# Source code for picos.expressions.set_rsoc

# coding: utf-8

# ------------------------------------------------------------------------------
# Copyright (C) 2019 Maximilian Stahlberg
# Based on the original picos.expressions module by Guillaume Sagnol.
#
# This file is part of PICOS.
#
# PICOS is free software: you can redistribute it and/or modify
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# PICOS 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 General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program.  If not, see <http://www.gnu.org/licenses/>.
# ------------------------------------------------------------------------------

"""Implements :class:RotatedSecondOrderCone."""

import operator
from collections import namedtuple

from .. import glyphs
from ..apidoc import api_end, api_start
from ..constraints import RSOCConstraint
from .data import convert_operands
from .exp_affine import AffineExpression
from .expression import refine_operands, validate_prediction
from .set import Set

_API_START = api_start(globals())
# -------------------------------

[docs]class RotatedSecondOrderCone(Set):
r"""The (narrowed or widened) rotated second order cone.

.. _rotatedcone:

For :math:n \in \mathbb{Z}_{\geq 3} and :math:p \in \mathbb{R}_{> 0},
represents the convex cone

.. math::

\mathcal{R}_{p}^n = \left\{
x \in \mathbb{R}^n
~\middle|~
p x_1 x_2 \geq \sum_{i = 3}^n x_i^2 \land x_1, x_2 \geq 0
\right\}.

For :math:p = 2, this is the standard rotated second order cone
:math:\mathcal{R}^n obtained by rotating the
:class:second order cone <picos.SecondOrderCone> :math:\mathcal{Q}^n
by :math:\frac{\pi}{4} in the :math:(x_1, x_2) plane.

The default instance of this class has :math:p = 1, which can be
understood as a narrowed version of the standard cone. This is more
convenient for defining the primal problem but it should be noted that
:math:\mathcal{R}_{1}^n is not self-dual, so working with
:math:p = 2 may seem more natural when the dual problem is of interest.

:Dual cone:

The dual cone is

.. math::

\left(\mathcal{R}_{p}^n\right)^* = \left\{
x \in \mathbb{R}^n
~\middle|~
\frac{4}{p} x_1 x_2 \geq \sum_{i = 2}^n x_i^2 \land x_1, x_2 \geq 0
\right\}=\mathcal{R}_{4/p}^n.

The cone is thus self-dual for :math:p = 2.
"""

[docs]    def __init__(self, p=1):
"""Construct a rotated second order cone.

:param float p:
The positive factor :math:p in the definition.
"""
try:
p = float(p)
except Exception as error:
raise TypeError("Failed to load the parameter 'p' as a float: {}"
.format(error))

if p <= 0:
raise ValueError("The parameter 'p' must be positive.")

self._p = p

typeStr = "Rotated Second Order Cone"
if p < 2:
typeStr = "Narrowed " + typeStr
elif p > 2:
typeStr = "Widened " + typeStr

symbStr = glyphs.set(glyphs.sep(
glyphs.col_vectorize("u", "v", "x"), glyphs.and_(
glyphs.le(
glyphs.squared(glyphs.norm("x")),
glyphs.clever_mul(glyphs.scalar(p), glyphs.mul("u", "v"))),
glyphs.ge("u", 0))))

Set.__init__(self, typeStr, symbStr)

@property
def p(self):
"""A narrowing (:math:p < 2) or widening (:math:p > 2) factor."""
return self._p

def _get_variables(self):
return set()

def _replace_variables(self):
return self

Subtype = namedtuple("Subtype", ())

def _get_subtype(self):
return self.Subtype()

@classmethod
def _predict(cls, subtype, relation, other):
assert isinstance(subtype, cls.Subtype)

if relation == operator.__rshift__:
if issubclass(other.clstype, AffineExpression):
if other.subtype.dim >= 3:
return RSOCConstraint.make_type(other.subtype.dim - 2)

return NotImplemented

@convert_operands()
@validate_prediction
@refine_operands()
def __rshift__(self, element):
if isinstance(element, AffineExpression):
if len(element) < 3:
raise TypeError("Elements of the rotated second order cone must"
" be at least three-dimensional.")

element = element.vec

return RSOCConstraint(element[2:], self.p * element[0], element[1])
else:
return NotImplemented

# --------------------------------------
__all__ = api_end(_API_START, globals())