#  ___________________________________________________________________________
#
#  Pyomo: Python Optimization Modeling Objects
#  Copyright 2017 National Technology and Engineering Solutions of Sandia, LLC
#  Under the terms of Contract DE-NA0003525 with National Technology and 
#  Engineering Solutions of Sandia, LLC, the U.S. Government retains certain 
#  rights in this software.
#  This software is distributed under the 3-clause BSD License.
#  ___________________________________________________________________________

"""Transformation to detect variables fixed by bounds and fix them."""
from math import fabs

from pyomo.core.base.transformation import TransformationFactory
from pyomo.common.collections import ComponentMap
from pyomo.common.config import (ConfigBlock, ConfigValue, NonNegativeFloat,
                                 add_docstring_list)
from pyomo.core.base.var import Var
from pyomo.core.expr.numvalue import value
from pyomo.core.plugins.transform.hierarchy import IsomorphicTransformation


@TransformationFactory.register(
        'contrib.detect_fixed_vars',
        doc="Detect variables that are de-facto fixed but not considered fixed.")
class FixedVarDetector(IsomorphicTransformation):
    """Detects variables that are de-facto fixed but not considered fixed.

    For each variable :math:`v` found on the model, check to see if its lower
    bound :math:`v^{LB}` is within some tolerance of its upper bound
    :math:`v^{UB}`. If so, fix the variable to the value of :math:`v^{LB}`.

    Keyword arguments below are specified for the ``apply_to`` and
    ``create_using`` functions.

    """

    CONFIG = ConfigBlock("FixedVarDetector")
    CONFIG.declare("tmp", ConfigValue(
        default=False, domain=bool,
        description="True to store the set of transformed variables and "
        "their old values so that they can be restored."
    ))
    CONFIG.declare("tolerance", ConfigValue(
        default=1E-13, domain=NonNegativeFloat,
        description="tolerance on bound equality (LB == UB)"
    ))

    __doc__ = add_docstring_list(__doc__, CONFIG)

    def _apply_to(self, instance, **kwargs):
        config = self.CONFIG(kwargs)

        if config.tmp:
            instance._xfrm_detect_fixed_vars_old_values = ComponentMap()

        for var in instance.component_data_objects(
                ctype=Var, descend_into=True):
            if var.fixed or var.lb is None or var.ub is None:
                # if the variable is already fixed, or if it is missing a
                # bound, we skip it.
                continue
            if fabs(value(var.lb) - value(var.ub)) <= config.tolerance:
                if config.tmp:
                    instance._xfrm_detect_fixed_vars_old_values[var] = \
                        var.value
                var.fix(var.lb)

    def revert(self, instance):
        """Revert variables fixed by the transformation."""
        for var, var_value in instance._xfrm_detect_fixed_vars_old_values.items():
            var.unfix()
            var.set_value(var_value)

        del instance._xfrm_detect_fixed_vars_old_values
