[Global]: loading init

deps/libtommath/s_mp_sqr.c

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+#include "tommath_private.h"
+#ifdef S_MP_SQR_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+/* low level squaring, b = a*a, HAC pp.596-597, Algorithm 14.16 */
+mp_err s_mp_sqr(const mp_int *a, mp_int *b)
+{
+   mp_int   t;
+   int      ix, pa;
+   mp_err   err;
+
+   pa = a->used;
+   if ((err = mp_init_size(&t, (2 * pa) + 1)) != MP_OKAY) {
+      return err;
+   }
+
+   /* default used is maximum possible size */
+   t.used = (2 * pa) + 1;
+
+   for (ix = 0; ix < pa; ix++) {
+      mp_digit u;
+      int iy;
+
+      /* first calculate the digit at 2*ix */
+      /* calculate double precision result */
+      mp_word r = (mp_word)t.dp[2*ix] +
+                  ((mp_word)a->dp[ix] * (mp_word)a->dp[ix]);
+
+      /* store lower part in result */
+      t.dp[ix+ix] = (mp_digit)(r & (mp_word)MP_MASK);
+
+      /* get the carry */
+      u           = (mp_digit)(r >> (mp_word)MP_DIGIT_BIT);
+
+      for (iy = ix + 1; iy < pa; iy++) {
+         /* first calculate the product */
+         r       = (mp_word)a->dp[ix] * (mp_word)a->dp[iy];
+
+         /* now calculate the double precision result, note we use
+          * addition instead of *2 since it's easier to optimize.
+          */
+         /* Some architectures and/or compilers seem to prefer a bit-shift nowadays */
+         r       = (mp_word)t.dp[ix + iy] + (r<<1) + (mp_word)u;
+
+         /* store lower part */
+         t.dp[ix + iy] = (mp_digit)(r & (mp_word)MP_MASK);
+
+         /* get carry */
+         u       = (mp_digit)(r >> (mp_word)MP_DIGIT_BIT);
+      }
+      /* propagate upwards */
+      while (u != 0uL) {
+         mp_digit tmp;
+         /*
+            "u" can get bigger than MP_DIGIT_MAX and would need a bigger type
+            for the sum (mp_word). That is costly if mp_word is not a native
+            integer but a bigint from the compiler library. We do a manual
+            multiword addition instead.
+          */
+         /* t.dp[ix + iy] has been masked off by MP_MASK and is hence of the correct size
+            and we can just add the lower part of "u". Carry is guaranteed to fit into
+            the type used for mp_digit, too, so we can extract it later. */
+         tmp = t.dp[ix + iy] + (u & MP_MASK);
+         /* t.dp[ix + iy] is set to the result minus the carry, carry is still in "tmp" */
+         t.dp[ix + iy] = tmp & MP_MASK;
+         /* Add high part of "u" and the carry from "tmp" to get the next "u" */
+         u = (u >> MP_DIGIT_BIT) + (tmp >> MP_DIGIT_BIT);
+         ++iy;
+      }
+   }
+
+   mp_clamp(&t);
+   mp_exch(&t, b);
+   mp_clear(&t);
+   return MP_OKAY;
+}
+#endif