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/* See LICENSE file for copyright and license details. */
#include "internals.h"
#define tb libzahl_tmp_pow_b
#define tc libzahl_tmp_pow_c
void
zpow(z_t a, z_t b, z_t c)
{
/*
* Exponentiation by squaring.
*
* 7↑19 = 7↑10011₂ = 7↑2⁰ ⋅ 7↑2¹ ⋅ 7↑2⁴ where a↑2↑(n + 1) = (a↑2↑n)².
*/
size_t i, j, n, bits;
zahl_char_t x;
if (EXPECT(zsignum(c) <= 0, 0)) {
if (zzero(c)) {
if (zzero(b))
FAILURE(EDOM); /* Indeterminate form: 0:th power of 0 */
zsetu(a, 1);
} else if (zzero(b)) {
FAILURE(EDOM); /* Undefined form: Division by 0 */
} else {
SET_SIGNUM(a, 0);
}
return;
} else if (EXPECT(zzero(b), 0)) {
SET_SIGNUM(a, 0);
return;
}
bits = zbits(c);
n = FLOOR_BITS_TO_CHARS(bits);
zset(tb, b);
zset(tc, c);
zsetu(a, 1);
for (i = 0; i < n; i++) { /* Remember, n is floored. */
x = tc->chars[i];
for (j = BITS_PER_CHAR; j--; x >>= 1) {
if (x & 1)
zmul(a, a, tb);
zsqr(tb, tb);
}
}
x = tc->chars[i];
for (; x; x >>= 1) {
if (x & 1)
zmul(a, a, tb);
zsqr(tb, tb);
}
}
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