# Copyright (c) Microsoft Corporation.
# Licensed under the MIT license.

"""Thie file handles "slice" commonly used in mixed-operation.

The ``slice_type`` we support here, is "slice" or "list of slice".
The reason is that sometimes (e.g., in multi-head attention),
the tensor slice could be from multiple parts. This type is extensible.
We can support arbitrary masks in future if we need them.

To slice a tensor, we need ``multidim_slice``,
which is simply a tuple consists of ``slice_type``.

Usually in python programs, the variable put into slice's start, stop and step
should be integers (or NoneType).
But in our case, it could also be a dict from integer to float,
representing a distribution of integer. When that happens,
we convert a "slice with some weighted values", to a "weighted slice".
To this end, we track the computation with ``MaybeWeighted``,
and replay the computation with each possible value.
Meanwhile, we record their weights.
Note that ``MaybeWeighted`` is also extensible.
We can support more types of objects on slice in future.

The fixed/weighted slice is fed into ``_slice_weight``,
which interprets the slice and apply it on a tensor.
"""

import operator
from typing import Tuple, Union, List, Dict, Callable, Optional, Iterator, TypeVar, Any, Generic, cast

import numpy as np
import torch

T = TypeVar('T')

slice_type = Union[slice, List[slice]]
multidim_slice = Tuple[slice_type, ...]

scalar_or_scalar_dict = Union[T, Dict[T, float]]
int_or_int_dict = scalar_or_scalar_dict[int]

_value_fn_type = Optional[Callable[[int_or_int_dict], int]]


def zeros_like(arr: T) -> T:
    if isinstance(arr, np.ndarray):
        return np.zeros_like(arr)
    elif isinstance(arr, torch.Tensor):
        return torch.zeros_like(arr)
    else:
        raise TypeError(f'Unsupported type for {arr}: {type(arr)}')


def _eliminate_list_slice(shape: tuple, slice_: multidim_slice) -> multidim_slice:
    # get rid of list of slice
    result = []
    for i in range(len(slice_)):
        if isinstance(slice_[i], list):
            # convert list of slices to mask
            mask = np.zeros(shape[i], dtype=np.bool)
            for sl in slice_[i]:
                mask[sl] = 1
            result.append(mask)
        else:
            result.append(slice_[i])
    return tuple(result)


def _slice_weight(weight: T, slice_: Union[multidim_slice, List[Tuple[multidim_slice, float]]]) -> T:
    # slice_ can be a tuple of slice, e.g., ([3:6], [2:4])
    # or tuple of slice -> float, e.g. {([3:6],): 0.6, ([2:4],): 0.3}

    if isinstance(slice_, list):
        # for weighted case, we get the corresponding masks. e.g.,
        # {([3:6],): 0.6, ([2:4],): 0.3} => [0, 0, 0.3, 0.9, 0.6, 0.6] (if the whole length is 6)
        # this mask is broadcasted and multiplied onto the weight

        masks = []

        # the accepted argument is list of tuple here
        # because slice can't be key of dict
        for sl, wt in slice_:
            # create a mask with weight w
            with torch.no_grad():
                mask = zeros_like(weight)
                mask[_eliminate_list_slice(weight.shape, sl)] = 1

            # track gradients here
            masks.append((mask * wt))

        masks = sum(masks)

        return masks * weight

    else:
        # for unweighted case, we slice it directly.

        def _do_slice(arr, slice_):
            return arr[_eliminate_list_slice(arr.shape, slice_)]

        # sometimes, we don't need slice.
        # this saves an op on computational graph, which will hopefully make training faster

        # Use a dummy array to check this. Otherwise it would be too complex.
        dummy_arr = np.zeros(weight.shape, dtype=np.bool)
        no_effect = _do_slice(dummy_arr, slice_).shape == dummy_arr.shape

        if no_effect:
            return weight

        return _do_slice(weight, slice_)


class Slicable(Generic[T]):
    """Wraps the weight so that in can be sliced with a ``multidim_slice``.
    The value within the slice can be instances of :class:`MaybeWeighted`.

    Examples
    --------
    >>> weight = conv2d.weight
    >>> Slicable(weight)[:MaybeWeighted({32: 0.4, 64: 0.6})]
    Tensor of shape (64, 64, 3, 3)
    """

    def __init__(self, weight: T):
        if not isinstance(weight, np.ndarray) and not torch.is_tensor(weight):
            raise TypeError(f'Unsuppoted weight type: {type(weight)}')
        self.weight = weight

    def __getitem__(self, index: Union[slice_type, multidim_slice]) -> T:
        if not isinstance(index, tuple):
            index = (index, )
        index = cast(multidim_slice, index)

        # Get the dict value in index's leafs
        # There can be at most one dict
        leaf_dict: Optional[Dict[int, float]] = None
        for maybe_weighted in _iterate_over_multidim_slice(index):
            for d in maybe_weighted.leaf_values():
                if isinstance(d, dict):
                    if leaf_dict is None:
                        leaf_dict = d
                    elif leaf_dict is not d:
                        raise ValueError('There can be at most one distinct dict in leaf values.')

        if leaf_dict is None:
            # in case of simple types with no dict
            res_index = _evaluate_multidim_slice(index)
        else:
            # there is a dict, iterate over dict
            res_index = []
            for val, wt in leaf_dict.items():
                res_index_item = _evaluate_multidim_slice(index, lambda _: val)
                res_index.append((res_index_item, wt))

        return _slice_weight(self.weight, res_index)


class MaybeWeighted:
    """Wrap a value (int or dict with int keys), so that the computation on it can be replayed.
    It builds a binary tree. If ``value`` is not None, it's a leaf node.
    Otherwise, it has left sub-tree and right sub-tree and an operation.

    Only support basic arithmetic operations: ``+``, ``-``, ``*``, ``//``.
    """

    def __init__(self,
                 value: Optional[int_or_int_dict] = None, *,
                 lhs: Optional[Union['MaybeWeighted', int]] = None,
                 rhs: Optional[Union['MaybeWeighted', int]] = None,
                 operation: Optional[Callable[[int, int], int]] = None):
        if operation is None:
            if not isinstance(value, (int, dict)):
                raise TypeError(f'Unsupported value type: {type(value)}')
        self.value = value
        self.lhs = lhs
        self.rhs = rhs
        self.operation = operation

    def leaf_values(self) -> Iterator[Dict[int, float]]:
        """Iterate over values on leaf nodes."""
        if self.value is not None:
            yield self.value
        else:
            if isinstance(self.lhs, MaybeWeighted):
                yield from self.lhs.leaf_values()
            if isinstance(self.rhs, MaybeWeighted):
                yield from self.rhs.leaf_values()

    def evaluate(self, value_fn: _value_fn_type = None) -> int:
        """Evaluate the value on root node, after replacing every value on leaf node with ``value_fn``.
        If ``value_fn`` is none, no replacement will happen and the raw value will be used.
        """
        if self.value is not None:
            if value_fn is not None:
                return value_fn(self.value)
            return self.value
        else:
            if isinstance(self.lhs, MaybeWeighted):
                eval_lhs = self.lhs.evaluate(value_fn)
            else:
                eval_lhs = self.lhs
            if isinstance(self.rhs, MaybeWeighted):
                eval_rhs = self.rhs.evaluate(value_fn)
            else:
                eval_rhs = self.rhs
            return self.operation(eval_lhs, eval_rhs)

    def __repr__(self):
        if self.value is not None:
            return f'{self.__class__.__name__}({self.value})'
        return f'{self.__class__.__name__}(lhs={self.lhs}, rhs={self.rhs}, op={self.operation})'

    def __add__(self, other: Any) -> 'MaybeWeighted':
        return MaybeWeighted(lhs=self, rhs=other, operation=operator.add)

    def __radd__(self, other: Any) -> 'MaybeWeighted':
        return MaybeWeighted(lhs=other, rhs=self, operation=operator.add)

    def __sub__(self, other: Any) -> 'MaybeWeighted':
        return MaybeWeighted(lhs=self, rhs=other, operation=operator.sub)

    def __rsub__(self, other: Any) -> 'MaybeWeighted':
        return MaybeWeighted(lhs=other, rhs=self, operation=operator.sub)

    def __mul__(self, other: Any) -> 'MaybeWeighted':
        return MaybeWeighted(lhs=self, rhs=other, operation=operator.mul)

    def __rmul__(self, other: Any) -> 'MaybeWeighted':
        return MaybeWeighted(lhs=other, rhs=self, operation=operator.mul)

    def __floordiv__(self, other: Any) -> 'MaybeWeighted':
        return MaybeWeighted(lhs=self, rhs=other, operation=operator.floordiv)

    def __rfloordiv__(self, other: Any) -> 'MaybeWeighted':
        return MaybeWeighted(lhs=other, rhs=self, operation=operator.floordiv)


def _iterate_over_slice_type(s: slice_type):
    if isinstance(s, list):
        for se in s:
            yield from _iterate_over_slice_type(se)
    else:
        # s must be a "slice" now
        if isinstance(s.start, MaybeWeighted):
            yield s.start
        if isinstance(s.stop, MaybeWeighted):
            yield s.stop
        if isinstance(s.step, MaybeWeighted):
            yield s.step


def _iterate_over_multidim_slice(ms: multidim_slice):
    """Get :class:`MaybeWeighted` instances in ``ms``."""
    for s in ms:
        if s is not None:
            yield from _iterate_over_slice_type(s)


def _evaluate_slice_type(s: slice_type, value_fn: _value_fn_type = None):
    if isinstance(s, list):
        return [_evaluate_slice_type(se, value_fn) for se in s]
    else:
        return slice(
            s.start.evaluate(value_fn) if isinstance(s.start, MaybeWeighted) else s.start,
            s.stop.evaluate(value_fn) if isinstance(s.stop, MaybeWeighted) else s.stop,
            s.step.evaluate(value_fn) if isinstance(s.step, MaybeWeighted) else s.step
        )


def _evaluate_multidim_slice(ms: multidim_slice, value_fn: _value_fn_type = None):
    """Wraps :meth:`MaybeWeighted.evaluate` to evaluate the whole ``multidim_slice``."""
    res = []
    for s in ms:
        if s is not None:
            res.append(_evaluate_slice_type(s, value_fn))
        else:
            res.append(None)
    return tuple(res)
