# Copyright (c) Open-MMLab. All rights reserved.
import functools
import os
import subprocess
from collections import OrderedDict

import torch
import torch.multiprocessing as mp
from torch import distributed as dist
from torch._utils import (_flatten_dense_tensors, _take_tensors,
                          _unflatten_dense_tensors)

from mmcv.utils import TORCH_VERSION


def init_dist(launcher, backend='nccl', **kwargs):
    if mp.get_start_method(allow_none=True) is None:
        mp.set_start_method('spawn')
    if launcher == 'pytorch':
        _init_dist_pytorch(backend, **kwargs)
    elif launcher == 'mpi':
        _init_dist_mpi(backend, **kwargs)
    elif launcher == 'slurm':
        _init_dist_slurm(backend, **kwargs)
    else:
        raise ValueError(f'Invalid launcher type: {launcher}')


def _init_dist_pytorch(backend, **kwargs):
    # TODO: use local_rank instead of rank % num_gpus
    rank = int(os.environ['RANK'])
    num_gpus = torch.cuda.device_count()
    torch.cuda.set_device(rank % num_gpus)
    dist.init_process_group(backend=backend, **kwargs)


def _init_dist_mpi(backend, **kwargs):
    raise NotImplementedError


def _init_dist_slurm(backend, port=None):
    """Initialize slurm distributed training environment.

    If argument ``port`` is not specified, then the master port will be system
    environment variable ``MASTER_PORT``. If ``MASTER_PORT`` is not in system
    environment variable, then a default port ``29500`` will be used.

    Args:
        backend (str): Backend of torch.distributed.
        port (int, optional): Master port. Defaults to None.
    """
    proc_id = int(os.environ['SLURM_PROCID'])
    ntasks = int(os.environ['SLURM_NTASKS'])
    node_list = os.environ['SLURM_NODELIST']
    num_gpus = torch.cuda.device_count()
    torch.cuda.set_device(proc_id % num_gpus)
    addr = subprocess.getoutput(
        f'scontrol show hostname {node_list} | head -n1')
    # specify master port
    if port is not None:
        os.environ['MASTER_PORT'] = str(port)
    elif 'MASTER_PORT' in os.environ:
        pass  # use MASTER_PORT in the environment variable
    else:
        # 29500 is torch.distributed default port
        os.environ['MASTER_PORT'] = '29500'
    # use MASTER_ADDR in the environment variable if it already exists
    if 'MASTER_ADDR' not in os.environ:
        os.environ['MASTER_ADDR'] = addr
    os.environ['WORLD_SIZE'] = str(ntasks)
    os.environ['LOCAL_RANK'] = str(proc_id % num_gpus)
    os.environ['RANK'] = str(proc_id)
    dist.init_process_group(backend=backend)


def get_dist_info():
    if TORCH_VERSION < '1.0':
        initialized = dist._initialized
    else:
        if dist.is_available():
            initialized = dist.is_initialized()
        else:
            initialized = False
    if initialized:
        rank = dist.get_rank()
        world_size = dist.get_world_size()
    else:
        rank = 0
        world_size = 1
    return rank, world_size


def master_only(func):

    @functools.wraps(func)
    def wrapper(*args, **kwargs):
        rank, _ = get_dist_info()
        if rank == 0:
            return func(*args, **kwargs)

    return wrapper


def allreduce_params(params, coalesce=True, bucket_size_mb=-1):
    """Allreduce parameters.

    Args:
        params (list[torch.Parameters]): List of parameters or buffers of a
            model.
        coalesce (bool, optional): Whether allreduce parameters as a whole.
            Defaults to True.
        bucket_size_mb (int, optional): Size of bucket, the unit is MB.
            Defaults to -1.
    """
    _, world_size = get_dist_info()
    if world_size == 1:
        return
    params = [param.data for param in params]
    if coalesce:
        _allreduce_coalesced(params, world_size, bucket_size_mb)
    else:
        for tensor in params:
            dist.all_reduce(tensor.div_(world_size))


def allreduce_grads(params, coalesce=True, bucket_size_mb=-1):
    """Allreduce gradients.

    Args:
        params (list[torch.Parameters]): List of parameters of a model
        coalesce (bool, optional): Whether allreduce parameters as a whole.
            Defaults to True.
        bucket_size_mb (int, optional): Size of bucket, the unit is MB.
            Defaults to -1.
    """
    grads = [
        param.grad.data for param in params
        if param.requires_grad and param.grad is not None
    ]
    _, world_size = get_dist_info()
    if world_size == 1:
        return
    if coalesce:
        _allreduce_coalesced(grads, world_size, bucket_size_mb)
    else:
        for tensor in grads:
            dist.all_reduce(tensor.div_(world_size))


def _allreduce_coalesced(tensors, world_size, bucket_size_mb=-1):
    if bucket_size_mb > 0:
        bucket_size_bytes = bucket_size_mb * 1024 * 1024
        buckets = _take_tensors(tensors, bucket_size_bytes)
    else:
        buckets = OrderedDict()
        for tensor in tensors:
            tp = tensor.type()
            if tp not in buckets:
                buckets[tp] = []
            buckets[tp].append(tensor)
        buckets = buckets.values()

    for bucket in buckets:
        flat_tensors = _flatten_dense_tensors(bucket)
        dist.all_reduce(flat_tensors)
        flat_tensors.div_(world_size)
        for tensor, synced in zip(
                bucket, _unflatten_dense_tensors(flat_tensors, bucket)):
            tensor.copy_(synced)
