Import code from previous AssetBuilder version

thirdparty/libtommath/bn_fast_s_mp_sqr.c

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+#include "tommath_private.h"
+#ifdef BN_FAST_S_MP_SQR_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
+ */
+
+/* the jist of squaring...
+ * you do like mult except the offset of the tmpx [one that
+ * starts closer to zero] can't equal the offset of tmpy.
+ * So basically you set up iy like before then you min it with
+ * (ty-tx) so that it never happens.  You double all those
+ * you add in the inner loop
+
+After that loop you do the squares and add them in.
+*/
+
+int fast_s_mp_sqr(const mp_int *a, mp_int *b)
+{
+   int       olduse, res, pa, ix, iz;
+   mp_digit   W[MP_WARRAY], *tmpx;
+   mp_word   W1;
+
+   /* grow the destination as required */
+   pa = a->used + a->used;
+   if (b->alloc < pa) {
+      if ((res = mp_grow(b, pa)) != MP_OKAY) {
+         return res;
+      }
+   }
+
+   /* number of output digits to produce */
+   W1 = 0;
+   for (ix = 0; ix < pa; ix++) {
+      int      tx, ty, iy;
+      mp_word  _W;
+      mp_digit *tmpy;
+
+      /* clear counter */
+      _W = 0;
+
+      /* get offsets into the two bignums */
+      ty = MIN(a->used-1, ix);
+      tx = ix - ty;
+
+      /* setup temp aliases */
+      tmpx = a->dp + tx;
+      tmpy = a->dp + ty;
+
+      /* this is the number of times the loop will iterrate, essentially
+         while (tx++ < a->used && ty-- >= 0) { ... }
+       */
+      iy = MIN(a->used-tx, ty+1);
+
+      /* now for squaring tx can never equal ty
+       * we halve the distance since they approach at a rate of 2x
+       * and we have to round because odd cases need to be executed
+       */
+      iy = MIN(iy, ((ty-tx)+1)>>1);
+
+      /* execute loop */
+      for (iz = 0; iz < iy; iz++) {
+         _W += (mp_word)*tmpx++ * (mp_word)*tmpy--;
+      }
+
+      /* double the inner product and add carry */
+      _W = _W + _W + W1;
+
+      /* even columns have the square term in them */
+      if (((unsigned)ix & 1u) == 0u) {
+         _W += (mp_word)a->dp[ix>>1] * (mp_word)a->dp[ix>>1];
+      }
+
+      /* store it */
+      W[ix] = _W & MP_MASK;
+
+      /* make next carry */
+      W1 = _W >> (mp_word)DIGIT_BIT;
+   }
+
+   /* setup dest */
+   olduse  = b->used;
+   b->used = a->used+a->used;
+
+   {
+      mp_digit *tmpb;
+      tmpb = b->dp;
+      for (ix = 0; ix < pa; ix++) {
+         *tmpb++ = W[ix] & MP_MASK;
+      }
+
+      /* clear unused digits [that existed in the old copy of c] */
+      for (; ix < olduse; ix++) {
+         *tmpb++ = 0;
+      }
+   }
+   mp_clamp(b);
+   return MP_OKAY;
+}
+#endif
+
+/* ref:         HEAD -> master, tag: v1.1.0 */
+/* git commit:  08549ad6bc8b0cede0b357a9c341c5c6473a9c55 */
+/* commit time: 2019-01-28 20:32:32 +0100 */