#include "pszip.h"

/*
 * Bit hack source:
 * https://graphics.stanford.edu/~seander/bithacks.html#NextBitPermutation
 */

void pszip_iterate_bithack01(mpz_t w, const mpz_t v)
{
    mpz_t vmo, t, tpo, ct, nct, act, actmo;
    mp_limb_t *actmo_l;
    size_t actmo_s;

    mpz_inits(vmo, t, tpo, ct, nct, act, actmo, 0);

    /* Gets v's least significant 0 bits set to 1.
     * t = v | (v - 1); */
    mpz_sub_ui(vmo, v, 1);
    mpz_ior(t, v, vmo);

    /* Next set to 1 the most significant bit to change,
     * set to 0 the least significant ones, and add the necessary 1 bits.
     * w = (t + 1) | (((~t & -~t) - 1) >> (__builtin_ctz(v) + 1));
     * __builtin_ctz(v) -> mpz_scan1(v, 0) */
    mpz_add_ui(tpo, t, 1);
    mpz_com(ct, t);
    mpz_neg(nct, ct);
    mpz_and(act, ct, nct);
    mpz_sub_ui(actmo, act, 1);

    /* the bitshift requires obtaining direct memory access */
    actmo_s = mpz_size(actmo);
    actmo_l = mpz_limbs_modify(actmo, actmo_s);
    mpn_rshift(actmo_l, actmo_l, actmo_s, (mpz_scan1(v, 0) + 1));
    mpz_limbs_finish(actmo, actmo_s);

    mpz_ior(w, tpo, actmo);

    mpz_clears(vmo, t, tpo, ct, nct, act, actmo, 0);
}