import builtins
import io
import re
import sys
import traceback
import warnings
from collections import Counter, defaultdict
from collections.abc import Callable, Iterable, Sequence
from keyword import iskeyword
from operator import itemgetter
from tempfile import NamedTemporaryFile
from textwrap import dedent, indent
from typing import (
    TYPE_CHECKING,
    Any,
    NoReturn,
    Optional,
    TextIO,
    TypeVar,
    cast,
)

import numpy as np

from pytensor import config, utils
from pytensor.graph.basic import Apply, Constant, Variable
from pytensor.graph.fg import FunctionGraph


if TYPE_CHECKING:
    from pytensor.graph.op import (
        BasicThunkType,
        InputStorageType,
        OutputStorageType,
        StorageCellType,
        StorageMapType,
    )


def map_storage(
    fgraph: FunctionGraph,
    order: Iterable[Apply],
    input_storage: Optional["InputStorageType"] = None,
    output_storage: Optional["OutputStorageType"] = None,
    storage_map: Optional["StorageMapType"] = None,
) -> tuple["InputStorageType", "OutputStorageType", "StorageMapType"]:
    """Ensure there is storage (a length-1 list) for inputs, outputs, and interior nodes.

    Parameters
    ----------
    fgraph
        The current fgraph. This function uses the inputs and outputs
        attributes.
    order
        An iterable over Apply instances (in program running order).
    input_storage
        None or existing input storage (see below).
    output_storage
        None or existing output storage (see below).

    Returns
    -------
    3-tuple
        List of storage for inputs, list of storage for outputs, and
        the `storage_map`.

    Extended summary
    ----------------
    This function iterates over the nodes in `order` and ensures that for every
    input and output `Variable`, there is a unique storage container. This is
    returned as a dictionary Variable -> storage called the `storage_map`.

    This function also returns `input_storage`, which is a list of storages
    corresponding to fgraph.inputs.
    This function also returns `output_storage`, which is a list of storages
    corresponding to fgraph.outputs.

    """
    # each Apply argument's data is stored in a list of length 1 (these lists act like pointers)

    if storage_map is None:
        storage_map = {}

    # input_storage is a list of data-containers for the inputs.
    if input_storage is None:
        input_storage = [[None] for input in fgraph.inputs]
    else:
        assert len(fgraph.inputs) == len(input_storage)

    # add input storage into storage_map
    for r, storage in zip(fgraph.inputs, input_storage, strict=True):
        if r in storage_map:
            assert storage_map[r] is storage, (
                "Given input_storage conflicts "
                "with storage in given storage_"
                "map. Given input_storage: ",
                storage,
                "Storage in storage_map: ",
                storage_map[r],
            )
        else:
            storage_map[r] = storage
    #     for orphan in fgraph.orphans:
    #         if not isinstance(orphan, Constant):
    #             raise TypeError("Cannot link a graph with non-constant orphans.", orphan)
    #         storage_map[orphan] = [orphan.data]

    # allocate output storage
    if output_storage is not None:
        assert len(fgraph.outputs) == len(output_storage)
        for r, storage in zip(fgraph.outputs, output_storage, strict=True):
            if r in storage_map:
                assert storage_map[r] is storage, (
                    "Given output_storage confl"
                    "icts with storage in given"
                    " storage_map. Given output"
                    "_storage: ",
                    storage,
                    "Storage in storage_map: ",
                    storage_map[r],
                )
            else:
                storage_map[r] = storage

    # allocate storage for intermediate computation
    for node in order:
        for r in node.inputs:
            if r not in storage_map:
                assert isinstance(r, Constant)
                storage_map[r] = [r.data]
        for r in node.outputs:
            storage_map.setdefault(r, [None])
    for r in fgraph.outputs:
        if isinstance(r, Constant):
            storage_map.setdefault(r, [r.data])

    # extract output storage
    if output_storage is None:
        output_storage = [storage_map[r] for r in fgraph.outputs]

    return input_storage, output_storage, storage_map


def streamline(
    fgraph: FunctionGraph,
    thunks: Sequence[Callable[[], None]],
    order: Sequence[Apply],
    post_thunk_old_storage: list["StorageCellType"] | None = None,
    no_recycling: list["StorageCellType"] | None = None,
    nice_errors: bool = True,
) -> "BasicThunkType":
    """Construct a single thunk that runs a list of thunks.

    Parameters
    ----------
    fgraph
    thunks
        The list of program instructions.
    order
        The list of apply instances that gave rise to the thunks
        (same order as thunks).
    post_thunk_old_storage
        A list (corresponding to thunks, order) whose elements are lists of
        storage cells, that should be cleared after running thecorresponding
        thunk. A value of None disables this functionality.
    no_recycling
        Storage elements that cannot be 'recycled' by repeatedly executing the
        program. These storage elements are cleared before re-running.
    nice_errors
        Run in such a way that the double-traceback is printed. This costs a
        bit of performance in the inner python loop.

    """
    if no_recycling is None:
        no_recycling = []

    if len(thunks) != len(order):
        raise ValueError(
            "Length of thunks and order must match", (len(thunks), len(order))
        )

    if post_thunk_old_storage:
        if len(thunks) != len(post_thunk_old_storage):
            raise ValueError(
                "Length of thunks and post_thunk_old_storage must match",
                (len(thunks), len(post_thunk_old_storage)),
            )

        def streamline_default_f():
            for x in no_recycling:
                x[0] = None
            try:
                # strict=False because we are in a hot loop
                for thunk, node, old_storage in zip(
                    thunks, order, post_thunk_old_storage, strict=False
                ):
                    thunk()
                    for old_s in old_storage:
                        old_s[0] = None
            except Exception:
                raise_with_op(fgraph, node, thunk)

        f = streamline_default_f
    elif nice_errors:

        def streamline_nice_errors_f():
            for x in no_recycling:
                x[0] = None
            try:
                # zip strict not specified because we are in a hot loop
                for thunk, node in zip(thunks, order):
                    thunk()
            except Exception:
                raise_with_op(fgraph, node, thunk)

        f = streamline_nice_errors_f
    else:
        # don't worry about raise_with_op, just go a little faster.
        # there is a mix of python and c thunks
        def streamline_fast_f():
            for x in no_recycling:
                x[0] = None
            for thunk in thunks:
                thunk()

        f = streamline_fast_f
    return f


def gc_helper(node_list: list[Apply]):
    """
    Return the set of Variable instances which are computed by node_list.
    Parameters
    ----------
    node_list
        List of Apply instances in program execution order.

    Returns
    -------
    2-tuple
        FIRST, the set of Variable instances which are computed by node_list,
        and SECOND a dictionary that maps each Variable instance to the last
        node to use Variable as an input.

    Extended Summary
    ----------------
    This is used to allow garbage collection within graphs.

    It ignores view_map and destroy_map. This isn't needed as python
    have reference count. In PyTensor gc, we should not take into
    account view_map and destroy_map as if the thunk decided to create
    a new output, we would delay uselessly its gc by Python.

    """
    # for freeing memory
    last_user = {}
    computed = set()
    for node in node_list:
        for input in node.inputs:
            last_user[input] = node
        for output in node.outputs:
            computed.add(output)
    return computed, last_user


def raise_with_op(
    fgraph: FunctionGraph, node: Apply, thunk=None, exc_info=None, storage_map=None
) -> NoReturn:
    """
    Re-raise an exception while annotating the exception object with
    debug info.

    Parameters
    ----------
    fgraph
        The `FunctionGraph` that contains `node`.
    node : Apply node
        The Apply node object that resulted in the raised exception.
    thunk :
        An optional thunk.
    exc_info : tuple, optional
        A tuple containing the exception type, exception object and
        associated traceback, as would be returned by a call to
        `sys.exc_info` (which is done if ``None`` is passed).
    storage_map: dict, optional
        storage map of the pytensor function that resulted in the
        raised exception.

    Notes
    -----
    This re-raises the exception described by `exc_info` (or the last
    one raised, if `exc_info` is omitted) and annotates the exception
    object with several new members which may be helpful for debugging
    PyTensor graphs. They are:

     * ``__op_instance__``: The `Op` that is responsible for the exception
       being raised.
     * ``__thunk_trace__``: A traceback corresponding to the code that
       actually generated the exception, if it is available.
     * ``__applynode_index__``: The index of the `Apply` node corresponding
       to this `Op` in ``op.fgraph.toposort``.

    The exception is not annotated if it is of type `KeyboardInterrupt`.

    TODO: Make this work with linker defined schedule
    """
    verbosity = config.exception_verbosity

    if exc_info is None:
        exc_info = sys.exc_info()
    exc_type, exc_value, exc_trace = exc_info
    if exc_type is KeyboardInterrupt:
        # print a simple traceback from KeyboardInterrupt
        raise exc_value.with_traceback(exc_trace)

    trace = getattr(node.outputs[0].tag, "trace", ())

    exc_value.__thunk_trace__ = trace
    exc_value.__op_instance__ = node
    topo = fgraph.toposort()
    if node in topo:
        node_index = topo.index(node)
    else:
        node_index = None
    exc_value.__applynode_index__ = node_index

    hints = []
    detailed_err_msg = f"\nApply node that caused the error: {node}"
    if exc_value.__applynode_index__ is not None:
        detailed_err_msg += f"\nToposort index: {node_index}"

    types = [getattr(ipt, "type", "No type") for ipt in node.inputs]
    detailed_err_msg += f"\nInputs types: {types}\n"

    shapes: list | str
    strides: list | str
    scalar_values: list | str

    if thunk is not None:
        if hasattr(thunk, "inputs"):
            shapes = [getattr(ipt[0], "shape", "No shapes") for ipt in thunk.inputs]
            strides = [getattr(ipt[0], "strides", "No strides") for ipt in thunk.inputs]
            scalar_values = []
            for ipt in thunk.inputs:
                if getattr(ipt[0], "size", -1) <= 5:
                    scalar_values.append(ipt[0])
                else:
                    scalar_values.append("not shown")
        else:
            shapes = "The thunk doesn't have an `inputs` attributes."
            strides = "So we can't access the strides of the input values"
            scalar_values = "and we can't print its scalar input values"
        clients = [[c[0] for c in fgraph.clients[var]] for var in node.outputs]
        detailed_err_msg += (
            f"Inputs shapes: {shapes}"
            f"\nInputs strides: {strides}"
            f"\nInputs values: {scalar_values}"
        )
        if verbosity == "high":
            inpts = [
                getattr(getattr(i[0], "dtype", ""), "num", "") for i in thunk.inputs
            ]
            detailed_err_msg += f"\nInputs type_num: {inpts}"

        detailed_err_msg += f"\nOutputs clients: {clients}\n"
    else:
        hints.append(
            "HINT: Use a linker other than the C linker to"
            " print the inputs' shapes and strides."
        )

    # Print node backtraces
    tr = getattr(node.outputs[0].tag, "trace", [])
    if isinstance(tr, list) and len(tr) > 0:
        detailed_err_msg += "\nBacktrace when the node is created "
        detailed_err_msg += (
            "(use PyTensor flag traceback__limit=N to make it longer):\n"
        )

        # Print separate message for each element in the list of batcktraces
        sio = io.StringIO()
        for subtr in tr:
            traceback.print_list(subtr, sio)
        detailed_err_msg += str(sio.getvalue())
    else:
        hints.append(
            "HINT: Re-running with most PyTensor optimizations disabled could"
            " provide a back-trace showing when this node was created. This can"
            " be done by setting the PyTensor flag"
            " 'optimizer=fast_compile'. If that does not work,"
            " PyTensor optimizations can be disabled with 'optimizer=None'."
        )

    if verbosity == "high":
        import pytensor.printing

        f = io.StringIO()
        pytensor.printing.debugprint(node, file=f, stop_on_name=True, print_type=True)
        detailed_err_msg += "\nDebug print of the apply node: \n"
        detailed_err_msg += f.getvalue()

    # Prints output_map
    if verbosity == "high" and storage_map is not None:
        detailed_err_msg += "\nStorage map footprint:\n"
        shared_input_list = [
            item
            for item in fgraph.inputs
            if isinstance(item, pytensor.compile.SharedVariable)
        ]
        nonshared_input_list = [
            item
            for item in fgraph.inputs
            if not isinstance(item, pytensor.compile.SharedVariable)
        ]
        storage_map_list: list = []
        total_size = 0
        total_size_inputs = 0
        for k in storage_map:
            storage_map_item: list = []

            # storage_map_item[0]: the variable
            storage_map_item.append(str(k))

            # storage_map_item[1]: the shape
            shapeinfo = None
            if hasattr(storage_map[k][0], "shape"):
                shapeinfo = storage_map[k][0].shape
                if len(shapeinfo) != 0:
                    storage_map_item.append(shapeinfo)
                else:
                    storage_map_item.append(tuple())
            else:
                storage_map_item.append(None)

            # storage_map_item[2]: itemsize
            # storage_map_item[3]: bytes
            if hasattr(storage_map[k][0], "dtype"):
                dtype = storage_map[k][0].dtype
                storage_map_item.append(np.dtype(dtype).itemsize)
                if shapeinfo is None:
                    storage_map_item.append(-1)
                else:
                    sz = np.dtype(dtype).itemsize * np.prod(shapeinfo)
                    storage_map_item.append(sz)
                    total_size += sz
                    if k.owner is None:
                        total_size_inputs += sz
                    else:
                        # If it is a view, don't count it twice.
                        vmap = k.owner.op.view_map
                        if vmap:
                            out_idx = k.owner.outputs.index(k)
                            data = storage_map[k][0]
                            if out_idx in vmap:
                                assert len(vmap[out_idx]) == 1
                                input_data = storage_map[
                                    k.owner.inputs[vmap[out_idx][0]]
                                ][0]
                                if k.type.may_share_memory(data, input_data):
                                    total_size -= sz
                        # If it is a destroyed input, the input
                        # shouldn't be in the storage_map anymore
                        # except if there is a special flag used. So
                        # we still must check it.
                        dmap = k.owner.op.destroy_map
                        if dmap:
                            out_idx = k.owner.outputs.index(k)
                            data = storage_map[k][0]
                            if out_idx in dmap:
                                assert len(dmap[out_idx]) == 1
                                input_data = storage_map[
                                    k.owner.inputs[dmap[out_idx][0]]
                                ][0]
                                if k.type.may_share_memory(data, input_data):
                                    total_size -= sz
            else:
                bytes = sys.getsizeof(storage_map[k][0])
                storage_map_item.append(bytes)
                storage_map_item.append(-1)

            # Flag of shared val
            # storage_map_item[4]
            if k in shared_input_list:
                storage_map_item.append(True)
            elif k in nonshared_input_list:
                storage_map_item.append(False)
            else:
                storage_map_item.append(None)
            storage_map_list.append(storage_map_item)

        storage_map_list.sort(key=itemgetter(3), reverse=True)
        for item in storage_map_list:
            if item[3] == -1:
                continue
            detailed_err_msg += " - " + item[0] + ", "
            if item[4] is True:
                detailed_err_msg += "Shared Input, "
            elif item[4] is False:
                detailed_err_msg += "Input, "
            if item[1] is not None:
                detailed_err_msg += f"Shape: {item[1]}, "
            detailed_err_msg += f"ElemSize: {item[2]} Byte(s)"
            if item[3] is not None:
                detailed_err_msg += f", TotalSize: {item[3]} Byte(s)\n"
            else:
                detailed_err_msg += "\n"
        detailed_err_msg += f" TotalSize: {total_size} Byte(s) {total_size / 1024 / 1024 / 1024:.3f} GB\n"
        detailed_err_msg += f" TotalSize inputs: {total_size_inputs} Byte(s) {total_size_inputs / 1024 / 1024 / 1024:.3f} GB\n"

    else:
        hints.append(
            "HINT: Use the PyTensor flag `exception_verbosity=high`"
            " for a debug print-out and storage map footprint of this Apply node."
        )

    try:
        exc_value = exc_type(
            str(exc_value) + detailed_err_msg + "\n" + "\n".join(hints)
        )
    except TypeError:
        warnings.warn(
            f"{exc_type} error does not allow us to add an extra error message"
        )
        # Some exception need extra parameter in inputs. So forget the
        # extra long error message in that case.
    raise exc_value.with_traceback(exc_trace)


def __log_thunk_trace(value, handler: io.TextIOWrapper) -> None:
    """
    Log PyTensor's diagnostic stack trace for an exception.

    Uses custom attributes that are added to trace objects by raise_with_op.
    """

    def write(msg):
        print(f"log_thunk_trace: {msg.strip()}", file=handler)

    if hasattr(value, "__thunk_trace__"):
        trace2 = value.__thunk_trace__
        write("There was a problem executing an Op.")
        if trace2 is None:
            write("Could not find where this Op was defined.")
            write(
                " * You might have instantiated this Op "
                "directly instead of using a constructor."
            )
            write(
                " * The Op you constructed might have been"
                " optimized. Try turning off optimizations."
            )
        elif trace2:
            write("Definition in: ")
            for line in traceback.format_list(trace2):
                write(line)
            write(
                "For the full definition stack trace set"
                " the PyTensor flags `traceback__limit` to -1"
            )


def register_thunk_trace_excepthook(handler: TextIO = sys.stdout) -> None:
    """Adds the `__log_thunk_trace` except hook to the collection in `pytensor.utils`.

    Parameters
    ----------
    handler
        Target for printing the output.
    """

    def wrapper(type, value, trace):
        __log_thunk_trace(value, handler)

    utils.add_excepthook(wrapper)


register_thunk_trace_excepthook()


def compile_function_src(
    src: str,
    function_name: str,
    global_env: dict[Any, Any] | None = None,
    local_env: dict[Any, Any] | None = None,
) -> Callable:
    with NamedTemporaryFile(delete=False) as f:
        filename = f.name
        f.write(src.encode())

    if global_env is None:
        global_env = {}

    if local_env is None:
        local_env = {}

    mod_code = compile(src, filename, mode="exec")
    exec(mod_code, global_env, local_env)

    res = cast(Callable, local_env[function_name])
    res.__source__ = src  # type: ignore
    return res


def get_name_for_object(x: Any) -> str:
    """Get the name for an arbitrary object."""

    if isinstance(x, Variable) and x.name:
        name = re.sub("[^0-9a-zA-Z]+", "_", x.name)
        name = (
            name
            if (
                name.isidentifier()
                and not iskeyword(name)
                and name not in dir(builtins)
            )
            else ""
        )
    else:
        name = re.sub(r"(?<!^)(?=[A-Z])", "_", getattr(x, "__name__", "")).lower()

    if not name or (not name.isidentifier() or iskeyword(name)):
        # Try to get snake-case out of the type name
        name = re.sub(r"(?<!^)(?=[A-Z])", "_", type(x).__name__).lower()

    assert name.isidentifier() and not iskeyword(name)

    return name


def unique_name_generator(
    external_names: list[str] | None = None, suffix_sep: str = "_"
) -> Callable:
    """Create a function that generates unique names."""

    if external_names is None:
        external_names = []

    T = TypeVar("T")

    def unique_name(
        x: T,
        force_unique=False,
        names_counter=Counter(external_names),
        objs_to_names: dict[T, str] = {},
    ) -> str:
        if not force_unique and x in objs_to_names:
            return objs_to_names[x]

        name = get_name_for_object(x)

        name_suffix = names_counter.get(name, "")
        if name_suffix:
            local_name = f"{name}{suffix_sep}{name_suffix}"
            names_counter.update((name,))
        else:
            local_name = name

        names_counter.update((local_name,))
        objs_to_names[x] = local_name

        return local_name

    return unique_name


def fgraph_to_python(
    fgraph: FunctionGraph,
    op_conversion_fn: Callable,
    *,
    type_conversion_fn: Callable = lambda x, **kwargs: x,
    order: list[Apply] | None = None,
    storage_map: Optional["StorageMapType"] = None,
    fgraph_name: str = "fgraph_to_python",
    global_env: dict[Any, Any] | None = None,
    local_env: dict[Any, Any] | None = None,
    get_name_for_object: Callable[[Any], str] = get_name_for_object,
    squeeze_output: bool = False,
    unique_name: Callable | None = None,
    **kwargs,
) -> Callable:
    """Convert a `FunctionGraph` into a regular Python function.

    Parameters
    ==========
    fgraph
        The `FunctionGraph` to convert.
    op_conversion_fn
        A callable used to convert nodes inside `fgraph` based on their `Op`
        types.  It must have the signature
        ``(op: Op, node: Apply=None, storage_map: Dict[Variable, List[Optional[Any]]]=None, **kwargs)``.
    type_conversion_fn
        A callable used to convert the values in `storage_map`.  It must have
        the signature
        ``(value: Optional[Any], variable: Variable=None, storage: List[Optional[Any]]=None, **kwargs)``.
    order
        The `order` argument to `map_storage`.
    storage_map
        The `storage_map` argument to `map_storage`.
    fgraph_name
        The name used for the resulting function.
    global_env
        The global environment used when the function is constructed.
        The default is an empty ``dict``.
    local_env
        The local environment used when the function is constructed.
        The default is ``locals()``.
    get_name_for_object
        A function used to provide names for the objects referenced within the
        generated function.
    unique_name
        A function to make random function names for generated code
    squeeze_output
        If the `FunctionGraph` has only one output and this option is
        ``True``, return the single output instead of a tuple with the output.
    **kwargs
        The remaining keywords are passed to `python_conversion_fn`
    """

    if order is None:
        order = fgraph.toposort()

    if storage_map is None:
        storage_map = {}

    if not unique_name:
        unique_name = unique_name_generator([fgraph_name])

    # make sure we plumb this through
    kwargs["unique_name"] = unique_name

    if global_env is None:
        global_env = {}

    tipifiyed_vars = set()

    body_assigns = []
    for node in order:
        node_input_names = []
        for inp in node.inputs:
            local_input_name = unique_name(inp)
            is_constant = isinstance(inp, Constant)
            input_storage = storage_map.setdefault(
                inp,
                [
                    inp.data  # type: ignore[attr-defined]
                    if is_constant
                    else None
                ],
            )
            if (
                is_constant or input_storage[0] is not None
            ) and inp not in tipifiyed_vars:
                # Constants need to be assigned locally and referenced
                # FIXME: This is converting shared variables, but these may change later,
                #  so this one-time conversion is wasteful / not robust
                global_env[local_input_name] = type_conversion_fn(
                    input_storage[0], variable=inp, storage=input_storage, **kwargs
                )
                tipifiyed_vars.add(inp)
                # TODO: We could attempt to use the storage arrays directly
                #  Otherwise we're doubling the memory footprint of constants
                # E.g. `local_input_name = f"{local_input_name}[0]"`
            node_input_names.append(local_input_name)

        node_output_names = [unique_name(v) for v in node.outputs]

        compiled_func = op_conversion_fn(
            node.op, node=node, storage_map=storage_map, **kwargs
        )
        # Create a local alias with a unique name
        local_compiled_func_name = unique_name(compiled_func)
        global_env[local_compiled_func_name] = compiled_func

        assign_str = f"{', '.join(node_output_names)} = {local_compiled_func_name}({', '.join(node_input_names)})"
        assign_comment_str = f"{indent(str(node), '# ')}"
        assign_block_str = f"{assign_comment_str}\n{assign_str}"
        body_assigns.append(assign_block_str)

    # Handle `Constant`-only outputs (these don't have associated `Apply`
    # nodes, so the above isn't applicable)
    for out in fgraph.outputs:
        if isinstance(out, Constant):
            local_output_name = unique_name(out)
            if local_output_name not in global_env:
                output_storage = storage_map.setdefault(
                    out, [None if not isinstance(out, Constant) else out.data]
                )
                global_env[local_output_name] = type_conversion_fn(
                    output_storage[0],
                    variable=out,
                    storage=output_storage,
                    **kwargs,
                )

    fgraph_input_names = [unique_name(v) for v in fgraph.inputs]
    fgraph_output_names = [unique_name(v) for v in fgraph.outputs]
    joined_body_assigns = indent("\n".join(body_assigns), "    ")

    if len(fgraph_output_names) == 1:
        fgraph_return_src = (
            fgraph_output_names[0] if squeeze_output else f"({fgraph_output_names[0]},)"
        )
    else:
        fgraph_return_src = ", ".join(fgraph_output_names)

    fgraph_def_src = dedent(
        f"""
    def {fgraph_name}({", ".join(fgraph_input_names)}):
{indent(joined_body_assigns, " " * 4)}
        return {fgraph_return_src}
    """
    ).strip()

    if local_env is None:
        local_env = locals()

    fgraph_def = compile_function_src(
        fgraph_def_src, fgraph_name, {**globals(), **global_env}, local_env
    )

    return fgraph_def


def get_destroy_dependencies(fgraph: FunctionGraph) -> dict[Apply, list[Variable]]:
    """Construct a ``dict`` of nodes to variables that are implicit dependencies induced by `Op.destroy_map` and `Op.view_map`

    These variable dependencies are in contrast to each node's inputs, which
    are _explicit_ dependencies.

    The variables in the values of this ``dict`` would be impossible to compute
    after the current key nodes are evaluated, because node.thunk() is going to
    destroy a common input variable needed by whatever node owns each variable
    in destroy_dependencies.
    """
    order = fgraph.orderings()
    destroy_dependencies = defaultdict(list)
    for node in fgraph.apply_nodes:
        for prereq in order.get(node, []):
            destroy_dependencies[node].extend(prereq.outputs)
    return destroy_dependencies
