Import code from previous AssetBuilder version

thirdparty/libtommath/bn_mp_expt_d_ex.c

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+#include "tommath_private.h"
+#ifdef BN_MP_EXPT_D_EX_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
+ */
+
+/* calculate c = a**b  using a square-multiply algorithm */
+int mp_expt_d_ex(const mp_int *a, mp_digit b, mp_int *c, int fast)
+{
+   int     res;
+   unsigned int x;
+
+   mp_int  g;
+
+   if ((res = mp_init_copy(&g, a)) != MP_OKAY) {
+      return res;
+   }
+
+   /* set initial result */
+   mp_set(c, 1uL);
+
+   if (fast != 0) {
+      while (b > 0u) {
+         /* if the bit is set multiply */
+         if ((b & 1u) != 0u) {
+            if ((res = mp_mul(c, &g, c)) != MP_OKAY) {
+               mp_clear(&g);
+               return res;
+            }
+         }
+
+         /* square */
+         if (b > 1u) {
+            if ((res = mp_sqr(&g, &g)) != MP_OKAY) {
+               mp_clear(&g);
+               return res;
+            }
+         }
+
+         /* shift to next bit */
+         b >>= 1;
+      }
+   } else {
+      for (x = 0; x < (unsigned)DIGIT_BIT; x++) {
+         /* square */
+         if ((res = mp_sqr(c, c)) != MP_OKAY) {
+            mp_clear(&g);
+            return res;
+         }
+
+         /* if the bit is set multiply */
+         if ((b & ((mp_digit)1 << (DIGIT_BIT - 1))) != 0u) {
+            if ((res = mp_mul(c, &g, c)) != MP_OKAY) {
+               mp_clear(&g);
+               return res;
+            }
+         }
+
+         /* shift to next bit */
+         b <<= 1;
+      }
+   } /* if ... else */
+
+   mp_clear(&g);
+   return MP_OKAY;
+}
+#endif
+
+/* ref:         HEAD -> master, tag: v1.1.0 */
+/* git commit:  08549ad6bc8b0cede0b357a9c341c5c6473a9c55 */
+/* commit time: 2019-01-28 20:32:32 +0100 */