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"""Extra support for type hinting and generics.

Provides additional support for annotation and static type checking.
Extends the specifications of the abstract base classes in gmxapi.abc.
"""

from typing import Tuple, TypeVar, Generic, NewType, Type

import gmxapi.abc

# Use SourceTypeVar and ResultTypeVar for static type hints, annotations, and as a parameter to generics.
# Use valid_source_types and valid_result_types for run-time type checking.
ResultTypeVar = TypeVar('ResultTypeVar', *(str, bool, int, float, dict, gmxapi.abc.NDArray))
valid_result_types: Tuple[type] = ResultTypeVar.__constraints__

SourceTypeVar = TypeVar('SourceTypeVar',
                        *(str, bool, int, float, dict, gmxapi.abc.NDArray))
valid_source_types: Tuple[type] = SourceTypeVar.__constraints__

# Place holder for type annotations of Context objects.
# TODO: Expand to support some static type checking.
_Context = TypeVar('_Context', bound=gmxapi.abc.Context)
# Type variable that binds to subclasses of the forward referenced OperationImplentation ABC.
_Op = TypeVar('_Op', bound=gmxapi.abc.OperationImplementation)

# Note: We could make many types generic in SourceContext and TargetContext and
# compose functionality into instances of a helpful base class with the option of
# subclassing the generic base with concrete types. If we want to reserve the word
# "Context" for an execution context, then we can say that a SourceResource or
# TargetEnvironment can each be either a Context or an OperationExecutionDirector.
#
# We can't
# generally add type-hinting details in terms of a specialized generic, but we
# can require a particular version of a generic and we can use the inferred
# type parameter value in limited cases, including to fully specify other generics.
# For instance, we can't assert that a generic with a particular parameter has
# additional attributes, but we can overload a function that produces an object
# with those attributes. More generally, using some sort of identity function like
# that may be the solution to the type erasure of a generic abstract base class.
# For static type checking of dynamically defined subclasses, we can use a decorator
# to reduce boiler plate if the type checker does a decent job of resolving closures.
# Ironically, there may be cases where we want to *avoid* annotating the return
# value to allow the static source analyzer to infer the dynamically typed return value.


class Future(Generic[ResultTypeVar]):
    """Generic result data."""
    def dtype(self) -> Type[ResultTypeVar]:
        ...

    def result(self) -> ResultTypeVar:
        ...


class OperationReference(gmxapi.abc.OperationReference, Generic[_Op]):
    """Object with an OperationReference interface.

    Generic version of AbstractOperationReference, parameterized by operation
    implementation.
    """


class OperationDirector(Generic[_Op, _Context]):
    """Generic abstract operation director.

    An operation director is instantiated for a specific operation and context
    by a dispatching factory to add a computational element to the work managed
    by the context.

    Note:
        This is a generic class, as defined with the Python `typing` module.
        If used as a base class, re-expression of the TypeVar parameters in class
        subscripts will cause the derived class to be generic unless regular
        classes (non-TypeVar) are given. Omission of the subscripts causes `Any`
        to be bound, which also results in a non-generic subclass.
    """


class OperationImplementation(Generic[_Op], gmxapi.abc.OperationImplementation):
    """Essential interface of an Operation implementation.

    Describe the essential features of an Operation that can be registered with
    gmxapi to support building and executing work graphs in gmxapi compatible
    execution contexts.
    """
    # The executable part of an operation consumes a distinct resource type.
    # The resource type may be opaque, because it is created through a factory
    # and only used in passing to a function object.
    ResourceType = NewType('ResourceType', object)

    def name(self) -> str:
        """The name of the operation.

        Generally, this corresponds to a callable attribute of a Python module
        (named by namespace()) that acts as a factory for new operation instances.
        It is also used by Context implementations to locate code supporting
        the operation.
        """
        ...

    def namespace(self) -> str:
        """The namespace of the operation.

        Generally, the namespace corresponds to a Python module importable in
        the execution environment.
        """

    # Note that this indicates that the ABC requires subclasses to provide a generic function,
    # which _is_ the factory behavior we are trying to specify.
    # TODO: Confirm that this type-checks correctly.
    # We may need to look more at how the type checking is implemented to see how
    # to do this right.
    def director(self, context: _Context) -> OperationDirector[_Op, _Context]:
        """Factory to get an OperationDirector appropriate for the context."""
        ...
