/*
    Copyright (C) 2009 William Hart

    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 "gmpcompat.h"
#include "fmpz.h"

void
fmpz_mod(fmpz_t f, const fmpz_t g, const fmpz_t h)
{
    fmpz c1 = *g;
    fmpz c2 = *h;

    if (!COEFF_IS_MPZ(c1))      /* g is small */
    {
        if (!COEFF_IS_MPZ(c2))  /* h is also small */
        {
            slong r;
            if (c2 < WORD(0))
                c2 = -c2;
            if (c1 < WORD(0))
            {
                r = c2 - (-c1 % c2);    /* C doesn't correctly handle negative mods */
                if (r == c2)
                    r = 0;
            }
            else
                r = c1 % c2;

            fmpz_set_si(f, r);
        }
        else                    /* h is large and g is small */
        {
            if (c1 < WORD(0))
            {
                fmpz_abs(f, h);
                fmpz_sub_ui(f, f, -c1);
            }
            else
                fmpz_set_ui(f, c1);
        }
    }
    else                        /* g is large */
    {
        if (!COEFF_IS_MPZ(c2))  /* h is small */
        {
            if (c2 < WORD(0))
                fmpz_set_si(f, flint_mpz_fdiv_ui(COEFF_TO_PTR(c1), -c2));
            else
                fmpz_set_ui(f, flint_mpz_fdiv_ui(COEFF_TO_PTR(c1), c2));
        }
        else                    /* both are large */
        {
            if (MPZ_WANT_FLINT_DIVISION(COEFF_TO_PTR(c1), COEFF_TO_PTR(c2)))
            {
                _fmpz_mod_newton(f, g, h);
            }
            else
            {
                mpz_ptr mf = _fmpz_promote(f);
                mpz_mod(mf, COEFF_TO_PTR(c1), COEFF_TO_PTR(c2));
                _fmpz_demote_val(f);    /* reduction mod h may result in small value */
            }
        }
    }
}
