constructor.py

r"""
Factories to construct Function Fields

AUTHORS:

- William Stein (2010): initial version

- Maarten Derickx (2011-09-11): added ``FunctionField_polymod_Constructor``,
  use ``@cached_function``

- Julian Rueth (2011-09-14): replaced ``@cached_function`` with
  ``UniqueFactory``

EXAMPLES::

    sage: K.<x> = FunctionField(QQ); K
    Rational function field in x over Rational Field
    sage: L.<x> = FunctionField(QQ); L
    Rational function field in x over Rational Field
    sage: K is L
    True
"""
#*****************************************************************************
#       Copyright (C) 2010 William Stein <[email protected]>
#       Copyright (C) 2011 Maarten Derickx <[email protected]>
#       Copyright (C) 2011 Julian Rueth <[email protected]>
#
#  Distributed under the terms of the GNU General Public License (GPL)
#  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.structure.factory import UniqueFactory

class FunctionFieldFactory(UniqueFactory):
    """
    Return the function field in one variable with constant field ``F``. The
    function field returned is unique in the sense that if you call this
    function twice with the same base field and name then you get the same
    python object back.

    INPUT:

    - ``F`` -- a field

    - ``names`` -- name of variable as a string or a tuple containing a string

    EXAMPLES::

        sage: K.<x> = FunctionField(QQ); K
        Rational function field in x over Rational Field
        sage: L.<y> = FunctionField(GF(7)); L
        Rational function field in y over Finite Field of size 7
        sage: R.<z> = L[]
        sage: M.<z> = L.extension(z^7-z-y); M
        Function field in z defined by z^7 + 6*z + 6*y

    TESTS::

        sage: K.<x> = FunctionField(QQ)
        sage: L.<x> = FunctionField(QQ)
        sage: K is L
        True
        sage: M.<x> = FunctionField(GF(7))
        sage: K is M
        False
        sage: N.<y> = FunctionField(QQ)
        sage: K is N
        False
    """
    def create_key(self,F,names):
        """
        Given the arguments and keywords, create a key that uniquely
        determines this object.

        EXAMPLES::

            sage: K.<x> = FunctionField(QQ) # indirect doctest
        """
        if not isinstance(names,tuple):
            names=(names,)
        return (F,names)

    def create_object(self,version,key,**extra_args):
        """
        Create the object from the key and extra arguments. This is only
        called if the object was not found in the cache.

        EXAMPLES::

            sage: K.<x> = FunctionField(QQ)
            sage: L.<x> = FunctionField(QQ)
            sage: K is L
            True
        """
        from function_field import RationalFunctionField
        return RationalFunctionField(key[0],names=key[1])

FunctionField=FunctionFieldFactory("sage.rings.function_field.constructor.FunctionField")

class FunctionFieldPolymodFactory(UniqueFactory):
    """
    Create a function field defined as an extension of another
    function field by adjoining a root of a univariate polynomial.
    The returned function field is unique in the sense that if you
    call this function twice with an equal ``polynomial`` and ``names``
    it returns the same python object in both calls.

    INPUT:

    - ``polynomial`` -- a univariate polynomial over a function field

    - ``names`` -- variable names (as a tuple of length 1 or string)

    - ``category`` -- a category (defaults to category of function fields)

    EXAMPLES::

        sage: K.<x> = FunctionField(QQ)
        sage: R.<y>=K[]
        sage: y2 = y*1
        sage: y2 is y
        False
        sage: L.<w>=K.extension(x-y^2)
        sage: M.<w>=K.extension(x-y2^2)
        sage: L is M
        True
    """
    def create_key(self,polynomial,names):
        """
        Given the arguments and keywords, create a key that uniquely
        determines this object.

        EXAMPLES::

            sage: K.<x> = FunctionField(QQ)
            sage: R.<y>=K[]
            sage: L.<w> = K.extension(x-y^2) # indirect doctest
        """
        if names is None:
            names=polynomial.variable_name()
        if not isinstance(names,tuple):
            names=(names,)
        return (polynomial,names)

    def create_object(self,version,key,**extra_args):
        """
        Create the object from the key and extra arguments. This is only
        called if the object was not found in the cache.

        EXAMPLES::

            sage: K.<x> = FunctionField(QQ)
            sage: R.<y>=K[]
            sage: L.<w> = K.extension(x-y^2) # indirect doctest
            sage: y2 = y*1
            sage: M.<w> = K.extension(x-y2^2) # indirect doctest
            sage: L is M
            True
        """
        from function_field import FunctionField_polymod
        return FunctionField_polymod(key[0],names=key[1])

FunctionField_polymod=FunctionFieldPolymodFactory("sage.rings.function_field.constructor.FunctionField_polymod")