static void two_complement_bitop(mp_int *a, mp_int *b, mp_int *c,
int (*mp_bitop)(mp_int *, mp_int *, mp_int *)) {
mp_int d;
if (SIGN(a) ^ SIGN(b)) {
/* exactly one of them is negative, so need to perform
* some magic. tommath stores a sign bit, but Perl 6 expects
* 2's complement */
mp_init(&d);
if (MP_NEG == SIGN(a)) {
grow_and_negate(a, USED(b), &d);
mp_bitop(&d, b, c);
} else {
grow_and_negate(b, USED(a), &d);
mp_bitop(a, &d, c);
}
if (DIGIT(c, USED(c) - 1) & ((mp_digit)1<<(mp_digit)(DIGIT_BIT - 1))) {
grow_and_negate(c, c->used, &d);
mp_copy(&d, c);
mp_neg(c, c);
}
mp_clear(&d);
} else {
mp_bitop(a, b, c);
}
}
static void two_complement_bitop(mp_int *a, mp_int *b, mp_int *c,
int (*mp_bitop)(mp_int *, mp_int *, mp_int *)) {
mp_int d;
mp_int e;
mp_int *f;
mp_int *g;
f = a;
g = b;
if (MP_NEG == SIGN(a)) {
mp_init(&d);
grow_and_negate(a, USED(b), &d);
f = &d;
}
if (MP_NEG == SIGN(b)) {
mp_init(&e);
grow_and_negate(b, USED(a), &e);
g = &e;
}
/* f and g now guaranteed to each point to positive bigints containing
* a 2s compliment representation of the values in a and b. If either
* a or b was negative, the representation is one tomath "digit" longer
* than it need be and sign extended.
*/
mp_bitop(f, g, c);
if (f == &d) mp_clear(&d);
if (g == &e) mp_clear(&e);
/* Use the fact that tomath clamps to detect results that should be
* signed. If we created extra tomath "digits" and they resulted in
* sign bits of 0, they have been clamped away. If the resulting sign
* bits were 1, they remain, and c will have more digits than either of
* original operands. Note this only works because we do not
* support NOR/NAND/NXOR, and so two zero sign bits can never create 1s.
*/
if (USED(c) > MAX(USED(a),USED(b))) {
int i;
for (i = 0; i < USED(c); i++) {
DIGIT(c, i) = (~DIGIT(c, i)) & MP_MASK;
}
mp_add_d(c, 1, c);
mp_neg(c, c);
}
}