import argparse
import os
import shutil
import time

import torch
import torch.nn as nn
import torch.nn.parallel
import torch.backends.cudnn as cudnn
import torch.distributed as dist
import torch.optim
import torch.utils.data
import torch.utils.data.distributed
import torchvision.transforms as transforms
import torchvision.datasets as datasets
import torchvision.models as models

import numpy as np

try:
    from apex.parallel import DistributedDataParallel as DDP
    from apex.fp16_utils import *
except ImportError:
    raise ImportError("Please install apex from https://www.github.com/nvidia/apex to run this example.")

model_names = sorted(name for name in models.__dict__
                     if name.islower() and not name.startswith("__")
                     and callable(models.__dict__[name]))

parser = argparse.ArgumentParser(description='PyTorch ImageNet Training')
parser.add_argument('data', metavar='DIR',
                    help='path to dataset')
parser.add_argument('--arch', '-a', metavar='ARCH', default='resnet18',
                    choices=model_names,
                    help='model architecture: ' +
                    ' | '.join(model_names) +
                    ' (default: resnet18)')
parser.add_argument('-j', '--workers', default=4, type=int, metavar='N',
                    help='number of data loading workers (default: 4)')
parser.add_argument('--epochs', default=90, type=int, metavar='N',
                    help='number of total epochs to run')
parser.add_argument('--start-epoch', default=0, type=int, metavar='N',
                    help='manual epoch number (useful on restarts)')
parser.add_argument('-b', '--batch-size', default=256, type=int,
                    metavar='N', help='mini-batch size per process (default: 256)')
parser.add_argument('--lr', '--learning-rate', default=0.1, type=float,
                    metavar='LR', help='Initial learning rate.  Will be scaled by <global batch size>/256: args.lr = args.lr*float(args.batch_size*args.world_size)/256.  A warmup schedule will also be applied over the first 5 epochs.')
parser.add_argument('--momentum', default=0.9, type=float, metavar='M',
                    help='momentum')
parser.add_argument('--weight-decay', '--wd', default=1e-4, type=float,
                    metavar='W', help='weight decay (default: 1e-4)')
parser.add_argument('--print-freq', '-p', default=10, type=int,
                    metavar='N', help='print frequency (default: 10)')
parser.add_argument('--resume', default='', type=str, metavar='PATH',
                    help='path to latest checkpoint (default: none)')
parser.add_argument('-e', '--evaluate', dest='evaluate', action='store_true',
                    help='evaluate model on validation set')
parser.add_argument('--pretrained', dest='pretrained', action='store_true',
                    help='use pre-trained model')

parser.add_argument('--fp16', action='store_true',
                    help='Run model fp16 mode.')
parser.add_argument('--static-loss-scale', type=float, default=1,
                    help='Static loss scale, positive power of 2 values can improve fp16 convergence.')
parser.add_argument('--prof', dest='prof', action='store_true',
                    help='Only run 10 iterations for profiling.')
parser.add_argument('--deterministic', action='store_true')

parser.add_argument("--local_rank", default=0, type=int)
parser.add_argument('--sync_bn', action='store_true',
                    help='enabling apex sync BN.')

cudnn.benchmark = True

def fast_collate(batch):
    imgs = [img[0] for img in batch]
    targets = torch.tensor([target[1] for target in batch], dtype=torch.int64)
    w = imgs[0].size[0]
    h = imgs[0].size[1]
    tensor = torch.zeros( (len(imgs), 3, h, w), dtype=torch.uint8 )
    for i, img in enumerate(imgs):
        nump_array = np.asarray(img, dtype=np.uint8)
        tens = torch.from_numpy(nump_array)
        if(nump_array.ndim < 3):
            nump_array = np.expand_dims(nump_array, axis=-1)
        nump_array = np.rollaxis(nump_array, 2)

        tensor[i] += torch.from_numpy(nump_array)
        
    return tensor, targets

best_prec1 = 0
args = parser.parse_args()

if args.deterministic:
    cudnn.benchmark = False
    cudnn.deterministic = True
    torch.manual_seed(args.local_rank)

def main():
    global best_prec1, args

    args.distributed = False
    if 'WORLD_SIZE' in os.environ:
        args.distributed = int(os.environ['WORLD_SIZE']) > 1

    args.gpu = 0
    args.world_size = 1

    if args.distributed:
        args.gpu = args.local_rank % torch.cuda.device_count()
        torch.cuda.set_device(args.gpu)
        torch.distributed.init_process_group(backend='nccl',
                                             init_method='env://')
        args.world_size = torch.distributed.get_world_size()

    if args.fp16:
        assert torch.backends.cudnn.enabled, "fp16 mode requires cudnn backend to be enabled."

    if args.static_loss_scale != 1.0:
        if not args.fp16:
            print("Warning:  static_loss_scale != 1.0 is only necessary with --fp16. "
                  "Resetting static_loss_scale to 1.0")
            args.static_loss_scale = 1.0

    # create model
    if args.pretrained:
        print("=> using pre-trained model '{}'".format(args.arch))
        model = models.__dict__[args.arch](pretrained=True)
    else:
        print("=> creating model '{}'".format(args.arch))
        model = models.__dict__[args.arch]()

    if args.sync_bn:
        import apex
        print("using apex synced BN")
        model = apex.parallel.convert_syncbn_model(model)

    model = model.cuda()
    if args.fp16:
        model = network_to_half(model)
    if args.distributed:
        # By default, apex.parallel.DistributedDataParallel overlaps communication with 
        # computation in the backward pass.
        # model = DDP(model)
        # delay_allreduce delays all communication to the end of the backward pass.
        model = DDP(model, delay_allreduce=True)

    global model_params, master_params
    if args.fp16:
        model_params, master_params = prep_param_lists(model)
    else:
        master_params = list(model.parameters())

    # define loss function (criterion) and optimizer
    criterion = nn.CrossEntropyLoss().cuda()

    # Scale learning rate based on global batch size
    args.lr = args.lr*float(args.batch_size*args.world_size)/256. 
    optimizer = torch.optim.SGD(master_params, args.lr,
                                momentum=args.momentum,
                                weight_decay=args.weight_decay)

    # Optionally resume from a checkpoint
    if args.resume:
        # Use a local scope to avoid dangling references
        def resume():
            if os.path.isfile(args.resume):
                print("=> loading checkpoint '{}'".format(args.resume))
                checkpoint = torch.load(args.resume, map_location = lambda storage, loc: storage.cuda(args.gpu))
                args.start_epoch = checkpoint['epoch']
                best_prec1 = checkpoint['best_prec1']
                model.load_state_dict(checkpoint['state_dict'])
                if args.fp16:
                    saved_master_params = checkpoint['master_params']
                    for master, saved in zip(master_params, saved_master_params):
                        master.data.copy_(saved.data) 
                optimizer.load_state_dict(checkpoint['optimizer'])
                print("=> loaded checkpoint '{}' (epoch {})"
                      .format(args.resume, checkpoint['epoch']))
            else:
                print("=> no checkpoint found at '{}'".format(args.resume))
        resume()

    # Data loading code
    traindir = os.path.join(args.data, 'train')
    valdir = os.path.join(args.data, 'val')

    if(args.arch == "inception_v3"):
        crop_size = 299
        val_size = 320 # I chose this value arbitrarily, we can adjust.
    else:
        crop_size = 224
        val_size = 256

    train_dataset = datasets.ImageFolder(
        traindir,
        transforms.Compose([
            transforms.RandomResizedCrop(crop_size),
            transforms.RandomHorizontalFlip(),
            # transforms.ToTensor(), Too slow
            # normalize,
        ]))
    val_dataset = datasets.ImageFolder(valdir, transforms.Compose([
            transforms.Resize(val_size),
            transforms.CenterCrop(crop_size),
        ]))

    train_sampler = None
    val_sampler = None
    if args.distributed:
        train_sampler = torch.utils.data.distributed.DistributedSampler(train_dataset)
        val_sampler = torch.utils.data.distributed.DistributedSampler(val_dataset)

    train_loader = torch.utils.data.DataLoader(
        train_dataset, batch_size=args.batch_size, shuffle=(train_sampler is None),
        num_workers=args.workers, pin_memory=True, sampler=train_sampler, collate_fn=fast_collate)

    val_loader = torch.utils.data.DataLoader(
        val_dataset,
        batch_size=args.batch_size, shuffle=False,
        num_workers=args.workers, pin_memory=True,
        sampler=val_sampler,
        collate_fn=fast_collate)

    if args.evaluate:
        validate(val_loader, model, criterion)
        return

    for epoch in range(args.start_epoch, args.epochs):
        if args.distributed:
            train_sampler.set_epoch(epoch)

        # train for one epoch
        train(train_loader, model, criterion, optimizer, epoch)
        if args.prof:
            break
        # evaluate on validation set
        prec1 = validate(val_loader, model, criterion)

        # remember best prec@1 and save checkpoint
        if args.local_rank == 0:
            is_best = prec1 > best_prec1
            best_prec1 = max(prec1, best_prec1)
            # Use local scope to avoid dangling references
            def create_and_save_checkpoint():
                checkpoint_dict = {
                    'epoch': epoch + 1,
                    'arch': args.arch,
                    'state_dict': model.state_dict(),
                    'best_prec1': best_prec1,
                    'optimizer' : optimizer.state_dict(),
                }
                if args.fp16:
                    checkpoint_dict['master_params'] = master_params
                save_checkpoint(checkpoint_dict, is_best)
            create_and_save_checkpoint()

class data_prefetcher():
    def __init__(self, loader):
        self.loader = iter(loader)
        self.stream = torch.cuda.Stream()
        self.mean = torch.tensor([0.485 * 255, 0.456 * 255, 0.406 * 255]).cuda().view(1,3,1,1)
        self.std = torch.tensor([0.229 * 255, 0.224 * 255, 0.225 * 255]).cuda().view(1,3,1,1)
        if args.fp16:
            self.mean = self.mean.half()
            self.std = self.std.half()
        self.preload()

    def preload(self):
        try:
            self.next_input, self.next_target = next(self.loader)
        except StopIteration:
            self.next_input = None
            self.next_target = None
            return
        with torch.cuda.stream(self.stream):
            self.next_input = self.next_input.cuda(non_blocking=True)
            self.next_target = self.next_target.cuda(non_blocking=True)
            if args.fp16:
                self.next_input = self.next_input.half()
            else:
                self.next_input = self.next_input.float()
            self.next_input = self.next_input.sub_(self.mean).div_(self.std)
            
    def next(self):
        torch.cuda.current_stream().wait_stream(self.stream)
        input = self.next_input
        target = self.next_target
        self.preload()
        return input, target


def train(train_loader, model, criterion, optimizer, epoch):
    batch_time = AverageMeter()
    data_time = AverageMeter()
    losses = AverageMeter()
    top1 = AverageMeter()
    top5 = AverageMeter()

    # switch to train mode
    model.train()
    end = time.time()

    prefetcher = data_prefetcher(train_loader)
    input, target = prefetcher.next()
    i = -1
    while input is not None:
        i += 1

        adjust_learning_rate(optimizer, epoch, i, len(train_loader))

        if args.prof:
            if i > 10:
                break
        # measure data loading time
        data_time.update(time.time() - end)

        # compute output
        output = model(input)
        loss = criterion(output, target)

        # measure accuracy and record loss
        prec1, prec5 = accuracy(output.data, target, topk=(1, 5))

        if args.distributed:
            reduced_loss = reduce_tensor(loss.data)
            prec1 = reduce_tensor(prec1)
            prec5 = reduce_tensor(prec5)
        else:
            reduced_loss = loss.data

        losses.update(to_python_float(reduced_loss), input.size(0))
        top1.update(to_python_float(prec1), input.size(0))
        top5.update(to_python_float(prec5), input.size(0))

        loss = loss*args.static_loss_scale
        # compute gradient and do SGD step
        if args.fp16:
            model.zero_grad()
            loss.backward()
            model_grads_to_master_grads(model_params, master_params)
            if args.static_loss_scale != 1:
                for param in master_params:
                    param.grad.data = param.grad.data/args.static_loss_scale
            optimizer.step()
            master_params_to_model_params(model_params, master_params)
        else:
            optimizer.zero_grad()
            loss.backward()
            optimizer.step()

        torch.cuda.synchronize()
        # measure elapsed time
        batch_time.update(time.time() - end)

        end = time.time()
        input, target = prefetcher.next()

        if args.local_rank == 0 and i % args.print_freq == 0 and i > 1:
            print('Epoch: [{0}][{1}/{2}]\t'
                  'Time {batch_time.val:.3f} ({batch_time.avg:.3f})\t'
                  'Speed {3:.3f} ({4:.3f})\t'
                  'Data {data_time.val:.3f} ({data_time.avg:.3f})\t'
                  'Loss {loss.val:.4f} ({loss.avg:.4f})\t'
                  'Prec@1 {top1.val:.3f} ({top1.avg:.3f})\t'
                  'Prec@5 {top5.val:.3f} ({top5.avg:.3f})'.format(
                   epoch, i, len(train_loader),
                   args.world_size * args.batch_size / batch_time.val,
                   args.world_size * args.batch_size / batch_time.avg,
                   batch_time=batch_time,
                   data_time=data_time, loss=losses, top1=top1, top5=top5))


def validate(val_loader, model, criterion):
    batch_time = AverageMeter()
    losses = AverageMeter()
    top1 = AverageMeter()
    top5 = AverageMeter()

    # switch to evaluate mode
    model.eval()

    end = time.time()

    prefetcher = data_prefetcher(val_loader)
    input, target = prefetcher.next()
    i = -1
    while input is not None:
        i += 1

        # compute output
        with torch.no_grad():
            output = model(input)
            loss = criterion(output, target)

        # measure accuracy and record loss
        prec1, prec5 = accuracy(output.data, target, topk=(1, 5))

        if args.distributed:
            reduced_loss = reduce_tensor(loss.data)
            prec1 = reduce_tensor(prec1)
            prec5 = reduce_tensor(prec5)
        else:
            reduced_loss = loss.data

        losses.update(to_python_float(reduced_loss), input.size(0))
        top1.update(to_python_float(prec1), input.size(0))
        top5.update(to_python_float(prec5), input.size(0))

        # measure elapsed time
        batch_time.update(time.time() - end)
        end = time.time()

        if args.local_rank == 0 and i % args.print_freq == 0:
            print('Test: [{0}/{1}]\t'
                  'Time {batch_time.val:.3f} ({batch_time.avg:.3f})\t'
                  'Speed {2:.3f} ({3:.3f})\t'
                  'Loss {loss.val:.4f} ({loss.avg:.4f})\t'
                  'Prec@1 {top1.val:.3f} ({top1.avg:.3f})\t'
                  'Prec@5 {top5.val:.3f} ({top5.avg:.3f})'.format(
                   i, len(val_loader),
                   args.world_size * args.batch_size / batch_time.val,
                   args.world_size * args.batch_size / batch_time.avg,
                   batch_time=batch_time, loss=losses,
                   top1=top1, top5=top5))

        input, target = prefetcher.next()

    print(' * Prec@1 {top1.avg:.3f} Prec@5 {top5.avg:.3f}'
          .format(top1=top1, top5=top5))

    return top1.avg


def save_checkpoint(state, is_best, filename='checkpoint.pth.tar'):
    torch.save(state, filename)
    if is_best:
        shutil.copyfile(filename, 'model_best.pth.tar')


class AverageMeter(object):
    """Computes and stores the average and current value"""
    def __init__(self):
        self.reset()

    def reset(self):
        self.val = 0
        self.avg = 0
        self.sum = 0
        self.count = 0

    def update(self, val, n=1):
        self.val = val
        self.sum += val * n
        self.count += n
        self.avg = self.sum / self.count


def adjust_learning_rate(optimizer, epoch, step, len_epoch):
    """LR schedule that should yield 76% converged accuracy with batch size 256"""
    factor = epoch // 30

    if epoch >= 80:
        factor = factor + 1

    lr = args.lr*(0.1**factor)

    """Warmup"""
    if epoch < 5:
        lr = lr*float(1 + step + epoch*len_epoch)/(5.*len_epoch)

    # if(args.local_rank == 0):
    #     print("epoch = {}, step = {}, lr = {}".format(epoch, step, lr))

    for param_group in optimizer.param_groups:
        param_group['lr'] = lr


def accuracy(output, target, topk=(1,)):
    """Computes the precision@k for the specified values of k"""
    maxk = max(topk)
    batch_size = target.size(0)

    _, pred = output.topk(maxk, 1, True, True)
    pred = pred.t()
    correct = pred.eq(target.view(1, -1).expand_as(pred))

    res = []
    for k in topk:
        correct_k = correct[:k].view(-1).float().sum(0, keepdim=True)
        res.append(correct_k.mul_(100.0 / batch_size))
    return res


def reduce_tensor(tensor):
    rt = tensor.clone()
    dist.all_reduce(rt, op=dist.reduce_op.SUM)
    rt /= args.world_size
    return rt

if __name__ == '__main__':
    main()
