Import code from previous AssetBuilder version

thirdparty/libtommath/bn_mp_rshd.c

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+#include "tommath_private.h"
+#ifdef BN_MP_RSHD_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
+ */
+
+/* shift right a certain amount of digits */
+void mp_rshd(mp_int *a, int b)
+{
+   int     x;
+
+   /* if b <= 0 then ignore it */
+   if (b <= 0) {
+      return;
+   }
+
+   /* if b > used then simply zero it and return */
+   if (a->used <= b) {
+      mp_zero(a);
+      return;
+   }
+
+   {
+      mp_digit *bottom, *top;
+
+      /* shift the digits down */
+
+      /* bottom */
+      bottom = a->dp;
+
+      /* top [offset into digits] */
+      top = a->dp + b;
+
+      /* this is implemented as a sliding window where
+       * the window is b-digits long and digits from
+       * the top of the window are copied to the bottom
+       *
+       * e.g.
+
+       b-2 | b-1 | b0 | b1 | b2 | ... | bb |   ---->
+                   /\                   |      ---->
+                    \-------------------/      ---->
+       */
+      for (x = 0; x < (a->used - b); x++) {
+         *bottom++ = *top++;
+      }
+
+      /* zero the top digits */
+      for (; x < a->used; x++) {
+         *bottom++ = 0;
+      }
+   }
+
+   /* remove excess digits */
+   a->used -= b;
+}
+#endif
+
+/* ref:         HEAD -> master, tag: v1.1.0 */
+/* git commit:  08549ad6bc8b0cede0b357a9c341c5c6473a9c55 */
+/* commit time: 2019-01-28 20:32:32 +0100 */