r"""This module contains the :term:`algebra of relations` and related functionality.
A :term:`relation` is also a :term:`set` (of :term:`couplet`\s), and inherits all operations
of the :term:`algebra of sets`. These are provided in :mod:`~.algebras.sets`.
"""
# $Id: relations.py 22702 2015-07-28 20:20:56Z jaustell $
# Copyright Algebraix Data Corporation 2015 - $Date: 2015-07-28 15:20:56 -0500 (Tue, 28 Jul 2015) $
#
# This file is part of algebraixlib <http://github.com/AlgebraixData/algebraixlib>.
#
# algebraixlib is free software: you can redistribute it and/or modify it under the terms of version
# 3 of the GNU Lesser General Public License as published by the Free Software Foundation.
#
# algebraixlib is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without
# even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
# Lesser General Public License for more details.
#
# You should have received a copy of the GNU Lesser General Public License along with algebraixlib.
# If not, see <http://www.gnu.org/licenses/>.
# --------------------------------------------------------------------------------------------------
from functools import partial
import algebraixlib.algebras.couplets as _couplets
import algebraixlib.algebras.sets as _sets
import algebraixlib.mathobjects as _mo
import algebraixlib.extension as _extension
import algebraixlib.structure as _structure
from algebraixlib.undef import make_or_raise_undef as _make_or_raise_undef
# --------------------------------------------------------------------------------------------------
[docs]class Algebra:
"""Provide the operations and relations that are members of the :term:`algebra of relations`.
This class contains only static member functions. Its main purpose is to provide a namespace for
and highlight the operations and relations that belong to the algebra of relations. All member
functions are also available at the enclosing module scope.
"""
# ----------------------------------------------------------------------------------------------
# Unary algebra operations.
@staticmethod
[docs] def transpose(rel: 'P(M x M)', _checked=True) -> 'P(M x M)':
"""Return a relation where all couplets have their left and right components swapped.
:return: The :term:`unary extension` of :term:`transposition` from the
:term:`algebra of couplets` to the :term:`algebra of relations`, applied to the
:term:`relation` ``rel``, or `Undef()` if ``rel`` is not a relation.
"""
if _checked:
if not is_member(rel):
return _make_or_raise_undef()
else:
assert is_member(rel)
return _extension.unary_extend(rel, partial(_couplets.transpose, _checked=False),
_checked=False).cache_is_relation(True)
# ----------------------------------------------------------------------------------------------
# Binary algebra operations.
@staticmethod
[docs] def compose(rel1: 'P(M x M)', rel2: 'P(M x M)', _checked=True) -> 'P(M x M)':
r"""Return the composition of ``rel1`` with ``rel2``.
:return: The :term:`binary extension` of :term:`composition` from the :term:`algebra of
couplets` to the :term:`algebra of relations`, applied to the :term:`relation`\s
``rel1`` and ``rel2``, or `Undef()` if ``rel1`` or ``rel2`` are not relations.
"""
if _checked:
if not is_member(rel1):
return _make_or_raise_undef()
if not is_member(rel2):
return _make_or_raise_undef()
else:
assert is_member(rel1)
assert is_member(rel2)
return _extension.binary_extend(rel1, rel2, partial(
_couplets.compose, _checked=False), _checked=False).cache_is_relation(True)
@staticmethod
[docs] def functional_union(rel1: 'P(M x M)', rel2: 'P(M x M)', _checked=True) -> 'P(M x M)':
r"""Return the union of ``rel1`` and ``rel2`` if it is a function, otherwise `Undef()`.
:return: The :term:`functional union` of the :term:`relation`\s ``rel1`` and ``rel2``;
that is, the :term:`union` if the result is a :term:`function`, otherwise
`Undef()`. Also return `Undef()` if ``rel1`` or ``rel2`` are not relations.
"""
if _checked:
if not is_member(rel1):
return _make_or_raise_undef()
if not is_member(rel2):
return _make_or_raise_undef()
else:
assert is_member(rel1)
assert is_member(rel2)
rel_union = _sets.union(rel1, rel2, _checked=False).cache_is_relation(True)
if not is_functional(rel_union, _checked=False):
return _make_or_raise_undef(2)
return rel_union
@staticmethod
[docs] def right_functional_union(rel1: 'P(M x M)', rel2: 'P(M x M)', _checked=True) -> 'P(M x M)':
r"""Return the union of ``rel1`` and ``rel2`` if it is right-functional, otherwise
`Undef()`.
:return: The :term:`right-functional union` of the :term:`relation`\s ``rel1`` and
``rel2``; that is, the :term:`union` if the result is :term:`right-functional`,
otherwise `Undef()`. Also return `Undef()` if ``rel1`` or ``rel2`` are not relations.
"""
if _checked:
if not is_member(rel1):
return _make_or_raise_undef()
if not is_member(rel2):
return _make_or_raise_undef()
else:
assert is_member(rel1)
assert is_member(rel2)
rel_union = _sets.union(rel1, rel2, _checked=False).cache_is_relation(True)
if not is_right_functional(rel_union, _checked=False):
return _make_or_raise_undef(2)
return rel_union
# For convenience, make the members of class Algebra (they are all static functions) available at
# the module level.
#: Convenience redirection to `Algebra.transpose`.
transpose = Algebra.transpose
#: Convenience redirection to `Algebra.compose`.
compose = Algebra.compose
#: Convenience redirection to `Algebra.functional_union`.
functional_union = Algebra.functional_union
#: Convenience redirection to `Algebra.right_functional_union`.
right_functional_union = Algebra.right_functional_union
# --------------------------------------------------------------------------------------------------
# Metadata functions.
[docs]def get_name() -> str:
"""Return the name and :term:`ground set` of this :term:`algebra` in string form."""
return 'Relations(M): {ground_set}'.format(ground_set=str(get_ground_set()))
[docs]def get_ground_set() -> _structure.Structure:
"""Return the :term:`ground set` of this :term:`algebra`."""
return _structure.PowerSet(_couplets.get_ground_set())
[docs]def get_absolute_ground_set() -> _structure.Structure:
"""Return the :term:`absolute ground set` of this :term:`algebra`."""
return _structure.PowerSet(_couplets.get_absolute_ground_set())
[docs]def is_member(obj: _mo.MathObject) -> bool:
"""Return whether ``obj`` is a member of the :term:`ground set` of this :term:`algebra`.
.. note:: This function may call :meth:`~.MathObject.get_ground_set` on ``obj``. The result
of this operation is cached.
"""
_mo.raise_if_not_mathobject(obj)
if not obj.cached_is_relation and not obj.cached_is_not_relation:
obj.cache_is_relation(obj.get_ground_set().is_subset(get_ground_set()))
return obj.cached_is_relation
[docs]def is_absolute_member(obj: _mo.MathObject) -> bool:
"""Return whether ``obj`` is a member of the :term:`absolute ground set` of this algebra.
:return: ``True`` if ``obj`` is an :term:`absolute relation`, ``False`` if not.
.. note:: This function calls :meth:`~.MathObject.get_ground_set` on ``obj``."""
_mo.raise_if_not_mathobject(obj)
return obj.get_ground_set().is_subset(get_absolute_ground_set())
# --------------------------------------------------------------------------------------------------
# Related operations, not formally part of the algebra.
[docs]def get_lefts(rel: 'P(M x M)', _checked=True) -> 'P( M )':
"""Return the set of the left components of all couplets in the relation ``rel``.
:return: The :term:`left set` of the :term:`relation` ``rel`` or `Undef()` if ``rel`` is not a
relation.
"""
if _checked:
if not is_member(rel):
return _make_or_raise_undef()
else:
assert is_member(rel)
return _mo.Set((e.left for e in rel), direct_load=True)
[docs]def get_rights(rel: 'P(M x M)', _checked=True) -> 'P( M )':
"""Return the set of the right components of all couplets in the relation ``rel``.
:return: The :term:`right set` of the :term:`relation` ``rel`` or `Undef()` if ``rel`` is not a
relation.
"""
if _checked:
if not is_member(rel):
return _make_or_raise_undef()
else:
assert is_member(rel)
return _mo.Set((e.right for e in rel), direct_load=True)
[docs]def get_rights_for_left(rel: 'P(M x M)', left: '( M )', _checked=True) -> '( M )':
"""Return the set of the right components of all couplets in the relation ``rel`` associated
with the :term:`left component` ``left``.
:return: The :term:`right set` of the :term:`relation` ``rel`` associated with the :term:`left
component` or `Undef()` if ``rel`` is not a :term:`relation`.
"""
if _checked:
if not is_member(rel):
return _make_or_raise_undef()
left = _mo.auto_convert(left)
else:
assert is_member(rel)
assert isinstance(left, _mo.MathObject)
return _mo.Set((elem.right for elem in rel if elem.left == left), direct_load=True)
[docs]def get_right(rel: 'P(M x M)', left: '( M )', _checked=True) -> '( M )':
r"""Return the right component of the couplet that has a left component of ``left``.
In general, use with :term:`function`\s; that is, :term:`relation`\s where all
:term:`left component`\s appear at most once.
:return: The :term:`right component` of the :term:`couplet` that has a :term:`left component`
of ``left``, or `Undef()` if there is not exactly one couplet with the left component
``left`` in ``rel`` or ``rel`` is not a :term:`relation`.
"""
if _checked:
if not is_member(rel):
return _make_or_raise_undef()
else:
assert is_member(rel)
left = _mo.auto_convert(left)
result = None
for elem in rel:
assert isinstance(elem, _mo.Couplet)
if elem.left == left:
if result is not None:
return _make_or_raise_undef() # Early Undef() exit if more than one found.
result = elem.right
if result is None:
return _make_or_raise_undef() # Undef() exit if none found.
return result
[docs]def get_left(rel: 'P(M x M)', right: '( M )', _checked=True) -> '( M )':
r"""Return the left component of the couplet that has a right component of ``right``.
In general, use with :term:`right-functional` :term:`relation`\s; that is, relations
where all :term:`right component`\s appear at most once.
:return: The :term:`left component` of the :term:`couplet` that has a :term:`right component`
of ``right``, or `Undef()` if there is not exactly one couplet with the right component
``right`` in ``rel`` or ``rel`` is not a :term:`relation`.
"""
if _checked:
if not is_member(rel):
return _make_or_raise_undef()
else:
assert is_member(rel)
right = _mo.auto_convert(right)
result = None
for elem in rel:
assert isinstance(elem, _mo.Couplet)
if elem.right == right:
if result is not None:
return _make_or_raise_undef() # Early Undef() exit if more than one found.
result = elem.left
if result is None:
return _make_or_raise_undef() # Undef() exit if none found.
return result
[docs]def is_functional(rel, _checked=True) -> bool:
"""Return whether ``rel`` is left-functional (is a function).
:return: ``True`` if ``rel`` is a :term:`function`, ``False`` if not, or `Undef()` if ``rel`` is
not a :term:`relation`.
"""
if _checked:
if not is_member(rel):
return _make_or_raise_undef()
else:
assert is_member(rel)
if not rel.cached_is_functional and not rel.cached_is_not_functional:
left_set = get_lefts(rel, _checked=False)
rel.cache_is_functional(left_set.cardinality == rel.cardinality)
return rel.cached_is_functional
[docs]def is_right_functional(rel, _checked=True) -> bool:
"""Return whether ``rel`` is right-functional.
:return: ``True`` if ``rel`` is :term:`right-functional`, ``False`` if not, or `Undef()` if
``rel`` is not a :term:`relation`.
"""
if _checked:
if not is_member(rel):
return _make_or_raise_undef()
else:
assert is_member(rel)
if not rel.cached_is_right_functional and not rel.cached_is_not_right_functional:
right_set = get_rights(rel, _checked=False)
rel.cache_is_right_functional(right_set.cardinality == rel.cardinality)
return rel.cached_is_right_functional
[docs]def is_reflexive(rel, _checked=True) -> bool:
"""Return whether ``rel`` is reflexive.
:return: ``True`` if ``rel`` is :term:`reflexive`, ``False`` if it is not, or `Undef()` if
``rel`` is not a :term:`relation`.
"""
if _checked:
if not is_member(rel):
return _make_or_raise_undef()
else:
assert is_member(rel)
if not rel.cached_is_reflexive and not rel.cached_is_not_reflexive:
reflexive = all(couplet.is_reflexive() for couplet in rel)
rel.cache_is_reflexive(reflexive)
return rel.cached_is_reflexive
[docs]def is_symmetric(rel, _checked=True) -> bool:
"""Return whether ``rel`` is symmetric.
:return: ``True`` if ``rel`` is :term:`symmetric`, ``False`` if it is not, or `Undef()` if
``rel`` is not a :term:`relation`.
"""
if _checked:
if not is_member(rel):
return _make_or_raise_undef()
else:
assert is_member(rel)
if not rel.cached_is_symmetric and not rel.cached_is_not_symmetric:
symmetric = all(rel.has_element(
_couplets.transpose(couplet, _checked=False)) for couplet in rel)
rel.cache_is_symmetric(symmetric)
return rel.cached_is_symmetric
[docs]def is_transitive(rel, _checked=True) -> bool:
"""Return whether ``rel`` is transitive.
:return: ``True`` if ``rel`` is :term:`transitive`, ``False`` if it is not, or `Undef()` if
``rel`` is not a :term:`relation`.
"""
if _checked:
if not is_member(rel):
return _make_or_raise_undef()
else:
assert is_member(rel)
if not rel.cached_is_transitive and not rel.cached_is_not_transitive:
transitive = True
for couplet1 in rel:
for couplet2 in rel:
if couplet1.left == couplet2.right:
if not rel.has_element(_mo.Couplet(couplet2.left, couplet1.right)):
transitive = False
break
rel.cache_is_transitive(transitive)
return rel.cached_is_transitive
[docs]def fill_lefts(rel: 'P(M x M)', renames: 'P(M x M)', _checked=True) -> 'P(M x M)':
r"""Return the left components in ``rel`` that are missing in ``renames`` as a diagonal
unioned with ``renames``.
The purpose is to create a :term:`relation` that can be used with the :term:`composition`
operation to change (rename) one or more :term:`left component`\s and leave the rest alone.
:param rel: The :term:`relation` that provides the full :term:`left set`.
:param renames: A relation where the :term:`right component`\s are meant to be
:term:`composition` 'origins' and the :term:`left component`\s composition 'targets'.
:return: A relation that contains all members of ``renames`` unioned with a :term:`diagonal`
that consists of all left components in ``rel`` that are missing in ``renames``.
"""
if _checked:
if not is_member(rel):
return _make_or_raise_undef()
if not is_member(renames):
return _make_or_raise_undef()
else:
assert is_member(rel)
assert is_member(renames)
missing_lefts = _sets.minus(get_lefts(rel, _checked=False),
get_rights(renames, _checked=False), _checked=False)
diag_missing_lefts = diag(*missing_lefts, _checked=False)
return _sets.union(renames, diag_missing_lefts, _checked=False).cache_is_relation(True)
[docs]def rename(rel: 'P(M x M)', renames: 'P(M x M)', _checked=True) -> 'P(M x M)':
r"""Return a relation where left components in ``rel`` are renamed according to ``renames``.
:param rel: The :term:`relation` with the :term:`left component`\s to rename.
:param renames: A relation where the :term:`right component`\s are the current left components
in ``rel`` and the left components are the new left components to use in ``rel``.
:return: A version of ``rel`` where some left components of the member :term:`couplet`\s are
changed (renamed), according to ``renames``.
"""
if _checked:
if not is_member(rel):
return _make_or_raise_undef()
if not is_member(renames):
return _make_or_raise_undef()
else:
assert is_member(rel)
assert is_member(renames)
renames_complete = fill_lefts(rel, renames, _checked=False)
result = compose(rel, renames_complete, _checked=False)
return result
[docs]def swap(rel: 'P(M x M)', swaps: 'P(M x M)', _checked=True) -> 'P(M x M)':
r"""Return a relation where components in ``rel`` are swapped according to ``swaps``.
:param rel: The :term:`relation` with the :term:`left component`\s to swap.
:param swaps: A relation where both :term:`right component`\s and left components are current
left components in ``rel``. These left components are swapped.
:return: A version of ``rel`` where some left components of the member :term:`couplet`\s are
swapped, according to ``swaps``.
"""
if _checked:
if not is_member(rel):
return _make_or_raise_undef()
if not is_member(swaps):
return _make_or_raise_undef()
else:
assert is_member(rel)
assert is_member(swaps)
renames = _sets.union(swaps, transpose(swaps, _checked=False), _checked=False)
return rename(rel, renames, _checked=False)
[docs]def functional_add(rel: 'P(M x M)', element: 'M x M') -> 'P(M x M)':
"""Add ``element`` to ``rel`` and return the new relation.
:param rel: The source data. Must be a :term:`relation`. It must not contain a :term:`couplet`
with the same :term:`left component` as ``element``.
:param element: The element to be added to ``rel``. Must be a :class:`~.Couplet` and its
:term:`left component` must not be a left component in ``rel``.
:return: The new relation, composed of ``rel`` and ``element``.
"""
if not is_member(rel):
return _make_or_raise_undef()
if not isinstance(element, _mo.Couplet):
return _make_or_raise_undef()
if _sets.is_subset_of(_mo.Set(element.left), get_lefts(rel)):
return _make_or_raise_undef(2)
result_relation = _sets.union(rel, _mo.Set(element))
return result_relation
[docs]def from_dict(dict1: dict) -> 'P(M x M)':
r"""Return a :term:`relation` where the :term:`couplet`\s are the elements of ``dict1``."""
return _mo.Set((_mo.Couplet(left, right) for left, right in dict1.items()),
direct_load=True).cache_is_relation(True).cache_is_functional(True)
[docs]def diag(*args, _checked=True) -> 'P(M x M)':
"""Return the :term:`diagonal` of the set comprising the elements in ``*args``."""
return _mo.Set((_mo.Couplet(
elem, direct_load=True if not _checked else False) for elem in args),
direct_load=True).cache_is_relation(True).cache_is_functional(True)
[docs]def defined_at(rel, left, _checked=True):
"""Return ``rel`` if it has a :term:`couplet` with left component ``left`` else `Undef()`."""
if not get_rights_for_left(rel, left, _checked):
return _make_or_raise_undef(2)
return rel