# coding=utf-8
# Copyright (c) 2020, NVIDIA CORPORATION.  All rights reserved.
#
# 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.


# Parts of the code here are adapted from PyTorch
# repo: https://github.com/pytorch/pytorch

import contextlib

import torch
from torch import _C
from torch.cuda import _lazy_call, device as device_ctx_manager
from torch.utils.checkpoint import detach_variable

from megatron.memory import allocate_mem_buff

from .initialize import get_data_parallel_rank
from .initialize import get_tensor_model_parallel_group
from .initialize import get_tensor_model_parallel_rank
from .initialize import get_tensor_model_parallel_world_size


# Default name for the model parallel rng tracker.
_MODEL_PARALLEL_RNG_TRACKER_NAME = 'model-parallel-rng'


def _set_cuda_rng_state(new_state, device=-1):
    """Sets the random number generator state of the current GPU.

    Argumentss:
        new_state (torch.ByteTensor): The desired state
    This function is adapted from PyTorch repo (torch.cuda.set_rng_state)
    with a single change: the input state is not cloned. Cloning caused
    major performance issues for +4 GPU cases.
    """
    if hasattr(_C, '_cuda_setRNGState') and callable(_C._cuda_setRNGState):
        # older PyTorch
        def cb():
            with device_ctx_manager(device):
                _C._cuda_setRNGState(new_state)
    else:
        # newer PyTorch
        if device == -1:
            device = torch.device('cuda')
        elif isinstance(device, str):
            device = torch.device(device)
        elif isinstance(device, int):
            device = torch.device('cuda', device)

        def cb():
            idx = device.index
            if idx is None:
                idx = torch.cuda.current_device()
            default_generator = torch.cuda.default_generators[idx]
            default_generator.set_state(new_state)

    _lazy_call(cb)


def split_tensor_into_1d_equal_chunks(tensor, new_buffer=False):
    """Break a tensor into equal 1D chunks."""
    partition_size = torch.numel(tensor) // \
        get_tensor_model_parallel_world_size()
    start_index = partition_size * get_tensor_model_parallel_rank()
    end_index = start_index + partition_size
    if new_buffer:
        data = torch.empty(partition_size, dtype=tensor.dtype,
                           device=torch.cuda.current_device(),
                           requires_grad=False)
        data.copy_(tensor.view(-1)[start_index:end_index])
    else:
        data = tensor.view(-1)[start_index:end_index]
    return data
    

def gather_split_1d_tensor(tensor):
    """Opposite of above function, gather values from model parallel ranks."""
    world_size = get_tensor_model_parallel_world_size()
    numel = torch.numel(tensor)
    numel_gathered = world_size * numel
    gathered = torch.empty(numel_gathered, dtype=tensor.dtype,
                           device=torch.cuda.current_device(),
                           requires_grad=False)
    chunks = [gathered[i*numel:(i+1)*numel] for i in range(world_size)]
    torch.distributed.all_gather(chunks, tensor,
                                 group=get_tensor_model_parallel_group())
    return gathered

def _kernel_make_viewless_tensor(inp, requires_grad):
    '''Make a viewless tensor.

    View tensors have the undesirable side-affect of retaining a reference
    to the originally-viewed tensor, even after manually setting the '.data'
    field. This method creates a new tensor that links to the old tensor's
    data, without linking the viewed tensor, referenced via the '._base'
    field.
    '''
    out = torch.empty(
        (1,),
        dtype = inp.dtype,
        device = inp.device,
        requires_grad = requires_grad,
    )
    out.data = inp.data
    return out

class MakeViewlessTensor(torch.autograd.Function):
    '''
    Autograd function to make a viewless tensor.

    This function should be used in cases where the computation graph needs
    to be propagated, but we only want a viewless tensor (e.g.,
    ParallelTransformer's hidden_states). Call this function by passing
    'keep_graph = True' to 'make_viewless_tensor()'.
    '''
    @staticmethod
    def forward(ctx, inp, requires_grad):
        return _kernel_make_viewless_tensor(inp, requires_grad)
    @staticmethod
    def backward(ctx, grad_output):
        return grad_output, None

def make_viewless_tensor(inp, requires_grad, keep_graph):
    '''
    Entry-point for creating viewless tensors.

    This method should be used, rather than calling 'MakeViewlessTensor'
    or '_kernel_make_viewless_tensor' directly. This method acts as a
    switch for determining if an autograd function or a regular method
    should be used to create the tensor.
    '''

    # return tensor as-is, if not a 'view'
    if inp._base is None:
        return inp

    # create viewless tensor
    if keep_graph:
        return MakeViewlessTensor.apply(inp, requires_grad)
    else:
        return _kernel_make_viewless_tensor(inp, requires_grad)

def assert_viewless_tensor(tensor, extra_msg = None):
    '''Assert that a tensor is not a view (i.e., its '._base' field is
    not set).'''
    if isinstance(tensor, list):
        [ assert_viewless_tensor(t) for t in tensor ]
        return tensor
    if not isinstance(tensor, torch.Tensor):
        return tensor
    assert tensor._base is None, (
        "Ensure tensor._base is None before setting tensor.data or storing "
        "tensor to memory buffer. Otherwise, a memory leak will occur (and "
        "likely accumulate over iterations). %s"
    ) % extra_msg
    return tensor

def safely_set_viewless_tensor_data(tensor, new_data_tensor):
    '''Safely set tensor's '.data' field.

    Check first that the tensor is viewless (i.e., '._base' not set). If not,
    raise an exception.
    '''
    assert_viewless_tensor(tensor, extra_msg = "FYI, tensor._base has shape %s, and new_data_tensor has shape %s." % ("--" if tensor._base is None else tensor._base.shape, new_data_tensor.shape))
    tensor.data = new_data_tensor


class CudaRNGStatesTracker:
    """Tracker for the cuda RNG states.

    Using the `add` method, a cuda rng state is initialized based on
    the input `seed` and is assigned to `name`. Later, by forking the
    rng state, we can perform operations and return to our starting
    cuda state.
    """

    def __init__(self):
        # Map from a string name to the cuda rng state.
        self.states_ = {}
        # Seeds are just for book keeping and ensure no seed is set twice.
        self.seeds_ = set()

    def reset(self):
        """Set to the initial state (no tracker)."""
        self.states_ = {}
        self.seeds_ = set()

    def get_states(self):
        """Get rng states. Copy the dictionary so we have direct
        pointers to the states, not just a pointer to the dictionary."""
        states = {}
        for name in self.states_:
            states[name] = self.states_[name]
        return states

    def set_states(self, states):
        """Set the rng states. For efficiency purposes, we do not check
        the size of seed for compatibility."""
        self.states_ = states

    def add(self, name, seed):
        """Track the rng state."""
        # Check seed is not already used.
        if seed in self.seeds_:
            raise Exception('seed {} already exists'.format(seed))
        self.seeds_.add(seed)
        # Check that state is not already defined.
        if name in self.states_:
            raise Exception('cuda rng state {} already exists'.format(name))
        # Get the current rng state.
        orig_rng_state = torch.cuda.get_rng_state()
        # Set the new state and store it.
        torch.cuda.manual_seed(seed)
        self.states_[name] = torch.cuda.get_rng_state()
        # Reset rng state to what it was.
        _set_cuda_rng_state(orig_rng_state)

    @contextlib.contextmanager
    def fork(self, name=_MODEL_PARALLEL_RNG_TRACKER_NAME):
        """Fork the cuda rng state, perform operations, and exit with
        the original state."""
        # Check if we have added the state
        if name not in self.states_:
            raise Exception('cuda rng state {} is not added'.format(name))
        # Store current rng state.
        orig_cuda_rng_state = torch.cuda.get_rng_state()
        # Set rng state to the desired one
        _set_cuda_rng_state(self.states_[name])
        # Do the stuff we wanted to do.
        try:
            yield
        finally:
            # Update the current rng state for later use.
            self.states_[name] = torch.cuda.get_rng_state()
            # And set the state to the original state we started with.
            _set_cuda_rng_state(orig_cuda_rng_state)


# RNG tracker object.
_CUDA_RNG_STATE_TRACKER = CudaRNGStatesTracker()


def get_cuda_rng_tracker():
    """Get cuda rng tracker."""
    return _CUDA_RNG_STATE_TRACKER


def model_parallel_cuda_manual_seed(seed):
    """Initialize model parallel cuda seed.

    This function should be called after the model parallel is
    initialized. Also, no torch.cuda.manual_seed should be called
    after this function. Basically, this is replacement for that
    function.
    Two set of RNG states are tracked:
        default state: This is for data parallelism and is the same among a
                       set of model parallel GPUs but different across
                       different model paralle groups. This is used for
                       example for dropout in the non-tensor-model-parallel regions.
        tensor-model-parallel state: This state is different among a set of model
                              parallel GPUs, but the same across data parallel
                              groups. This is used for example for dropout in
                              model parallel regions.
    """
    # 2718 is just for fun and any POSITIVE value will work.
    offset = seed + 2718
    tensor_model_parallel_seed = offset + get_tensor_model_parallel_rank()
    # Data parallel gets the original seed.
    data_parallel_seed = seed

    if torch.distributed.get_rank() == 0:
        print('> initializing model parallel cuda seeds on global rank {}, '
              'model parallel rank {}, and data parallel rank {} with '
              'model parallel seed: {} and data parallel seed: {}'.format(
                  torch.distributed.get_rank(), get_tensor_model_parallel_rank(),
                  get_data_parallel_rank(), tensor_model_parallel_seed,
                  data_parallel_seed), flush=True)
    _CUDA_RNG_STATE_TRACKER.reset()
    # Set the default state.
    torch.cuda.manual_seed(data_parallel_seed)
    # and model parallel state.
    _CUDA_RNG_STATE_TRACKER.add(_MODEL_PARALLEL_RNG_TRACKER_NAME,
                                tensor_model_parallel_seed)


class CheckpointFunction(torch.autograd.Function):
    """This function is adapted from torch.utils.checkpoint with
       two main changes:
           1) torch.cuda.set_rng_state is replaced with `_set_cuda_rng_state`
           2) the states in the model parallel tracker are also properly
              tracked/set/reset.
    """
    @staticmethod
    def forward(ctx, run_function, distribute_checkpointed_activations, *args):
        ctx.run_function = run_function
        ctx.distribute_checkpointed_activations \
            = distribute_checkpointed_activations

        # Copy the rng states.
        ctx.fwd_cpu_rng_state = torch.get_rng_state()
        ctx.fwd_cuda_rng_state = torch.cuda.get_rng_state()
        ctx.fwd_cuda_rng_state_tracker = get_cuda_rng_tracker().get_states()

        with torch.no_grad():
            outputs = run_function(*args)

        # Divide hidden states across model parallel group and only keep
        # the chunk corresponding to the current rank.
        if distribute_checkpointed_activations:
            ctx.input_0_shape = args[0].data.shape
            safely_set_viewless_tensor_data(
                args[0],
                split_tensor_into_1d_equal_chunks(args[0].data, new_buffer=True))

        # Store everything.
        ctx.save_for_backward(*args)

        return outputs

    @staticmethod
    def backward(ctx, *args):
        if not torch.autograd._is_checkpoint_valid():
            raise RuntimeError("Checkpointing is not compatible with .grad(), "
                               "please use .backward() if possible")
        inputs = ctx.saved_tensors
        if ctx.distribute_checkpointed_activations:
            safely_set_viewless_tensor_data(
                inputs[0],
                gather_split_1d_tensor(inputs[0].data).view(ctx.input_0_shape))

        # Store the current states.
        bwd_cpu_rng_state = torch.get_rng_state()
        bwd_cuda_rng_state = torch.cuda.get_rng_state()
        bwd_cuda_rng_state_tracker = get_cuda_rng_tracker().get_states()

        # Set the states to what it used to be before the forward pass.
        torch.set_rng_state(ctx.fwd_cpu_rng_state)
        _set_cuda_rng_state(ctx.fwd_cuda_rng_state)
        get_cuda_rng_tracker().set_states(ctx.fwd_cuda_rng_state_tracker)

        # Compute the forward pass.
        detached_inputs = detach_variable(inputs)
        with torch.enable_grad():
            outputs = ctx.run_function(*detached_inputs)

        # Set the states back to what it was at the start of this function.
        torch.set_rng_state(bwd_cpu_rng_state)
        _set_cuda_rng_state(bwd_cuda_rng_state)
        get_cuda_rng_tracker().set_states(bwd_cuda_rng_state_tracker)

        if isinstance(outputs, torch.Tensor):
            outputs = (outputs,)
        torch.autograd.backward(outputs, args)
        grads = tuple(inp.grad if isinstance(inp, torch.Tensor) else inp
                      for inp in detached_inputs)
        return (None, None) + grads


def checkpoint(function, distribute_checkpointed_activations, *args):
    """Checkpoint a model or part of the model.
    This has been directly copied from torch.utils.checkpoint."""
    return CheckpointFunction.apply(function,
                                    distribute_checkpointed_activations, *args)
