c->used;
+   tmpa    = a->dp;
+   tmpc    = c->dp;
 
-  /* if a <= b simply fix the single digit */
-  if (((a->used == 1) && (a->dp[0] <= b)) || (a->used == 0)) {
-     if (a->used == 1) {
-        *tmpc++ = b - *tmpa;
-     } else {
-        *tmpc++ = b;
-     }
-     ix      = 1;
+   /* if a <= b simply fix the single digit */
+   if (((a->used == 1) && (a->dp[0] <= b)) || (a->used == 0)) {
+      if (a->used == 1) {
+         *tmpc++ = b - *tmpa;
+      } else {
+         *tmpc++ = b;
+      }
+      ix      = 1;
 
-     /* negative/1digit */
-     c->sign = MP_NEG;
-     c->used = 1;
-  } else {
-     /* positive/size */
-     c->sign = MP_ZPOS;
-     c->used = a->used;
+      /* negative/1digit */
+      c->sign = MP_NEG;
+      c->used = 1;
+   } else {
+      mp_digit mu = b;
 
-     /* subtract first digit */
-     *tmpc    = *tmpa++ - b;
-     mu       = *tmpc >> ((sizeof(mp_digit) * CHAR_BIT) - 1);
-     *tmpc++ &= MP_MASK;
+      /* positive/size */
+      c->sign = MP_ZPOS;
+      c->used = a->used;
 
-     /* handle rest of the digits */
-     for (ix = 1; ix < a->used; ix++) {
-        *tmpc    = *tmpa++ - mu;
-        mu       = *tmpc >> ((sizeof(mp_digit) * CHAR_BIT) - 1);
-        *tmpc++ &= MP_MASK;
-     }
-  }
+      /* subtract digits, mu is carry */
+      for (ix = 0; ix < a->used; ix++) {
+         *tmpc    = *tmpa++ - mu;
+         mu       = *tmpc >> (MP_SIZEOF_BITS(mp_digit) - 1u);
+         *tmpc++ &= MP_MASK;
+      }
+   }
 
-  /* zero excess digits */
-  while (ix++ < oldused) {
-     *tmpc++ = 0;
-  }
-  mp_clamp(c);
-  return MP_OKAY;
+   /* zero excess digits */
+   MP_ZERO_DIGITS(tmpc, oldused - ix);
+
+   mp_clamp(c);
+   return MP_OKAY;
 }
 
 #endif
-
-/* ref:         $Format:%D$ */
-/* git commit:  $Format:%H$ */
-/* commit time: $Format:%ai$ */