Import code from previous AssetBuilder version

thirdparty/libtommath/bn_mp_sub.c

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+#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.
+ *
+ * SPDX-License-Identifier: Unlicense
+ */
+
+/* high level subtraction (handles signs) */
+int mp_sub(const mp_int *a, const mp_int *b, mp_int *c)
+{
+   int     sa, sb, res;
+
+   sa = a->sign;
+   sb = b->sign;
+
+   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 */
+         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;
+}
+
+#endif
+
+/* ref:         HEAD -> master, tag: v1.1.0 */
+/* git commit:  08549ad6bc8b0cede0b357a9c341c5c6473a9c55 */
+/* commit time: 2019-01-28 20:32:32 +0100 */