# Copyright 2021 AlQuraishi Laboratory
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#      http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
import logging
import math
from functools import partial
from typing import Any, Callable, Dict, Iterable, List, Optional, Sequence, Tuple, Union

import torch

from .tensor_utils import tensor_tree_map, tree_map


def _fetch_dims(tree: Union[dict, list, tuple, torch.Tensor]) -> List[Tuple[int, ...]]:
    shapes = []
    if isinstance(tree, dict):
        for v in tree.values():
            shapes.extend(_fetch_dims(v))
    elif isinstance(tree, (list, tuple)):
        for t in tree:
            shapes.extend(_fetch_dims(t))
    elif isinstance(tree, torch.Tensor):
        shapes.append(tree.shape)
    else:
        raise ValueError("Not supported")

    return shapes


@torch.jit.ignore
def _flat_idx_to_idx(flat_idx: int, dims: Tuple[int, ...]) -> Tuple[int, ...]:
    idx = []
    for d in reversed(dims):
        idx.append(flat_idx % d)
        flat_idx = flat_idx // d

    return tuple(reversed(idx))


@torch.jit.ignore
def _get_minimal_slice_set(
    start: Sequence[int],
    end: Sequence[int],
    dims: Sequence[int],
    start_edges: Optional[Sequence[bool]] = None,
    end_edges: Optional[Sequence[bool]] = None,
) -> List[Tuple[slice, ...]]:
    """
    Produces an ordered sequence of tensor slices that, when used in sequence on a tensor with shape dims, yields
    tensors that contain every leaf in the contiguous range [start, end]. Care is taken to yield a short sequence of
    slices, and perhaps even the shortest possible (I'm pretty sure it's the latter).

    end is INCLUSIVE.
    """

    # start_edges and end_edges both indicate whether, starting from any given
    # dimension, the start/end index is at the top/bottom edge of the
    # corresponding tensor, modeled as a tree
    def reduce_edge_list(l: List[bool]) -> None:
        tally = True
        for i in range(len(l)):
            reversed_idx = -1 * (i + 1)
            l[reversed_idx] &= tally
            tally = l[reversed_idx]

    if start_edges is None:
        start_edges = [s == 0 for s in start]
        reduce_edge_list(start_edges)
    if end_edges is None:
        end_edges = [e == (d - 1) for e, d in zip(end, dims)]
        reduce_edge_list(end_edges)

    # Base cases. Either start/end are empty and we're done, or the final,
    # one-dimensional tensor can be simply sliced
    if len(start) == 0:
        return [tuple()]
    elif len(start) == 1:
        return [(slice(start[0], end[0] + 1),)]

    slices: List[Tuple[slice, ...]] = []
    path_list: List[slice] = []

    # Dimensions common to start and end can be selected directly
    for s, e in zip(start, end):
        if s == e:
            path_list.append(slice(s, s + 1))
        else:
            break

    path: Tuple[slice, ...] = tuple(path_list)
    divergence_idx = len(path)

    # start == end, and we're done
    if divergence_idx == len(dims):
        return [path]

    def upper() -> Tuple[Tuple[slice, ...], ...]:
        assert start_edges is not None
        assert end_edges is not None

        sdi = start[divergence_idx]
        return tuple(
            path + (slice(sdi, sdi + 1),) + s
            for s in _get_minimal_slice_set(
                start[divergence_idx + 1 :],
                [d - 1 for d in dims[divergence_idx + 1 :]],
                dims[divergence_idx + 1 :],
                start_edges=start_edges[divergence_idx + 1 :],
                end_edges=[True for _ in end_edges[divergence_idx + 1 :]],
            )
        )

    def lower() -> Tuple[Tuple[slice, ...], ...]:
        assert start_edges is not None
        assert end_edges is not None

        edi = end[divergence_idx]
        return tuple(
            path + (slice(edi, edi + 1),) + s
            for s in _get_minimal_slice_set(
                [0 for _ in start[divergence_idx + 1 :]],
                end[divergence_idx + 1 :],
                dims[divergence_idx + 1 :],
                start_edges=[True for _ in start_edges[divergence_idx + 1 :]],
                end_edges=end_edges[divergence_idx + 1 :],
            )
        )

    # If both start and end are at the edges of the subtree rooted at
    # divergence_idx, we can just select the whole subtree at once
    if start_edges[divergence_idx] and end_edges[divergence_idx]:
        slices.append(path + (slice(start[divergence_idx], end[divergence_idx] + 1),))
    # If just start is at the edge, we can grab almost all of the subtree,
    # treating only the ragged bottom edge as an edge case
    elif start_edges[divergence_idx]:
        slices.append(path + (slice(start[divergence_idx], end[divergence_idx]),))
        slices.extend(lower())
    # Analogous to the previous case, but the top is ragged this time
    elif end_edges[divergence_idx]:
        slices.extend(upper())
        slices.append(path + (slice(start[divergence_idx] + 1, end[divergence_idx] + 1),))
    # If both sides of the range are ragged, we need to handle both sides
    # separately. If there's contiguous meat in between them, we can index it
    # in one big chunk
    else:
        slices.extend(upper())
        middle_ground = end[divergence_idx] - start[divergence_idx]
        if middle_ground > 1:
            slices.append(path + (slice(start[divergence_idx] + 1, end[divergence_idx]),))
        slices.extend(lower())

    return slices


@torch.jit.ignore
def _chunk_slice(t: torch.Tensor, flat_start: int, flat_end: int, no_batch_dims: int) -> torch.Tensor:
    """
    Equivalent to

        t.reshape((-1,) + t.shape[no_batch_dims:])[flat_start:flat_end]

    but without the need for the initial reshape call, which can be memory-intensive in certain situations. The only
    reshape operations in this function are performed on sub-tensors that scale with (flat_end - flat_start), the chunk
    size.
    """

    batch_dims = t.shape[:no_batch_dims]
    start_idx = list(_flat_idx_to_idx(flat_start, batch_dims))
    # _get_minimal_slice_set is inclusive
    end_idx = list(_flat_idx_to_idx(flat_end - 1, batch_dims))

    # Get an ordered list of slices to perform
    slices = _get_minimal_slice_set(
        start_idx,
        end_idx,
        batch_dims,
    )

    sliced_tensors = [t[s] for s in slices]

    return torch.cat([s.view((-1,) + t.shape[no_batch_dims:]) for s in sliced_tensors])


def chunk_layer(
    layer: Callable,
    inputs: Dict[str, Any],
    chunk_size: int,
    no_batch_dims: int,
    low_mem: bool = False,
    _out: Any = None,
    _add_into_out: bool = False,
) -> Any:
    """
    Implements the "chunking" procedure described in section 1.11.8.

    Layer outputs and inputs are assumed to be simple "pytrees," consisting only of (arbitrarily nested) lists, tuples,
    and dicts with torch.Tensor leaves.

    Args:
        layer:
            The layer to be applied chunk-wise
        inputs:
            A (non-nested) dictionary of keyworded inputs. All leaves must be tensors and must share the same batch
            dimensions.
        chunk_size:
            The number of sub-batches per chunk. If multiple batch dimensions are specified, a "sub-batch" is defined
            as a single indexing of all batch dimensions simultaneously (s.t. the number of sub-batches is the product
            of the batch dimensions).
        no_batch_dims:
            How many of the initial dimensions of each input tensor can be considered batch dimensions.
        low_mem:
            Avoids flattening potentially large input tensors. Unnecessary in most cases, and is ever so slightly
            slower than the default setting.
    Returns:
        The reassembled output of the layer on the inputs.
    """
    if not (len(inputs) > 0):
        raise ValueError("Must provide at least one input")

    initial_dims = [shape[:no_batch_dims] for shape in _fetch_dims(inputs)]
    orig_batch_dims = tuple([max(s) for s in zip(*initial_dims)])

    def _prep_inputs(t: torch.Tensor) -> torch.Tensor:
        if not low_mem:
            if not sum(t.shape[:no_batch_dims]) == no_batch_dims:
                t = t.expand(orig_batch_dims + t.shape[no_batch_dims:])
            t = t.reshape(-1, *t.shape[no_batch_dims:])
        else:
            t = t.expand(orig_batch_dims + t.shape[no_batch_dims:])
        return t

    prepped_inputs: Dict[str, Any] = tensor_tree_map(_prep_inputs, inputs)
    prepped_outputs = None
    if _out is not None:
        prepped_outputs = tensor_tree_map(lambda t: t.view([-1] + list(t.shape[no_batch_dims:])), _out)

    flat_batch_dim = 1
    for d in orig_batch_dims:
        flat_batch_dim *= d

    no_chunks = flat_batch_dim // chunk_size + (flat_batch_dim % chunk_size != 0)

    def _select_chunk(t: torch.Tensor) -> torch.Tensor:
        return t[i : i + chunk_size] if t.shape[0] != 1 else t

    i = 0
    out = prepped_outputs
    for _ in range(no_chunks):
        # Chunk the input
        if not low_mem:
            select_chunk = _select_chunk
        else:
            select_chunk = partial(
                _chunk_slice,
                flat_start=i,
                flat_end=min(flat_batch_dim, i + chunk_size),
                no_batch_dims=len(orig_batch_dims),
            )

        chunks: Dict[str, Any] = tensor_tree_map(select_chunk, prepped_inputs)

        # Run the layer on the chunk
        output_chunk = layer(**chunks)

        # Allocate space for the output
        if out is None:
            out = tensor_tree_map(lambda t: t.new_zeros((flat_batch_dim,) + t.shape[1:]), output_chunk)

        # Put the chunk in its pre-allocated space
        if isinstance(output_chunk, dict):

            def assign(d1: dict, d2: dict) -> None:
                for k, v in d1.items():
                    if isinstance(v, dict):
                        assign(v, d2[k])
                    else:
                        if _add_into_out:
                            v[i : i + chunk_size] += d2[k]
                        else:
                            v[i : i + chunk_size] = d2[k]

            assign(out, output_chunk)
        elif isinstance(output_chunk, tuple):
            for x1, x2 in zip(out, output_chunk):
                if _add_into_out:
                    x1[i : i + chunk_size] += x2
                else:
                    x1[i : i + chunk_size] = x2
        elif isinstance(output_chunk, torch.Tensor):
            if _add_into_out:
                out[i : i + chunk_size] += output_chunk
            else:
                out[i : i + chunk_size] = output_chunk
        else:
            raise ValueError("Not supported")

        i += chunk_size

    out = tensor_tree_map(lambda t: t.view(orig_batch_dims + t.shape[1:]), out)

    return out


class ChunkSizeTuner:
    def __init__(
        self,
        # Heuristically, runtimes for most of the modules in the network
        # plateau earlier than this on all GPUs I've run the model on.
        max_chunk_size: int = 512,
    ):
        self.max_chunk_size = max_chunk_size
        self.cached_chunk_size: Optional[int] = None
        self.cached_arg_data: Optional[tuple] = None

    def _determine_favorable_chunk_size(self, fn: Callable, args: tuple, min_chunk_size: int) -> int:
        logging.info("Tuning chunk size...")

        if min_chunk_size >= self.max_chunk_size:
            return min_chunk_size

        candidates: List[int] = [2**l for l in range(int(math.log(self.max_chunk_size, 2)) + 1)]
        candidates = [c for c in candidates if c > min_chunk_size]
        candidates = [min_chunk_size] + candidates
        candidates[-1] += 4

        def test_chunk_size(chunk_size: int) -> bool:
            try:
                with torch.no_grad():
                    fn(*args, chunk_size=chunk_size)
                return True
            except RuntimeError:
                return False

        min_viable_chunk_size_index = 0
        i = len(candidates) - 1
        while i > min_viable_chunk_size_index:
            viable = test_chunk_size(candidates[i])
            if not viable:
                i = (min_viable_chunk_size_index + i) // 2
            else:
                min_viable_chunk_size_index = i
                i = (i + len(candidates) - 1) // 2

        return candidates[min_viable_chunk_size_index]

    def _compare_arg_caches(self, ac1: Iterable, ac2: Iterable) -> bool:
        consistent = True
        for a1, a2 in zip(ac1, ac2):
            assert type(ac1) == type(ac2)
            if isinstance(ac1, (list, tuple)):
                consistent &= self._compare_arg_caches(a1, a2)
            elif isinstance(ac1, dict):
                a1_items = [v for _, v in sorted(a1.items(), key=lambda x: x[0])]
                a2_items = [v for _, v in sorted(a2.items(), key=lambda x: x[0])]
                consistent &= self._compare_arg_caches(a1_items, a2_items)
            else:
                consistent &= a1 == a2

        return consistent

    def tune_chunk_size(
        self,
        representative_fn: Callable,
        args: tuple,
        min_chunk_size: int,
    ) -> int:
        consistent = True
        arg_data: tuple = tree_map(lambda a: a.shape if isinstance(a, torch.Tensor) else a, args, object)
        if self.cached_arg_data is not None:
            # If args have changed shape/value, we need to re-tune
            assert len(self.cached_arg_data) == len(arg_data)
            consistent = self._compare_arg_caches(self.cached_arg_data, arg_data)
        else:
            # Otherwise, we can reuse the precomputed value
            consistent = False

        if not consistent:
            self.cached_chunk_size = self._determine_favorable_chunk_size(
                representative_fn,
                args,
                min_chunk_size,
            )
            self.cached_arg_data = arg_data

        assert self.cached_chunk_size is not None

        return self.cached_chunk_size
