static GEN
gen_Z2X_Dixon(GEN F, GEN V, long N, void *E,
GEN lin(void *E, GEN F, GEN d, long N),
GEN lins(void *E, GEN F, GEN d, long N),
GEN invls(void *E, GEN d))
{
pari_sp av = avma;
long n, m;
GEN Xn, Xm, FXn, Vm;
if (N<BITS_IN_LONG)
{
ulong q = 1UL<<N;
return Flx_to_ZX(gen_Z2x_Dixon(ZXT_to_FlxT(F,q), ZX_to_Flx(V,q),N,E,lins,invls));
}
V = ZX_remi2n(V, N);
n = (N + 1)>>1; m = N - n;
F = ZXT_remi2n(F, N);
Xn = gen_Z2X_Dixon(F, V, n, E, lin, lins, invls);
FXn = lin(E, F, Xn, N);
Vm = ZX_shifti(ZX_sub(V, FXn), -n);
Xm = gen_Z2X_Dixon(F, Vm, m, E, lin, lins, invls);
return gerepileupto(av, ZX_remi2n(ZX_add(Xn, ZX_shifti(Xm, n)), N));
}
static long
nf_pick_prime(long ct, GEN nf, GEN polbase, long fl,
GEN *lt, GEN *Fa, GEN *pr, GEN *Tp)
{
GEN nfpol = gel(nf,1), dk, bad;
long maxf, n = degpol(nfpol), dpol = degpol(polbase), nbf = 0;
byteptr pt = diffptr;
ulong pp = 0;
*lt = leading_term(polbase); /* t_INT */
if (gcmp1(*lt)) *lt = NULL;
dk = absi(gel(nf,3));
bad = mulii(dk,gel(nf,4)); if (*lt) bad = mulii(bad, *lt);
/* FIXME: slow factorization of large polynomials over large Fq */
maxf = 1;
if (ct > 1) {
if (dpol > 100) /* tough */
{
if (n >= 20) maxf = 4;
}
else
{
if (n >= 15) maxf = 4;
}
}
for (ct = 5;;)
{
GEN aT, apr, ap, modpr, red;
long anbf;
pari_timer ti_pr;
GEN list, r = NULL, fa = NULL;
pari_sp av2 = avma;
if (DEBUGLEVEL>3) TIMERstart(&ti_pr);
for (;;)
{
NEXT_PRIME_VIADIFF_CHECK(pp, pt);
if (! umodiu(bad,pp)) continue;
ap = utoipos(pp);
list = (GEN)FpX_factor(nfpol, ap)[1];
if (maxf == 1)
{ /* deg.1 factors are best */
r = gel(list,1);
if (degpol(r) == 1) break;
}
else
{ /* otherwise, pick factor of largish degree */
long i, dr;
for (i = lg(list)-1; i > 0; i--)
{
r = gel(list,i); dr = degpol(r);
if (dr <= maxf) break;
}
if (i > 0) break;
}
avma = av2;
}
apr = primedec_apply_kummer(nf,r,1,ap);
modpr = zk_to_ff_init(nf,&apr,&aT,&ap);
red = modprX(polbase, nf, modpr);
if (!aT)
{ /* degree 1 */
red = ZX_to_Flx(red, pp);
if (!Flx_is_squarefree(red, pp)) { avma = av2; continue; }
anbf = fl? Flx_nbroots(red, pp): Flx_nbfact(red, pp);
}
else
{
GEN q;
if (!FqX_is_squarefree(red,aT,ap)) { avma = av2; continue; }
q = gpowgs(ap, degpol(aT));
anbf = fl? FqX_split_deg1(&fa, red, q, aT, ap)
: FqX_split_by_degree(&fa, red, q, aT, ap);
}
if (fl == 2 && anbf < dpol) return anbf;
if (anbf <= 1)
{
if (!fl) return anbf; /* irreducible */
if (!anbf) return 0; /* no root */
}
if (!nbf || anbf < nbf
|| (anbf == nbf && cmpii(gel(apr,4), gel(*pr,4)) > 0))
{
nbf = anbf;
*pr = apr;
*Tp = aT;
*Fa = fa;
}
else avma = av2;
if (DEBUGLEVEL>3)
fprintferr("%3ld %s at prime\n %Z\nTime: %ld\n",
anbf, fl?"roots": "factors", apr, TIMER(&ti_pr));
if (--ct <= 0) return nbf;
}
}