--- a/libtommath/bn_mp_sub.c	Wed Feb 28 22:11:39 2018 +0800
+++ b/libtommath/bn_mp_sub.c	Thu Oct 15 19:55:15 2020 +0800
@@ -1,59 +1,40 @@
-#include <tommath_private.h>
+#include "tommath_private.h"
 #ifdef BN_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 */
 
 /* high level subtraction (handles signs) */
-int
-mp_sub (mp_int * a, mp_int * b, mp_int * c)
+mp_err mp_sub(const mp_int *a, const mp_int *b, mp_int *c)
 {
-  int     sa, sb, res;
-
-  sa = a->sign;
-  sb = b->sign;
+   mp_sign sa = a->sign, sb = b->sign;
+   mp_err err;
 
-  if (sa != sb) {
-    /* subtract a negative from a positive, OR */
-    /* subtract a positive from a negative. */
-    /* In either case, ADD their magnitudes, */
-    /* and use the sign of the first number. */
-    c->sign = sa;
-    res = s_mp_add (a, b, c);
-  } else {
-    /* subtract a positive from a positive, OR */
-    /* subtract a negative from a negative. */
-    /* First, take the difference between their */
-    /* magnitudes, then... */
-    if (mp_cmp_mag (a, b) != MP_LT) {
-      /* Copy the sign from the first */
+   if (sa != sb) {
+      /* subtract a negative from a positive, OR */
+      /* subtract a positive from a negative. */
+      /* In either case, ADD their magnitudes, */
+      /* and use the sign of the first number. */
       c->sign = sa;
-      /* The first has a larger or equal magnitude */
-      res = s_mp_sub (a, b, c);
-    } else {
-      /* The result has the *opposite* sign from */
-      /* the first number. */
-      c->sign = (sa == MP_ZPOS) ? MP_NEG : MP_ZPOS;
-      /* The second has a larger magnitude */
-      res = s_mp_sub (b, a, c);
-    }
-  }
-  return res;
+      err = s_mp_add(a, b, c);
+   } else {
+      /* subtract a positive from a positive, OR */
+      /* subtract a negative from a negative. */
+      /* First, take the difference between their */
+      /* magnitudes, then... */
+      if (mp_cmp_mag(a, b) != MP_LT) {
+         /* Copy the sign from the first */
+         c->sign = sa;
+         /* The first has a larger or equal magnitude */
+         err = s_mp_sub(a, b, c);
+      } else {
+         /* The result has the *opposite* sign from */
+         /* the first number. */
+         c->sign = (sa == MP_ZPOS) ? MP_NEG : MP_ZPOS;
+         /* The second has a larger magnitude */
+         err = s_mp_sub(b, a, c);
+      }
+   }
+   return err;
 }
 
 #endif
-
-/* ref:         $Format:%D$ */
-/* git commit:  $Format:%H$ */
-/* commit time: $Format:%ai$ */