Merge pull request #502 from czurnieden/comba_cutoff_squaring

Raised upper limit for Comba squaring

Author: sjaeckel

demo/test.c

@@ -1828,6 +1828,51 @@ static int test_mp_root_n(void)
    return EXIT_FAILURE;
 }
 
+/* Less error-prone than -1 + 2^n with mp_2expt */
+static mp_err s_fill_with_ones(mp_int *a, int size)
+{
+   int i;
+   mp_err err = MP_OKAY;
+
+   mp_zero(a);
+
+   if ((err = mp_grow(a, size)) != MP_OKAY)        goto LTM_ERR;
+   for (i = 0; i < size; i++) {
+      a->dp[i] = (mp_digit)MP_MASK;
+      a->used++;
+   }
+
+LTM_ERR:
+   return err;
+}
+
+static int test_s_mp_sqr_comba(void)
+{
+   mp_int a, r1, r2;
+   int i, j;
+
+   DOR(mp_init_multi(&a, &r1, &r2, NULL));
+
+   for (i = 1; i <= MP_MAX_COMBA; i++) {
+      DO(s_fill_with_ones(&a, i));
+      DO(s_mp_sqr_comba(&a, &r1));
+      DO(s_mp_sqr(&a, &r2));
+      EXPECT(mp_cmp(&r1, &r2) == MP_EQ);
+      for (j = 0; j < 20; j++) {
+         DO(mp_rand(&a, i));
+         DO(s_mp_sqr_comba(&a, &r1));
+         DO(s_mp_sqr(&a, &r2));
+         EXPECT(mp_cmp(&r1, &r2) == MP_EQ);
+      }
+   }
+
+   mp_clear_multi(&a, &r1, &r2, NULL);
+   return EXIT_SUCCESS;
+LBL_ERR:
+   mp_clear_multi(&a, &r1, &r2, NULL);
+   return EXIT_FAILURE;
+}
+
 static int test_s_mp_mul_balance(void)
 {
    mp_int a, b, c;
@@ -2328,6 +2373,8 @@ static int unit_tests(int argc, char **argv)
       T1(mp_xor, MP_XOR),
       T3(s_mp_div_recursive, ONLY_PUBLIC_API, S_MP_DIV_RECURSIVE, S_MP_DIV_SCHOOL),
       T3(s_mp_div_small, ONLY_PUBLIC_API, S_MP_DIV_SMALL, S_MP_DIV_SCHOOL),
+      /* s_mp_mul_comba not (yet) testable because s_mp_mul branches to s_mp_mul_comba automatically */
+      T2(s_mp_sqr_comba, ONLY_PUBLIC_API, S_MP_SQR_COMBA),
       T2(s_mp_mul_balance, ONLY_PUBLIC_API, S_MP_MUL_BALANCE),
       T2(s_mp_mul_karatsuba, ONLY_PUBLIC_API, S_MP_MUL_KARATSUBA),
       T2(s_mp_sqr_karatsuba, ONLY_PUBLIC_API, S_MP_SQR_KARATSUBA),

mp_mul.c

@@ -23,7 +23,7 @@ mp_err mp_mul(const mp_int *a, const mp_int *b, mp_int *c)
    } else if ((a == b) &&
               MP_HAS(S_MP_SQR_COMBA) && /* can we use the fast comba multiplier? */
               (((a->used * 2) + 1) < MP_WARRAY) &&
-              (a->used < (MP_MAX_COMBA / 2))) {
+              (a->used <= MP_MAX_COMBA)) {
       err = s_mp_sqr_comba(a, c);
    } else if ((a == b) &&
               MP_HAS(S_MP_SQR)) {