/* -*-c-*-
*
- * $Id: mp-jacobi.c,v 1.2 1999/12/10 23:19:02 mdw Exp $
+ * $Id$
*
* Compute Jacobi symbol
*
* MA 02111-1307, USA.
*/
-/*----- Revision history --------------------------------------------------*
- *
- * $Log: mp-jacobi.c,v $
- * Revision 1.2 1999/12/10 23:19:02 mdw
- * Improve error-checking.
- *
- * Revision 1.1 1999/11/22 20:50:37 mdw
- * Add support for computing Jacobi symbols.
- *
- */
-
/*----- Header files ------------------------------------------------------*/
#include "mp.h"
{
int s = 1;
+ assert(MP_ODDP(n));
+
/* --- Take copies of the arguments --- */
a = MP_COPY(a);
/* --- Main recursive mess, flattened out into something nice --- */
for (;;) {
+ mpw nn;
+ size_t e;
/* --- Some simple special cases --- */
MP_SHRINK(a);
-
- if (MP_LEN(a) == 0) {
+ if (MP_ZEROP(a)) {
s = 0;
goto done;
}
- /* --- Find the power-of-two factor in @a@ --- */
-
- {
- mpscan sc;
- mpw nn;
- unsigned e;
-
- /* --- Scan for a set bit --- */
+ /* --- Main case with powers of two --- */
- MP_SCAN(&sc, a);
- e = 0;
- while (MP_STEP(&sc) && !MP_BIT(&sc))
- e++;
-
- /* --- Do the shift --- */
-
- if (e)
- a = mp_lsr(a, a, e);
-
- /* --- Maybe adjust the sign of @s@ --- */
-
- nn = n->v[0] & 7;
- if ((e & 1) && (nn == 3 || nn == 5))
- s = -s;
-
- if (MP_LEN(a) == 1 && a->v[0] == 1)
- goto done;
-
- if ((nn & 3) == 3 && (a->v[0] & 3) == 3)
- s = -s;
- }
+ a = mp_odd(a, a, &e);
+ nn = n->v[0] & 7;
+ if ((e & 1) && (nn == 3 || nn == 5))
+ s = -s;
+ if (MP_LEN(a) == 1 && a->v[0] == 1)
+ goto done;
+ if ((nn & 3) == 3 && (a->v[0] & 3) == 3)
+ s = -s;
/* --- Reduce and swap --- */