Import code from previous AssetBuilder version

thirdparty/libtommath/bn_mp_dr_reduce.c

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+#include "tommath_private.h"
+#ifdef BN_MP_DR_REDUCE_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
+ */
+
+/* reduce "x" in place modulo "n" using the Diminished Radix algorithm.
+ *
+ * Based on algorithm from the paper
+ *
+ * "Generating Efficient Primes for Discrete Log Cryptosystems"
+ *                 Chae Hoon Lim, Pil Joong Lee,
+ *          POSTECH Information Research Laboratories
+ *
+ * The modulus must be of a special format [see manual]
+ *
+ * Has been modified to use algorithm 7.10 from the LTM book instead
+ *
+ * Input x must be in the range 0 <= x <= (n-1)**2
+ */
+int mp_dr_reduce(mp_int *x, const mp_int *n, mp_digit k)
+{
+   int      err, i, m;
+   mp_word  r;
+   mp_digit mu, *tmpx1, *tmpx2;
+
+   /* m = digits in modulus */
+   m = n->used;
+
+   /* ensure that "x" has at least 2m digits */
+   if (x->alloc < (m + m)) {
+      if ((err = mp_grow(x, m + m)) != MP_OKAY) {
+         return err;
+      }
+   }
+
+   /* top of loop, this is where the code resumes if
+    * another reduction pass is required.
+    */
+top:
+   /* aliases for digits */
+   /* alias for lower half of x */
+   tmpx1 = x->dp;
+
+   /* alias for upper half of x, or x/B**m */
+   tmpx2 = x->dp + m;
+
+   /* set carry to zero */
+   mu = 0;
+
+   /* compute (x mod B**m) + k * [x/B**m] inline and inplace */
+   for (i = 0; i < m; i++) {
+      r         = ((mp_word)*tmpx2++ * (mp_word)k) + *tmpx1 + mu;
+      *tmpx1++  = (mp_digit)(r & MP_MASK);
+      mu        = (mp_digit)(r >> ((mp_word)DIGIT_BIT));
+   }
+
+   /* set final carry */
+   *tmpx1++ = mu;
+
+   /* zero words above m */
+   for (i = m + 1; i < x->used; i++) {
+      *tmpx1++ = 0;
+   }
+
+   /* clamp, sub and return */
+   mp_clamp(x);
+
+   /* if x >= n then subtract and reduce again
+    * Each successive "recursion" makes the input smaller and smaller.
+    */
+   if (mp_cmp_mag(x, n) != MP_LT) {
+      if ((err = s_mp_sub(x, n, x)) != MP_OKAY) {
+         return err;
+      }
+      goto top;
+   }
+   return MP_OKAY;
+}
+#endif
+
+/* ref:         HEAD -> master, tag: v1.1.0 */
+/* git commit:  08549ad6bc8b0cede0b357a9c341c5c6473a9c55 */
+/* commit time: 2019-01-28 20:32:32 +0100 */