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#include "tommath_private.h"
#ifdef S_MP_DIV_SCHOOL_C
/* LibTomMath, multiple-precision integer library -- Tom St Denis */
/* SPDX-License-Identifier: Unlicense */
/* integer signed division.
* c*b + d == a [e.g. a/b, c=quotient, d=remainder]
* HAC pp.598 Algorithm 14.20
*
* Note that the description in HAC is horribly
* incomplete. For example, it doesn't consider
* the case where digits are removed from 'x' in
* the inner loop. It also doesn't consider the
* case that y has fewer than three digits, etc..
*
* The overall algorithm is as described as
* 14.20 from HAC but fixed to treat these cases.
*/
mp_err s_mp_div_school(const mp_int *a, const mp_int *b, mp_int *c, mp_int *d)
{
mp_int q, x, y, t1, t2;
int n, t, i, norm;
bool neg;
mp_err err;
if ((err = mp_init_size(&q, a->used + 2)) != MP_OKAY) {
return err;
}
q.used = a->used + 2;
if ((err = mp_init(&t1)) != MP_OKAY) goto LBL_Q;
if ((err = mp_init(&t2)) != MP_OKAY) goto LBL_T1;
if ((err = mp_init_copy(&x, a)) != MP_OKAY) goto LBL_T2;
if ((err = mp_init_copy(&y, b)) != MP_OKAY) goto LBL_X;
/* fix the sign */
neg = (a->sign != b->sign);
x.sign = y.sign = MP_ZPOS;
/* normalize both x and y, ensure that y >= b/2, [b == 2**MP_DIGIT_BIT] */
norm = mp_count_bits(&y) % MP_DIGIT_BIT;
if (norm < (MP_DIGIT_BIT - 1)) {
norm = (MP_DIGIT_BIT - 1) - norm;
if ((err = mp_mul_2d(&x, norm, &x)) != MP_OKAY) goto LBL_Y;
if ((err = mp_mul_2d(&y, norm, &y)) != MP_OKAY) goto LBL_Y;
} else {
norm = 0;
}
/* note hac does 0 based, so if used==5 then its 0,1,2,3,4, e.g. use 4 */
n = x.used - 1;
t = y.used - 1;
/* while (x >= y*b**n-t) do { q[n-t] += 1; x -= y*b**{n-t} } */
/* y = y*b**{n-t} */
if ((err = mp_lshd(&y, n - t)) != MP_OKAY) goto LBL_Y;
while (mp_cmp(&x, &y) != MP_LT) {
++(q.dp[n - t]);
if ((err = mp_sub(&x, &y, &x)) != MP_OKAY) goto LBL_Y;
}
/* reset y by shifting it back down */
mp_rshd(&y, n - t);
/* step 3. for i from n down to (t + 1) */
for (i = n; i >= (t + 1); i--) {
if (i > x.used) {
continue;
}
/* step 3.1 if xi == yt then set q{i-t-1} to b-1,
* otherwise set q{i-t-1} to (xi*b + x{i-1})/yt */
if (x.dp[i] == y.dp[t]) {
q.dp[(i - t) - 1] = ((mp_digit)1 << (mp_digit)MP_DIGIT_BIT) - (mp_digit)1;
} else {
mp_word tmp;
tmp = (mp_word)x.dp[i] << (mp_word)MP_DIGIT_BIT;
tmp |= (mp_word)x.dp[i - 1];
tmp /= (mp_word)y.dp[t];
if (tmp > (mp_word)MP_MASK) {
tmp = MP_MASK;
}
q.dp[(i - t) - 1] = (mp_digit)(tmp & (mp_word)MP_MASK);
}
/* while (q{i-t-1} * (yt * b + y{t-1})) >
xi * b**2 + xi-1 * b + xi-2
do q{i-t-1} -= 1;
*/
q.dp[(i - t) - 1] = (q.dp[(i - t) - 1] + 1uL) & (mp_digit)MP_MASK;
do {
q.dp[(i - t) - 1] = (q.dp[(i - t) - 1] - 1uL) & (mp_digit)MP_MASK;
/* find left hand */
mp_zero(&t1);
t1.dp[0] = ((t - 1) < 0) ? 0u : y.dp[t - 1];
t1.dp[1] = y.dp[t];
t1.used = 2;
if ((err = mp_mul_d(&t1, q.dp[(i - t) - 1], &t1)) != MP_OKAY) goto LBL_Y;
/* find right hand */
t2.dp[0] = ((i - 2) < 0) ? 0u : x.dp[i - 2];
t2.dp[1] = x.dp[i - 1]; /* i >= 1 always holds */
t2.dp[2] = x.dp[i];
t2.used = 3;
} while (mp_cmp_mag(&t1, &t2) == MP_GT);
/* step 3.3 x = x - q{i-t-1} * y * b**{i-t-1} */
if ((err = mp_mul_d(&y, q.dp[(i - t) - 1], &t1)) != MP_OKAY) goto LBL_Y;
if ((err = mp_lshd(&t1, (i - t) - 1)) != MP_OKAY) goto LBL_Y;
if ((err = mp_sub(&x, &t1, &x)) != MP_OKAY) goto LBL_Y;
/* if x < 0 then { x = x + y*b**{i-t-1}; q{i-t-1} -= 1; } */
if (mp_isneg(&x)) {
if ((err = mp_copy(&y, &t1)) != MP_OKAY) goto LBL_Y;
if ((err = mp_lshd(&t1, (i - t) - 1)) != MP_OKAY) goto LBL_Y;
if ((err = mp_add(&x, &t1, &x)) != MP_OKAY) goto LBL_Y;
q.dp[(i - t) - 1] = (q.dp[(i - t) - 1] - 1uL) & MP_MASK;
}
}
/* now q is the quotient and x is the remainder
* [which we have to normalize]
*/
/* get sign before writing to c */
x.sign = mp_iszero(&x) ? MP_ZPOS : a->sign;
if (c != NULL) {
mp_clamp(&q);
mp_exch(&q, c);
c->sign = (neg ? MP_NEG : MP_ZPOS);
}
if (d != NULL) {
if ((err = mp_div_2d(&x, norm, &x, NULL)) != MP_OKAY) goto LBL_Y;
mp_exch(&x, d);
}
LBL_Y:
mp_clear(&y);
LBL_X:
mp_clear(&x);
LBL_T2:
mp_clear(&t2);
LBL_T1:
mp_clear(&t1);
LBL_Q:
mp_clear(&q);
return err;
}
#endif
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