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	zpow.c - libzahl - big integer library
	git clone git://git.suckless.org/libzahl
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	---
	zpow.c (1157B)
	---
	1 /* See LICENSE file for copyright and license details. */
	2 #include "internals.h"
	3
	4 #define tb libzahl_tmp_pow_b
	5 #define tc libzahl_tmp_pow_c
	6
	7
	8 void
	9 zpow(z_t a, z_t b, z_t c)
	10 {
	11 /*
	12 * Exponentiation by squaring.
	13 *
	14 * 7↑19 = 7↑10011₂ = 7↑2⁰ ⋅ 7↑2¹ ⋅ 7↑2⁴ whe…
	15 */
	16
	17 /* TODO use zpowu when possible */
	18
	19 size_t i, j, n, bits;
	20 zahl_char_t x;
	21 int neg;
	22
	23 if (unlikely(zsignum(c) <= 0)) {
	24 if (zzero(c)) {
	25 if (check(zzero(b)))
	26 libzahl_failure(-ZERROR_0_POW_0);
	27 zsetu(a, 1);
	28 } else if (check(zzero(b))) {
	29 libzahl_failure(-ZERROR_DIV_0);
	30 } else {
	31 SET_SIGNUM(a, 0);
	32 }
	33 return;
	34 } else if (unlikely(zzero(b))) {
	35 SET_SIGNUM(a, 0);
	36 return;
	37 }
	38
	39 bits = zbits(c);
	40 n = FLOOR_BITS_TO_CHARS(bits);
	41
	42 neg = znegative(b) && zodd(c);
	43 zabs(tb, b);
	44 zset(tc, c);
	45 zsetu(a, 1);
	46
	47 for (i = 0; i < n; i++) { /* Remember, n is floored. */
	48 x = tc->chars[i];
	49 for (j = BITS_PER_CHAR; j--; x >>= 1) {
	50 if (x & 1)
	51 zmul_ll(a, a, tb);
	52 zsqr_ll(tb, tb);
	53 }
	54 }
	55 x = tc->chars[i];
	56 for (; x; x >>= 1) {
	57 if (x & 1)
	58 zmul_ll(a, a, tb);
	59 zsqr_ll(tb, tb);
	60 }
	61
	62 if (neg)
	63 zneg(a, a);
	64 }