--- a/libtommath/bn_s_mp_sub.c	Wed Feb 28 22:11:39 2018 +0800
+++ b/libtommath/bn_s_mp_sub.c	Thu Oct 15 19:55:15 2020 +0800
@@ -1,89 +1,71 @@
-#include <tommath_private.h>
+#include "tommath_private.h"
 #ifdef BN_S_MP_SUB_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 */
 
 /* low level subtraction (assumes |a| > |b|), HAC pp.595 Algorithm 14.9 */
-int
-s_mp_sub (mp_int * a, mp_int * b, mp_int * c)
+mp_err s_mp_sub(const mp_int *a, const mp_int *b, mp_int *c)
 {
-  int     olduse, res, min, max;
+   int    olduse, min, max;
+   mp_err err;
 
-  /* find sizes */
-  min = b->used;
-  max = a->used;
+   /* find sizes */
+   min = b->used;
+   max = a->used;
 
-  /* init result */
-  if (c->alloc < max) {
-    if ((res = mp_grow (c, max)) != MP_OKAY) {
-      return res;
-    }
-  }
-  olduse = c->used;
-  c->used = max;
+   /* init result */
+   if (c->alloc < max) {
+      if ((err = mp_grow(c, max)) != MP_OKAY) {
+         return err;
+      }
+   }
+   olduse = c->used;
+   c->used = max;
 
-  {
-    mp_digit u, *tmpa, *tmpb, *tmpc;
-    int i;
+   {
+      mp_digit u, *tmpa, *tmpb, *tmpc;
+      int i;
 
-    /* alias for digit pointers */
-    tmpa = a->dp;
-    tmpb = b->dp;
-    tmpc = c->dp;
+      /* alias for digit pointers */
+      tmpa = a->dp;
+      tmpb = b->dp;
+      tmpc = c->dp;
 
-    /* set carry to zero */
-    u = 0;
-    for (i = 0; i < min; i++) {
-      /* T[i] = A[i] - B[i] - U */
-      *tmpc = (*tmpa++ - *tmpb++) - u;
+      /* set carry to zero */
+      u = 0;
+      for (i = 0; i < min; i++) {
+         /* T[i] = A[i] - B[i] - U */
+         *tmpc = (*tmpa++ - *tmpb++) - u;
 
-      /* U = carry bit of T[i]
-       * Note this saves performing an AND operation since
-       * if a carry does occur it will propagate all the way to the
-       * MSB.  As a result a single shift is enough to get the carry
-       */
-      u = *tmpc >> ((mp_digit)((CHAR_BIT * sizeof(mp_digit)) - 1));
+         /* U = carry bit of T[i]
+          * Note this saves performing an AND operation since
+          * if a carry does occur it will propagate all the way to the
+          * MSB.  As a result a single shift is enough to get the carry
+          */
+         u = *tmpc >> (MP_SIZEOF_BITS(mp_digit) - 1u);
 
-      /* Clear carry from T[i] */
-      *tmpc++ &= MP_MASK;
-    }
+         /* Clear carry from T[i] */
+         *tmpc++ &= MP_MASK;
+      }
 
-    /* now copy higher words if any, e.g. if A has more digits than B  */
-    for (; i < max; i++) {
-      /* T[i] = A[i] - U */
-      *tmpc = *tmpa++ - u;
+      /* now copy higher words if any, e.g. if A has more digits than B  */
+      for (; i < max; i++) {
+         /* T[i] = A[i] - U */
+         *tmpc = *tmpa++ - u;
 
-      /* U = carry bit of T[i] */
-      u = *tmpc >> ((mp_digit)((CHAR_BIT * sizeof(mp_digit)) - 1));
+         /* U = carry bit of T[i] */
+         u = *tmpc >> (MP_SIZEOF_BITS(mp_digit) - 1u);
 
-      /* Clear carry from T[i] */
-      *tmpc++ &= MP_MASK;
-    }
+         /* Clear carry from T[i] */
+         *tmpc++ &= MP_MASK;
+      }
 
-    /* clear digits above used (since we may not have grown result above) */
-    for (i = c->used; i < olduse; i++) {
-      *tmpc++ = 0;
-    }
-  }
+      /* clear digits above used (since we may not have grown result above) */
+      MP_ZERO_DIGITS(tmpc, olduse - c->used);
+   }
 
-  mp_clamp (c);
-  return MP_OKAY;
+   mp_clamp(c);
+   return MP_OKAY;
 }
 
 #endif
-
-/* ref:         $Format:%D$ */
-/* git commit:  $Format:%H$ */
-/* commit time: $Format:%ai$ */