Import code from previous AssetBuilder version

thirdparty/libtommath/bn_mp_mul_2.c

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+#include "tommath_private.h"
+#ifdef BN_MP_MUL_2_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
+ */
+
+/* b = a*2 */
+int mp_mul_2(const mp_int *a, mp_int *b)
+{
+   int     x, res, oldused;
+
+   /* grow to accomodate result */
+   if (b->alloc < (a->used + 1)) {
+      if ((res = mp_grow(b, a->used + 1)) != MP_OKAY) {
+         return res;
+      }
+   }
+
+   oldused = b->used;
+   b->used = a->used;
+
+   {
+      mp_digit r, rr, *tmpa, *tmpb;
+
+      /* alias for source */
+      tmpa = a->dp;
+
+      /* alias for dest */
+      tmpb = b->dp;
+
+      /* carry */
+      r = 0;
+      for (x = 0; x < a->used; x++) {
+
+         /* get what will be the *next* carry bit from the
+          * MSB of the current digit
+          */
+         rr = *tmpa >> (mp_digit)(DIGIT_BIT - 1);
+
+         /* now shift up this digit, add in the carry [from the previous] */
+         *tmpb++ = ((*tmpa++ << 1uL) | r) & MP_MASK;
+
+         /* copy the carry that would be from the source
+          * digit into the next iteration
+          */
+         r = rr;
+      }
+
+      /* new leading digit? */
+      if (r != 0u) {
+         /* add a MSB which is always 1 at this point */
+         *tmpb = 1;
+         ++(b->used);
+      }
+
+      /* now zero any excess digits on the destination
+       * that we didn't write to
+       */
+      tmpb = b->dp + b->used;
+      for (x = b->used; x < oldused; x++) {
+         *tmpb++ = 0;
+      }
+   }
+   b->sign = a->sign;
+   return MP_OKAY;
+}
+#endif
+
+/* ref:         HEAD -> master, tag: v1.1.0 */
+/* git commit:  08549ad6bc8b0cede0b357a9c341c5c6473a9c55 */
+/* commit time: 2019-01-28 20:32:32 +0100 */