"""
Splitting fields of polynomials over number fields

AUTHORS:

- Jeroen Demeyer (2014-01-02): initial version for :trac:`2217`

- Jeroen Demeyer (2014-01-03): add ``abort_degree`` argument, :trac:`15626`
"""

#*****************************************************************************
#       Copyright (C) 2014 Jeroen Demeyer <jdemeyer@cage.ugent.be>
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 2 of the License, or
# (at your option) any later version.
#                  http://www.gnu.org/licenses/
#*****************************************************************************

from sage.rings.integer import Integer
from sage.rings.arith import factorial
from sage.rings.number_field.all import NumberField
from sage.rings.number_field.number_field_base import is_NumberField
from sage.rings.polynomial.all import PolynomialRing
from sage.rings.rational_field import RationalField, is_RationalField
from sage.libs.pari.all import pari, PariError


class SplittingFieldAbort(Exception):
    """
    Special exception class to indicate an early abort of :func:`splitting_field`.

    EXAMPLES::

        sage: from sage.rings.number_field.splitting_field import SplittingFieldAbort
        sage: raise SplittingFieldAbort(20, 60)
        Traceback (most recent call last):
        ...
        SplittingFieldAbort: degree of splitting field is a multiple of 20
        sage: raise SplittingFieldAbort(12, 12)
        Traceback (most recent call last):
        ...
        SplittingFieldAbort: degree of splitting field equals 12
    """
    def __init__(self, div, mult):
        self.degree_divisor = div
        self.degree_multiple = mult
        if div == mult:
            msg = "degree of splitting field equals %s"%div
        else:
            msg = "degree of splitting field is a multiple of %s"%div
        super(SplittingFieldAbort, self).__init__(msg)

class SplittingData:
    """
    A class to store data for internal use in :func:`splitting_field`.
    It contains two attributes :attr:`pol` (polynomial), :attr:`dm`
    (degree multiple), where ``pol`` is a PARI polynomial and
    ``dm`` a Sage :class:`Integer`.

    ``dm`` is a multiple of the degree of the splitting field of
    ``pol`` over some field `E`. In :func:`splitting_field`, `E` is the
    field containing the current field `K` and all roots of other
    polynomials inside the list `L` with ``dm`` less than this ``dm``.
    """
    def __init__(self, _pol, _dm):
        self.pol = _pol
        self.dm = Integer(_dm)

    def key(self):
        """
        Return a sorting key. Compare first by degree bound, then by
        polynomial degree, then by discriminant.

        EXAMPLES::

            sage: from sage.rings.number_field.splitting_field import SplittingData
            sage: L = []
            sage: L.append(SplittingData(pari("x^2 + 1"), 1))
            sage: L.append(SplittingData(pari("x^3 + 1"), 1))
            sage: L.append(SplittingData(pari("x^2 + 7"), 2))
            sage: L.append(SplittingData(pari("x^3 + 1"), 2))
            sage: L.append(SplittingData(pari("x^3 + x^2 + x + 1"), 2))
            sage: L.sort(key=lambda x: x.key()); L
            [SplittingData(x^2 + 1, 1), SplittingData(x^3 + 1, 1), SplittingData(x^2 + 7, 2), SplittingData(x^3 + x^2 + x + 1, 2), SplittingData(x^3 + 1, 2)]
            sage: [x.key() for x in L]
            [(1, 2, 16), (1, 3, 729), (2, 2, 784), (2, 3, 256), (2, 3, 729)]
        """
        return (self.dm, self.poldegree(), self.pol.poldisc().norm().abs())

    def poldegree(self):
        """
        Return the degree of ``self.pol``

        EXAMPLES::

            sage: from sage.rings.number_field.splitting_field import SplittingData
            sage: SplittingData(pari("x^123 + x + 1"), 2).poldegree()
            123
        """
        return self.pol.poldegree()

    def __repr__(self):
        """
        TESTS::

            sage: from sage.rings.number_field.splitting_field import SplittingData
            sage: print SplittingData(pari("polcyclo(24)"), 2)
            SplittingData(x^8 - x^4 + 1, 2)
        """
        return "SplittingData(%s, %s)"%(self.pol, self.dm)

    def _repr_tuple(self):
        """
        TESTS::

            sage: from sage.rings.number_field.splitting_field import SplittingData
            sage: SplittingData(pari("polcyclo(24)"), 2)._repr_tuple()
            (8, 2)
        """
        return (self.poldegree(), self.dm)


def splitting_field(poly, name, map=False, degree_multiple=None, abort_degree=None, simplify=True, simplify_all=False):
    """
    Compute the splitting field of a given polynomial, defined over a
    number field.

    INPUT:

    - ``poly`` -- a monic polynomial over a number field

    - ``name`` -- a variable name for the number field

    - ``map`` -- (default: ``False``) also return an embedding of
      ``poly`` into the resulting field. Note that computing this
      embedding might be expensive.

    - ``degree_multiple`` -- a multiple of the absolute degree of
      the splitting field.  If ``degree_multiple`` equals the actual
      degree, this can enormously speed up the computation.

    - ``abort_degree`` -- abort by raising a :class:`SplittingFieldAbort`
      if it can be determined that the absolute degree of the splitting
      field is strictly larger than ``abort_degree``.

    - ``simplify`` -- (default: ``True``) during the algorithm, try
      to find a simpler defining polynomial for the intermediate
      number fields using PARI's ``polred()``.  This usually speeds
      up the computation but can also considerably slow it down.
      Try and see what works best in the given situation.

    - ``simplify_all`` -- (default: ``False``) If ``True``, simplify
      intermediate fields and also the resulting number field.

    OUTPUT:

    If ``map`` is ``False``, the splitting field as an absolute number
    field.  If ``map`` is ``True``, a tuple ``(K, phi)`` where ``phi``
    is an embedding of the base field in ``K``.

    EXAMPLES::

        sage: R.<x> = PolynomialRing(QQ)
        sage: K.<a> = (x^3 + 2).splitting_field(); K
        Number Field in a with defining polynomial x^6 + 3*x^5 + 6*x^4 + 11*x^3 + 12*x^2 - 3*x + 1
        sage: K.<a> = (x^3 - 3*x + 1).splitting_field(); K
        Number Field in a with defining polynomial x^3 - 3*x + 1

    The ``simplify`` and ``simplify_all`` flags usually yield
    fields defined by polynomials with smaller coefficients.
    By default, ``simplify`` is True and ``simplify_all`` is False.

    ::

        sage: (x^4 - x + 1).splitting_field('a', simplify=False)
        Number Field in a with defining polynomial x^24 - 2780*x^22 + 2*x^21 + 3527512*x^20 - 2876*x^19 - 2701391985*x^18 + 945948*x^17 + 1390511639677*x^16 + 736757420*x^15 - 506816498313560*x^14 - 822702898220*x^13 + 134120588299548463*x^12 + 362240696528256*x^11 - 25964582366880639486*x^10 - 91743672243419990*x^9 + 3649429473447308439427*x^8 + 14310332927134072336*x^7 - 363192569823568746892571*x^6 - 1353403793640477725898*x^5 + 24293393281774560140427565*x^4 + 70673814899934142357628*x^3 - 980621447508959243128437933*x^2 - 1539841440617805445432660*x + 18065914012013502602456565991
        sage: (x^4 - x + 1).splitting_field('a', simplify=True)
        Number Field in a with defining polynomial x^24 + 8*x^23 - 32*x^22 - 310*x^21 + 540*x^20 + 4688*x^19 - 6813*x^18 - 32380*x^17 + 49525*x^16 + 102460*x^15 - 129944*x^14 - 287884*x^13 + 372727*x^12 + 150624*x^11 - 110530*x^10 - 566926*x^9 + 1062759*x^8 - 779940*x^7 + 863493*x^6 - 1623578*x^5 + 1759513*x^4 - 955624*x^3 + 459975*x^2 - 141948*x + 53919
        sage: (x^4 - x + 1).splitting_field('a', simplify_all=True)
        Number Field in a with defining polynomial x^24 - 3*x^23 + 2*x^22 - x^20 + 4*x^19 + 32*x^18 - 35*x^17 - 92*x^16 + 49*x^15 + 163*x^14 - 15*x^13 - 194*x^12 - 15*x^11 + 163*x^10 + 49*x^9 - 92*x^8 - 35*x^7 + 32*x^6 + 4*x^5 - x^4 + 2*x^2 - 3*x + 1

    Reducible polynomials also work::

        sage: pol = (x^4 - 1)*(x^2 + 1/2)*(x^2 + 1/3)
        sage: pol.splitting_field('a', simplify_all=True)
        Number Field in a with defining polynomial x^8 - x^4 + 1

    Relative situation::

        sage: R.<x> = PolynomialRing(QQ)
        sage: K.<a> = NumberField(x^3 + 2)
        sage: S.<t> = PolynomialRing(K)
        sage: L.<b> = (t^2 - a).splitting_field()
        sage: L
        Number Field in b with defining polynomial t^6 + 2

    With ``map=True``, we also get the embedding of the base field
    into the splitting field::

        sage: L.<b>, phi = (t^2 - a).splitting_field(map=True)
        sage: phi
        Ring morphism:
          From: Number Field in a with defining polynomial x^3 + 2
          To:   Number Field in b with defining polynomial t^6 + 2
          Defn: a |--> b^2
        sage: (x^4 - x + 1).splitting_field('a', simplify_all=True, map=True)[1]
        Ring morphism:
          From: Rational Field
          To:   Number Field in a with defining polynomial x^24 - 3*x^23 + 2*x^22 - x^20 + 4*x^19 + 32*x^18 - 35*x^17 - 92*x^16 + 49*x^15 + 163*x^14 - 15*x^13 - 194*x^12 - 15*x^11 + 163*x^10 + 49*x^9 - 92*x^8 - 35*x^7 + 32*x^6 + 4*x^5 - x^4 + 2*x^2 - 3*x + 1
          Defn: 1 |--> 1

    We can enable verbose messages::

        sage: set_verbose(2)
        sage: K.<a> = (x^3 - x + 1).splitting_field()
        verbose 1 (...: splitting_field.py, splitting_field) Starting field: y
        verbose 1 (...: splitting_field.py, splitting_field) SplittingData to factor: [(3, 0)]
        verbose 2 (...: splitting_field.py, splitting_field) Done factoring (time = ...)
        verbose 1 (...: splitting_field.py, splitting_field) SplittingData to handle: [(2, 2), (3, 3)]
        verbose 1 (...: splitting_field.py, splitting_field) Bounds for absolute degree: [6, 6]
        verbose 2 (...: splitting_field.py, splitting_field) Handling polynomial x^2 + 23
        verbose 1 (...: splitting_field.py, splitting_field) New field before simplifying: x^2 + 23 (time = ...)
        verbose 1 (...: splitting_field.py, splitting_field) New field: y^2 - y + 6 (time = ...)
        verbose 2 (...: splitting_field.py, splitting_field) Converted polynomials to new field (time = ...)
        verbose 1 (...: splitting_field.py, splitting_field) SplittingData to factor: []
        verbose 2 (...: splitting_field.py, splitting_field) Done factoring (time = ...)
        verbose 1 (...: splitting_field.py, splitting_field) SplittingData to handle: [(3, 3)]
        verbose 1 (...: splitting_field.py, splitting_field) Bounds for absolute degree: [6, 6]
        verbose 2 (...: splitting_field.py, splitting_field) Handling polynomial x^3 - x + 1
        verbose 1 (...: splitting_field.py, splitting_field) New field: y^6 + 3*y^5 + 19*y^4 + 35*y^3 + 127*y^2 + 73*y + 271 (time = ...)
        sage: set_verbose(0)

    Try all Galois groups in degree 4. We use a quadratic base field
    such that ``polgalois()`` cannot be used::

        sage: R.<x> = PolynomialRing(QuadraticField(-11))
        sage: C2C2pol = x^4 - 10*x^2 + 1
        sage: C2C2pol.splitting_field('x')
        Number Field in x with defining polynomial x^8 + 24*x^6 + 608*x^4 + 9792*x^2 + 53824
        sage: C4pol = x^4 + x^3 + x^2 + x + 1
        sage: C4pol.splitting_field('x')
        Number Field in x with defining polynomial x^8 - x^7 - 2*x^6 + 5*x^5 + x^4 + 15*x^3 - 18*x^2 - 27*x + 81
        sage: D8pol = x^4 - 2
        sage: D8pol.splitting_field('x')
        Number Field in x with defining polynomial x^16 + 8*x^15 + 68*x^14 + 336*x^13 + 1514*x^12 + 5080*x^11 + 14912*x^10 + 35048*x^9 + 64959*x^8 + 93416*x^7 + 88216*x^6 + 41608*x^5 - 25586*x^4 - 60048*x^3 - 16628*x^2 + 12008*x + 34961
        sage: A4pol = x^4 - 4*x^3 + 14*x^2 - 28*x + 21
        sage: A4pol.splitting_field('x')
        Number Field in x with defining polynomial x^24 - 20*x^23 + 290*x^22 - 3048*x^21 + 26147*x^20 - 186132*x^19 + 1130626*x^18 - 5913784*x^17 + 26899345*x^16 - 106792132*x^15 + 371066538*x^14 - 1127792656*x^13 + 2991524876*x^12 - 6888328132*x^11 + 13655960064*x^10 - 23000783036*x^9 + 32244796382*x^8 - 36347834476*x^7 + 30850889884*x^6 - 16707053128*x^5 + 1896946429*x^4 + 4832907884*x^3 - 3038258802*x^2 - 200383596*x + 593179173
        sage: S4pol = x^4 + x + 1
        sage: S4pol.splitting_field('x')
        Number Field in x with defining polynomial x^48 ...

    Some bigger examples::

        sage: R.<x> = PolynomialRing(QQ)
        sage: pol15 = chebyshev_T(31, x) - 1    # 2^30*(x-1)*minpoly(cos(2*pi/31))^2
        sage: pol15.splitting_field('a')
        Number Field in a with defining polynomial x^15 - x^14 - 14*x^13 + 13*x^12 + 78*x^11 - 66*x^10 - 220*x^9 + 165*x^8 + 330*x^7 - 210*x^6 - 252*x^5 + 126*x^4 + 84*x^3 - 28*x^2 - 8*x + 1
        sage: pol48 = x^6 - 4*x^4 + 12*x^2 - 12
        sage: pol48.splitting_field('a')
        Number Field in a with defining polynomial x^48 ...

    If you somehow know the degree of the field in advance, you
    should add a ``degree_multiple`` argument.  This can speed up the
    computation, in particular for polynomials of degree >= 12 or
    for relative extensions::

        sage: pol15.splitting_field('a', degree_multiple=15)
        Number Field in a with defining polynomial x^15 + x^14 - 14*x^13 - 13*x^12 + 78*x^11 + 66*x^10 - 220*x^9 - 165*x^8 + 330*x^7 + 210*x^6 - 252*x^5 - 126*x^4 + 84*x^3 + 28*x^2 - 8*x - 1

    A value for ``degree_multiple`` which isn't actually a
    multiple of the absolute degree of the splitting field can
    either result in a wrong answer or the following exception::

        sage: pol48.splitting_field('a', degree_multiple=20)
        Traceback (most recent call last):
        ...
        ValueError: inconsistent degree_multiple in splitting_field()

    Compute the Galois closure as the splitting field of the defining polynomial::

        sage: R.<x> = PolynomialRing(QQ)
        sage: pol48 = x^6 - 4*x^4 + 12*x^2 - 12
        sage: K.<a> = NumberField(pol48)
        sage: L.<b> = pol48.change_ring(K).splitting_field()
        sage: L
        Number Field in b with defining polynomial x^48 ...

    Try all Galois groups over `\QQ` in degree 5 except for `S_5`
    (the latter is infeasible with the current implementation)::

        sage: C5pol = x^5 + x^4 - 4*x^3 - 3*x^2 + 3*x + 1
        sage: C5pol.splitting_field('x')
        Number Field in x with defining polynomial x^5 + x^4 - 4*x^3 - 3*x^2 + 3*x + 1
        sage: D10pol = x^5 - x^4 - 5*x^3 + 4*x^2 + 3*x - 1
        sage: D10pol.splitting_field('x')
        Number Field in x with defining polynomial x^10 - 28*x^8 + 216*x^6 - 681*x^4 + 902*x^2 - 401
        sage: AGL_1_5pol = x^5 - 2
        sage: AGL_1_5pol.splitting_field('x')
        Number Field in x with defining polynomial x^20 + 10*x^19 + 55*x^18 + 210*x^17 + 595*x^16 + 1300*x^15 + 2250*x^14 + 3130*x^13 + 3585*x^12 + 3500*x^11 + 2965*x^10 + 2250*x^9 + 1625*x^8 + 1150*x^7 + 750*x^6 + 400*x^5 + 275*x^4 + 100*x^3 + 75*x^2 + 25
        sage: A5pol = x^5 - x^4 + 2*x^2 - 2*x + 2
        sage: A5pol.splitting_field('x')
        Number Field in x with defining polynomial x^60 ...

    We can use the ``abort_degree`` option if we don't want to compute
    fields of too large degree (this can be used to check whether the
    splitting field has small degree)::

        sage: (x^5+x+3).splitting_field('b', abort_degree=119)
        Traceback (most recent call last):
        ...
        SplittingFieldAbort: degree of splitting field equals 120
        sage: (x^10+x+3).splitting_field('b', abort_degree=60)  # long time (10s on sage.math, 2014)
        Traceback (most recent call last):
        ...
        SplittingFieldAbort: degree of splitting field is a multiple of 180

    Use the ``degree_divisor`` attribute to recover the divisor of the
    degree of the splitting field or ``degree_multiple`` to recover a
    multiple::

        sage: from sage.rings.number_field.splitting_field import SplittingFieldAbort
        sage: try:  # long time (4s on sage.math, 2014)
        ....:     (x^8+x+1).splitting_field('b', abort_degree=60, simplify=False)
        ....: except SplittingFieldAbort as e:
        ....:     print e.degree_divisor
        ....:     print e.degree_multiple
        120
        1440

    TESTS::

        sage: from sage.rings.number_field.splitting_field import splitting_field
        sage: splitting_field(polygen(QQ), name='x', map=True, simplify_all=True)
        (Number Field in x with defining polynomial x, Ring morphism:
          From: Rational Field
          To:   Number Field in x with defining polynomial x
          Defn: 1 |--> 1)
    """
    from sage.misc.all import verbose, cputime

    degree_multiple = Integer(degree_multiple or 0)
    abort_degree = Integer(abort_degree or 0)

    # Kpol = PARI polynomial in y defining the extension found so far
    F = poly.base_ring()
    if is_RationalField(F):
        Kpol = pari("'y")
    else:
        Kpol = F.pari_polynomial("y")
    # Fgen = the generator of F as element of Q[y]/Kpol
    # (only needed if map=True)
    if map:
        Fgen = F.gen()._pari_()
    verbose("Starting field: %s"%Kpol)

    # L and Lred are lists of SplittingData.
    # L contains polynomials which are irreducible over K,
    # Lred contains polynomials which need to be factored.
    L = []
    Lred = [SplittingData(poly._pari_with_name(), degree_multiple)]

    # Main loop, handle polynomials one by one
    while True:
        # Absolute degree of current field K
        absolute_degree = Integer(Kpol.poldegree())

        # Compute minimum relative degree of splitting field
        rel_degree_divisor = Integer(1)
        for splitting in L:
            rel_degree_divisor = rel_degree_divisor.lcm(splitting.poldegree())

        # Check for early aborts
        abort_rel_degree = abort_degree//absolute_degree
        if abort_rel_degree and rel_degree_divisor > abort_rel_degree:
            raise SplittingFieldAbort(absolute_degree * rel_degree_divisor, degree_multiple)
        
        # First, factor polynomials in Lred and store the result in L
        verbose("SplittingData to factor: %s"%[s._repr_tuple() for s in Lred])
        t = cputime()
        for splitting in Lred:
            m = splitting.dm.gcd(degree_multiple).gcd(factorial(splitting.poldegree()))
            if m == 1:
                continue
            factors = Kpol.nffactor(splitting.pol)[0]
            for q in factors:
                d = q.poldegree()
                fac = factorial(d)
                # Multiple of the degree of the splitting field of q,
                # note that the degree equals fac iff the Galois group is S_n.
                mq = m.gcd(fac)
                if mq == 1:
                    continue
                # Multiple of the degree of the splitting field of q
                # over the field defined by adding square root of the
                # discriminant.
                # If the Galois group is contained in A_n, then mq_alt is
                # also the degree multiple over the current field K.
                # Here, we have equality if the Galois group is A_n.
                mq_alt = mq.gcd(fac//2)

                # If we are over Q, then use PARI's polgalois() to compute
                # these degrees exactly.
                if absolute_degree == 1:
                    try:
                        G = q.polgalois()
                    except PariError:
                        pass
                    else:
                        mq = Integer(G[0])
                        mq_alt = mq//2 if (G[1] == -1) else mq

                # In degree 4, use the cubic resolvent to refine the
                # degree bounds.
                if d == 4 and mq >= 12:  # mq equals 12 or 24
                    # Compute cubic resolvent
                    a0, a1, a2, a3, a4 = (q/q.pollead()).Vecrev()
                    assert a4 == 1
                    cubicpol = pari([4*a0*a2 - a1*a1 -a0*a3*a3, a1*a3 - 4*a0, -a2, 1]).Polrev()
                    cubicfactors = Kpol.nffactor(cubicpol)[0]
                    if len(cubicfactors) == 1:    # A4 or S4
                        # After adding a root of the cubic resolvent,
                        # the degree of the extension defined by q
                        # is a factor 3 smaller.
                        L.append(SplittingData(cubicpol, 3))
                        rel_degree_divisor = rel_degree_divisor.lcm(3)
                        mq = mq//3  # 4 or 8
                        mq_alt = 4
                    elif len(cubicfactors) == 2:  # C4 or D8
                        # The irreducible degree 2 factor is
                        # equivalent to x^2 - q.poldisc().
                        discpol = cubicfactors[1]
                        L.append(SplittingData(discpol, 2))
                        mq = mq_alt = 4
                    else:                         # C2 x C2
                        mq = mq_alt = 4

                if mq > mq_alt >= 3:
                    # Add quadratic resolvent x^2 - D to decrease
                    # the degree multiple by a factor 2.
                    discpol = pari([-q.poldisc(), 0, 1]).Polrev()
                    discfactors = Kpol.nffactor(discpol)[0]
                    if len(discfactors) == 1:
                        # Discriminant is not a square
                        L.append(SplittingData(discpol, 2))
                        rel_degree_divisor = rel_degree_divisor.lcm(2)
                    mq = mq_alt

                L.append(SplittingData(q, mq))
                rel_degree_divisor = rel_degree_divisor.lcm(q.poldegree())
                if abort_rel_degree and rel_degree_divisor > abort_rel_degree:
                    raise SplittingFieldAbort(absolute_degree * rel_degree_divisor, degree_multiple)
        verbose("Done factoring", t, level=2)

        if len(L) == 0:  # Nothing left to do
            break

        # Recompute absolute degree multiple
        new_degree_multiple = absolute_degree
        for splitting in L:
            new_degree_multiple *= splitting.dm
        degree_multiple = new_degree_multiple.gcd(degree_multiple)

        # Absolute degree divisor
        degree_divisor = rel_degree_divisor * absolute_degree

        # Sort according to degree to handle low degrees first
        L.sort(key=lambda x: x.key())
        verbose("SplittingData to handle: %s"%[s._repr_tuple() for s in L])
        verbose("Bounds for absolute degree: [%s, %s]"%(degree_divisor,degree_multiple))

        # Check consistency
        if degree_multiple % degree_divisor != 0:
            raise ValueError("inconsistent degree_multiple in splitting_field()")
        for splitting in L:
            # The degree of the splitting field must be a multiple of
            # the degree of the polynomial. Only do this check for
            # SplittingData with minimal dm, because the higher dm are
            # defined as relative degree over the splitting field of
            # the polynomials with lesser dm.
            if splitting.dm > L[0].dm:
                break
            if splitting.dm % splitting.poldegree() != 0:
                raise ValueError("inconsistent degree_multiple in splitting_field()")

        # Add a root of f = L[0] to construct the field N = K[x]/f(x)
        splitting = L[0]
        f = splitting.pol
        verbose("Handling polynomial %s"%(f.lift()), level=2)
        t = cputime()
        Npol, KtoN, k = Kpol.rnfequation(f, flag=1)

        # Make Npol monic integral primitive, store in Mpol
        # (after this, we don't need Npol anymore, only Mpol)
        Mdiv = pari(1)
        Mpol = Npol
        while True:
            denom = Integer(Mpol.pollead())
            if denom == 1:
                break
            denom = pari(denom.factor().radical_value())
            Mpol = (Mpol*(denom**Mpol.poldegree())).subst("x", pari([0,1/denom]).Polrev("x"))
            Mpol /= Mpol.content()
            Mdiv *= denom

        # We are finished for sure if we hit the degree bound
        finished = (Mpol.poldegree() >= degree_multiple)

        if simplify_all or (simplify and not finished):
            # Find a simpler defining polynomial Lpol for Mpol
            verbose("New field before simplifying: %s"%Mpol, t)
            t = cputime()
            M = Mpol.polred(flag=3)
            n = len(M[0])-1
            Lpol = M[1][n].change_variable_name("y")
            LtoM = M[0][n].change_variable_name("y").Mod(Mpol.change_variable_name("y"))
            MtoL = LtoM.modreverse()
        else:
            # Lpol = Mpol
            Lpol = Mpol.change_variable_name("y")
            MtoL = pari("'y")

        NtoL = MtoL/Mdiv
        KtoL = KtoN.lift().subst("x", NtoL).Mod(Lpol)
        Kpol = Lpol   # New Kpol (for next iteration)
        verbose("New field: %s"%Kpol, t)
        if map:
            t = cputime()
            Fgen = Fgen.lift().subst("y", KtoL)
            verbose("Computed generator of F in K", t, level=2)
        if finished:
            break

        t = cputime()

        # Convert f and elements of L from K to L and store in L
        # (if the polynomial is certain to remain irreducible) or Lred.
        Lold = L[1:]
        L = []
        Lred = []

        # First add f divided by the linear factor we obtained,
        # mg is the new degree multiple.
        mg = splitting.dm//f.poldegree()
        if mg > 1:
            g = [c.subst("y", KtoL).Mod(Lpol) for c in f.Vecrev().lift()]
            g = pari(g).Polrev()
            g /= pari([k*KtoL - NtoL, 1]).Polrev()  # divide linear factor
            Lred.append(SplittingData(g, mg))

        for splitting in Lold:
            g = [c.subst("y", KtoL) for c in splitting.pol.Vecrev().lift()]
            g = pari(g).Polrev()
            mg = splitting.dm
            if Integer(g.poldegree()).gcd(f.poldegree()) == 1:  # linearly disjoint fields
                L.append(SplittingData(g, mg))
            else:
                Lred.append(SplittingData(g, mg))
        verbose("Converted polynomials to new field", t, level=2)

    # Convert Kpol to Sage and construct the absolute number field
    Kpol = PolynomialRing(RationalField(), name=poly.variable_name())(Kpol/Kpol.pollead())
    K = NumberField(Kpol, name)
    if map:
        return K, F.hom(Fgen, K)
    else:
        return K
