// Copyright (c) 2008 Max-Planck-Institute Saarbruecken (Germany).
// All rights reserved.
//
// This file is part of CGAL (www.cgal.org)
//
// $URL: https://github.com/CGAL/cgal/blob/v6.1.1/Polynomial/include/CGAL/Polynomial/polynomial_gcd_implementations.h $
// $Id: include/CGAL/Polynomial/polynomial_gcd_implementations.h 08b27d3db14 $
// SPDX-License-Identifier: LGPL-3.0-or-later OR LicenseRef-Commercial
//
//
// Author(s)     : Michael Hemmer   <hemmer@informatik.uni-mainz.de>

#ifndef CGAL_POLYNOMIAL_GCD_IMPLEMENTATIONS_H
#define CGAL_POLYNOMIAL_GCD_IMPLEMENTATIONS_H


#include <CGAL/basic.h>
#include <CGAL/Polynomial.h>
#include <CGAL/Real_timer.h>
#include <CGAL/polynomial_utils.h>
#include <CGAL/Polynomial/hgdelta_update.h>
#include <CGAL/Polynomial/polynomial_gcd.h>


namespace CGAL {
namespace internal {

template <class NT>
inline
Polynomial<NT> gcd_utcf_UFD(
        Polynomial<NT> p1, Polynomial<NT> p2
) {
    // implemented using the subresultant algorithm for gcd computation
    // see [Cohen, 1993], algorithm 3.3.1
    // handle trivial cases
    if (p1.is_zero()){
        if (p2.is_zero()) return Polynomial<NT>(NT(1));
        else {
            return CGAL::canonicalize(p2);
        }
    }
    if (p2.is_zero()){
        return CGAL::canonicalize(p1);
    }
    if (p2.degree() > p1.degree()) {
        Polynomial<NT> p3 = p1; p1 = p2; p2 = p3;
    }

    // compute gcd of content
    NT p1c = p1.content(), p2c = p2.content();
    NT gcdcont = CGAL::gcd(p1c,p2c);

    // compute gcd of primitive parts
    p1 /= p1c; p2 /= p2c;

    NT dummy;
    Polynomial<NT> q, r;

    NT g = NT(1), h = NT(1);
    for (;;) {
        Polynomial<NT>::pseudo_division(p1, p2, q, r, dummy);
        if (r.is_zero()) { break; }

        if (r.degree() == 0) {
            return CGAL::canonicalize(Polynomial<NT>(gcdcont));
        }
        int delta = p1.degree() - p2.degree();
        p1 = p2;
        p2 = r / (g * ipower(h, delta));
        g = p1.lcoeff();
        // h = h^(1-delta) * g^delta
        CGAL::internal::hgdelta_update(h, g, delta);
    }

    p2 /= p2.content() * p2.unit_part();

    // combine both parts to proper gcd
    p2 *= gcdcont;

    return CGAL::canonicalize(p2);
}

template <class NT>
inline
Polynomial<NT> gcd_Euclidean_ring(
        Polynomial<NT> p1, Polynomial<NT> p2
) {
//    std::cout<<" gcd_Field"<<std::endl;
    // handle trivial cases
    if (p1.is_zero()){
        if (p2.is_zero()) return Polynomial<NT>(NT(1));
        else return p2 / p2.unit_part();
    }
    if (p2.is_zero())
        return p1 / p1.unit_part();
    if (p2.degree() > p1.degree()) {
        Polynomial<NT> p3 = p1; p1 = p2; p2 = p3;
    }

    Polynomial<NT> q, r;
    while (!p2.is_zero()) {
        Polynomial<NT>::euclidean_division(p1, p2, q, r);
        p1 = p2; p2 = r;
    }
    p1 /= p1.lcoeff();
    p1.simplify_coefficients();
    return p1;
}

template <class NT>
inline
NT content_utcf_(const Polynomial<NT>& p)
{
    typename Algebraic_structure_traits<NT>::Integral_division idiv;
    typename Algebraic_structure_traits<NT>::Unit_part upart;
    typedef typename Polynomial<NT>::const_iterator const_iterator;

    const_iterator it = p.begin(), ite = p.end();
    while (*it == NT(0)) it++;
    NT cont = idiv(*it, upart(*it));
    for( ; it != ite; it++) {
        if (cont == NT(1)) break;
        if (*it != NT(0)) cont = internal::gcd_utcf_(cont, *it);
    }

    return cont;
}


template <class NT>
inline
Polynomial<NT> gcd_utcf_Integral_domain( Polynomial<NT> p1, Polynomial<NT> p2){
  // std::cout<<" gcd_utcf_Integral_domain"<<std::endl;
  typedef Polynomial<NT> POLY;

  // handle trivial cases
    if (p1.is_zero()){
      if (p2.is_zero()){
            return Polynomial<NT>(NT(1));
        }else{
            return CGAL::canonicalize(p2);
        }
    }
    if (p2.is_zero()){
        return CGAL::canonicalize(p1);
    }

    if (p2.degree() > p1.degree()) {
        Polynomial<NT> p3 = p1; p1 = p2; p2 = p3;
    }

    // remove redundant scalar factors
    p1=CGAL::canonicalize(p1);
    p2=CGAL::canonicalize(p2);

    // compute content of p1 and p2
    NT p1c = internal::content_utcf_(p1);
    NT p2c = internal::content_utcf_(p2);


    // compute gcd of content
    NT gcdcont = internal::gcd_utcf_(p1c, p2c);

    // compute gcd of primitive parts
    p1 = integral_division_up_to_constant_factor(p1, POLY(p1c));
    p2 = integral_division_up_to_constant_factor(p2, POLY(p2c));


    Polynomial<NT> q, r;

    // TODO measure performance of both methods with respect to
    // univariat polynomials on Integeres
    // univariat polynomials on Sqrt_extension<Integer,Integer>
    // multivariat polynomials
    // May write specializations for different cases
#if 0
    // implemented using the subresultant algorithm for gcd computation
    // with respect to constant scalar factors
    // see [Cohen, 1993], algorithm 3.3.1
    NT g = NT(1), h = NT(1), dummy;
    for (;;) {
        Polynomial<NT>::pseudo_division(p1, p2, q, r, dummy);
        if (r.is_zero()) { break; }
        if (r.degree() == 0) { return Polynomial<NT>(gcdcont); }
        int delta = p1.degree() - p2.degree();
        p1 = p2;
        p2 = r / (g * ipower(h, delta));
        g = p1.lcoeff();
        // h = h^(1-delta) * g^delta
        CGAL::internal::hgdelta_update(h, g, delta);
    }
#else
    // implementation using just the 'naive' method
    // but performed much better as the one by Cohen
    // (for univariat polynomials with Sqrt_extension coeffs )
    NT dummy;
    for (;;) {
        Polynomial<NT>::pseudo_division(p1, p2, q, r, dummy);
        if (r.is_zero()) { break; }
        if (r.degree() == 0) { return Polynomial<NT>(gcdcont); }
        p1 = p2;
        p2 = r ;
        p2=CGAL::canonicalize(p2);
    }
#endif

    p2 = integral_division_up_to_constant_factor(p2, POLY(content_utcf_(p2)));

    // combine both parts to proper gcd
    p2 *= gcdcont;

    Polynomial<NT> result;

    // make poly unique
    result = CGAL::canonicalize(p2);
    return result;
}


}  // namespace internal

}  // namespace CGAL

#endif //CGAL_POLYNOMIAL_GCD_IMPLEMENTATIONS_H
