unitsystem.py

"""
Unit system for physical quantities; include definition of constants.
"""

from typing import Dict as tDict, Set as tSet

from sympy.core.add import Add
from sympy.core.function import (Derivative, Function)
from sympy.core.mul import Mul
from sympy.core.power import Pow
from sympy.core.singleton import S
from sympy.physics.units.dimensions import _QuantityMapper
from sympy.physics.units.quantities import Quantity

from .dimensions import Dimension


class UnitSystem(_QuantityMapper):
    """
    UnitSystem represents a coherent set of units.

    A unit system is basically a dimension system with notions of scales. Many
    of the methods are defined in the same way.

    It is much better if all base units have a symbol.
    """

    _unit_systems = {}  # type: tDict[str, UnitSystem]

    def __init__(self, base_units, units=(), name="", descr="", dimension_system=None, derived_units: tDict[Dimension, Quantity]={}):

        UnitSystem._unit_systems[name] = self

        self.name = name
        self.descr = descr

        self._base_units = base_units
        self._dimension_system = dimension_system
        self._units = tuple(set(base_units) | set(units))
        self._base_units = tuple(base_units)
        self._derived_units = derived_units

        super().__init__()

    def __str__(self):
        """
        Return the name of the system.

        If it does not exist, then it makes a list of symbols (or names) of
        the base dimensions.
        """

        if self.name != "":
            return self.name
        else:
            return "UnitSystem((%s))" % ", ".join(
                str(d) for d in self._base_units)

    def __repr__(self):
        return '<UnitSystem: %s>' % repr(self._base_units)

    def extend(self, base, units=(), name="", description="", dimension_system=None, derived_units: tDict[Dimension, Quantity]={}):
        """Extend the current system into a new one.

        Take the base and normal units of the current system to merge
        them to the base and normal units given in argument.
        If not provided, name and description are overridden by empty strings.
        """

        base = self._base_units + tuple(base)
        units = self._units + tuple(units)

        return UnitSystem(base, units, name, description, dimension_system, {**self._derived_units, **derived_units})

    def get_dimension_system(self):
        return self._dimension_system

    def get_quantity_dimension(self, unit):
        qdm = self.get_dimension_system()._quantity_dimension_map
        if unit in qdm:
            return qdm[unit]
        return super().get_quantity_dimension(unit)

    def get_quantity_scale_factor(self, unit):
        qsfm = self.get_dimension_system()._quantity_scale_factors
        if unit in qsfm:
            return qsfm[unit]
        return super().get_quantity_scale_factor(unit)

    @staticmethod
    def get_unit_system(unit_system):
        if isinstance(unit_system, UnitSystem):
            return unit_system

        if unit_system not in UnitSystem._unit_systems:
            raise ValueError(
                "Unit system is not supported. Currently"
                "supported unit systems are {}".format(
                    ", ".join(sorted(UnitSystem._unit_systems))
                )
            )

        return UnitSystem._unit_systems[unit_system]

    @staticmethod
    def get_default_unit_system():
        return UnitSystem._unit_systems["SI"]

    @property
    def dim(self):
        """
        Give the dimension of the system.

        That is return the number of units forming the basis.
        """
        return len(self._base_units)

    @property
    def is_consistent(self):
        """
        Check if the underlying dimension system is consistent.
        """
        # test is performed in DimensionSystem
        return self.get_dimension_system().is_consistent

    @property
    def derived_units(self) -> tDict[Dimension, Quantity]:
        return self._derived_units

    def get_dimensional_expr(self, expr):
        from sympy.physics.units import Quantity
        if isinstance(expr, Mul):
            return Mul(*[self.get_dimensional_expr(i) for i in expr.args])
        elif isinstance(expr, Pow):
            return self.get_dimensional_expr(expr.base) ** expr.exp
        elif isinstance(expr, Add):
            return self.get_dimensional_expr(expr.args[0])
        elif isinstance(expr, Derivative):
            dim = self.get_dimensional_expr(expr.expr)
            for independent, count in expr.variable_count:
                dim /= self.get_dimensional_expr(independent)**count
            return dim
        elif isinstance(expr, Function):
            args = [self.get_dimensional_expr(arg) for arg in expr.args]
            if all(i == 1 for i in args):
                return S.One
            return expr.func(*args)
        elif isinstance(expr, Quantity):
            return self.get_quantity_dimension(expr).name
        return S.One

    def _collect_factor_and_dimension(self, expr):
        """
        Return tuple with scale factor expression and dimension expression.
        """
        from sympy.physics.units import Quantity
        if isinstance(expr, Quantity):
            return expr.scale_factor, expr.dimension
        elif isinstance(expr, Mul):
            factor = 1
            dimension = Dimension(1)
            for arg in expr.args:
                arg_factor, arg_dim = self._collect_factor_and_dimension(arg)
                factor *= arg_factor
                dimension *= arg_dim
            return factor, dimension
        elif isinstance(expr, Pow):
            factor, dim = self._collect_factor_and_dimension(expr.base)
            exp_factor, exp_dim = self._collect_factor_and_dimension(expr.exp)
            if self.get_dimension_system().is_dimensionless(exp_dim):
                exp_dim = 1
            return factor ** exp_factor, dim ** (exp_factor * exp_dim)
        elif isinstance(expr, Add):
            factor, dim = self._collect_factor_and_dimension(expr.args[0])
            for addend in expr.args[1:]:
                addend_factor, addend_dim = \
                    self._collect_factor_and_dimension(addend)
                if dim != addend_dim:
                    raise ValueError(
                        'Dimension of "{}" is {}, '
                        'but it should be {}'.format(
                            addend, addend_dim, dim))
                factor += addend_factor
            return factor, dim
        elif isinstance(expr, Derivative):
            factor, dim = self._collect_factor_and_dimension(expr.args[0])
            for independent, count in expr.variable_count:
                ifactor, idim = self._collect_factor_and_dimension(independent)
                factor /= ifactor**count
                dim /= idim**count
            return factor, dim
        elif isinstance(expr, Function):
            fds = [self._collect_factor_and_dimension(
                arg) for arg in expr.args]
            return (expr.func(*(f[0] for f in fds)),
                    *(d[1] for d in fds))
        elif isinstance(expr, Dimension):
            return S.One, expr
        else:
            return expr, Dimension(1)

    def get_units_non_prefixed(self) -> tSet[Quantity]:
        """
        Return the units of the system that do not have a prefix.
        """
        return set(filter(lambda u: not u.is_prefixed and not u.is_physical_constant, self._units))