/*
    Copyright (C) 2019 Daniel Schultz

    This file is part of FLINT.

    FLINT is free software: you can redistribute it and/or modify it under
    the terms of the GNU Lesser General Public License (LGPL) as published
    by the Free Software Foundation; either version 3 of the License, or
    (at your option) any later version.  See <https://www.gnu.org/licenses/>.
*/

#include "nmod.h"
#include "nmod_poly.h"

/* split f assuming that f has degree(f) distinct nonzero roots in Fp */
void _nmod_poly_split_rabin(
    nmod_poly_t a,
    nmod_poly_t b,
    const nmod_poly_t f,
    nmod_poly_t t,
    nmod_poly_t t2,
    flint_rand_t randstate)
{
    FLINT_ASSERT(nmod_poly_degree(f) > 1);

    nmod_poly_reverse(t, f, f->length);
    nmod_poly_inv_series_newton(t2, t, t->length);

try_again:

    nmod_poly_zero(a);
    nmod_poly_set_coeff_ui(a, 1, 1);
    nmod_poly_set_coeff_ui(a, 0, n_randint(randstate, f->mod.n));
    nmod_poly_powmod_ui_binexp_preinv(t, a, (f->mod.n - 1)/2, f, t2);
    nmod_poly_sub_ui(t, t, 1);
    nmod_poly_gcd(a, t, f);

    FLINT_ASSERT(!nmod_poly_is_zero(a));

    if (0 >= nmod_poly_degree(a) || nmod_poly_degree(a) >= nmod_poly_degree(f))
    {
        goto try_again;
    }

    nmod_poly_divexact(b, f, a);

    /* deg a >= deg b */
    if (nmod_poly_degree(a) < nmod_poly_degree(b))
        nmod_poly_swap(a, b);

    return;
}

/*
    If P has deg(P) distinct nonzero roots of P, fill them in and return 1.
    Otherwise return 0. Function is undefined for zero P.
    The modulus of P is assumed to be prime.
*/
int nmod_poly_find_distinct_nonzero_roots(
    ulong * roots,
    const nmod_poly_t P)
{
    ulong a0, a1;
    int success;
    slong i, roots_idx, sp;
    nmod_poly_struct * a , * b;
    nmod_poly_t f, t, t2;
    nmod_poly_struct stack[FLINT_BITS + 1];
    flint_rand_t randstate;
    slong d = nmod_poly_degree(P);

    if (d < 2)
    {
        if (d == 1)
        {
            a0 = nmod_poly_get_coeff_ui(P, 0);
            a1 = nmod_poly_get_coeff_ui(P, 1);
            if (a0 == 0)
            {
                return 0;
            }
            roots[0] = nmod_mul(a0, nmod_inv(P->mod.n - a1, P->mod), P->mod);
        }
        return 1;
    }

    if (P->mod.n == 2)
        return 0;

    if (P->coeffs[0] == 0)
        return 0;

    flint_rand_init(randstate);
    nmod_poly_init_mod(t, P->mod);
    nmod_poly_init_mod(t2, P->mod);
    nmod_poly_init_mod(f, P->mod);
    for (i = 0; i <= FLINT_BITS; i++)
        nmod_poly_init_mod(stack + i, P->mod);

    roots_idx = 0;

    nmod_poly_make_monic(f, P);
    nmod_poly_reverse(t, f, f->length);
    nmod_poly_inv_series_newton(t2, t, t->length);

    a = stack + 0;
    nmod_poly_zero(a);
    nmod_poly_set_coeff_ui(a, 1, 1);
    nmod_poly_powmod_ui_binexp_preinv(t, a, (P->mod.n - 1)/2, f, t2);
    nmod_poly_sub_ui(t, t, 1);
    nmod_poly_gcd(a, t, f);

    b = stack + 1;
    nmod_poly_add_ui(t, t, 2);
    nmod_poly_gcd(b, t, f);

    if (nmod_poly_degree(b) + nmod_poly_degree(a) != d)
    {
        success = 0;
        goto cleanup;
    }

    /* deg a >= deg b */
    if (nmod_poly_degree(a) < nmod_poly_degree(b))
    {
        nmod_poly_swap(a, b);
    }

    sp = nmod_poly_degree(b) > 0 ? 2 : 1;
    while (sp > 0)
    {
        FLINT_ASSERT(sp < FLINT_BITS);
        sp--;
        nmod_poly_swap(f, stack + sp);

        FLINT_ASSERT(nmod_poly_degree(f) > 0);
        if (nmod_poly_degree(f) == 1)
        {
            a0 = nmod_poly_get_coeff_ui(f, 0);
            a1 = nmod_poly_get_coeff_ui(f, 1);
            FLINT_ASSERT(a0 != 0);
            FLINT_ASSERT(a1 == 1);
            roots[roots_idx] = P->mod.n - a0;
            roots_idx++;
        }
        else
        {
            _nmod_poly_split_rabin(stack + sp + 0, stack + sp + 1, f, t, t2, randstate);
            FLINT_ASSERT(FLINT_BIT_COUNT(nmod_poly_degree(stack + sp + 1)) <= FLINT_BITS - sp - 1);
            sp += 2;
        }
    }

    success = 1;

cleanup:

    flint_rand_clear(randstate);
    nmod_poly_clear(t);
    nmod_poly_clear(t2);
    nmod_poly_clear(f);
    for (i = 0; i <= FLINT_BITS; i++)
        nmod_poly_clear(stack + i);

    FLINT_ASSERT((!success) || roots_idx == d);

    return success;
}
