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-rw-r--r--libtommath/bn_s_mp_mul_high_digs.c77
1 files changed, 77 insertions, 0 deletions
diff --git a/libtommath/bn_s_mp_mul_high_digs.c b/libtommath/bn_s_mp_mul_high_digs.c
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+++ b/libtommath/bn_s_mp_mul_high_digs.c
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+#include <tommath.h>
+#ifdef BN_S_MP_MUL_HIGH_DIGS_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.
+ *
+ * The library is free for all purposes without any express
+ * guarantee it works.
+ *
+ * Tom St Denis, tomstdenis@iahu.ca, http://math.libtomcrypt.org
+ */
+
+/* multiplies |a| * |b| and does not compute the lower digs digits
+ * [meant to get the higher part of the product]
+ */
+int
+s_mp_mul_high_digs (mp_int * a, mp_int * b, mp_int * c, int digs)
+{
+ mp_int t;
+ int res, pa, pb, ix, iy;
+ mp_digit u;
+ mp_word r;
+ mp_digit tmpx, *tmpt, *tmpy;
+
+ /* can we use the fast multiplier? */
+#ifdef BN_FAST_S_MP_MUL_HIGH_DIGS_C
+ if (((a->used + b->used + 1) < MP_WARRAY)
+ && MIN (a->used, b->used) < (1 << ((CHAR_BIT * sizeof (mp_word)) - (2 * DIGIT_BIT)))) {
+ return fast_s_mp_mul_high_digs (a, b, c, digs);
+ }
+#endif
+
+ if ((res = mp_init_size (&t, a->used + b->used + 1)) != MP_OKAY) {
+ return res;
+ }
+ t.used = a->used + b->used + 1;
+
+ pa = a->used;
+ pb = b->used;
+ for (ix = 0; ix < pa; ix++) {
+ /* clear the carry */
+ u = 0;
+
+ /* left hand side of A[ix] * B[iy] */
+ tmpx = a->dp[ix];
+
+ /* alias to the address of where the digits will be stored */
+ tmpt = &(t.dp[digs]);
+
+ /* alias for where to read the right hand side from */
+ tmpy = b->dp + (digs - ix);
+
+ for (iy = digs - ix; iy < pb; iy++) {
+ /* calculate the double precision result */
+ r = ((mp_word)*tmpt) +
+ ((mp_word)tmpx) * ((mp_word)*tmpy++) +
+ ((mp_word) u);
+
+ /* get the lower part */
+ *tmpt++ = (mp_digit) (r & ((mp_word) MP_MASK));
+
+ /* carry the carry */
+ u = (mp_digit) (r >> ((mp_word) DIGIT_BIT));
+ }
+ *tmpt = u;
+ }
+ mp_clamp (&t);
+ mp_exch (&t, c);
+ mp_clear (&t);
+ return MP_OKAY;
+}
+#endif