# 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
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# distributed under the License is distributed on an "AS IS" BASIS,
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"""Pretrain VIT"""

import torch
import torch.nn.functional as F
from functools import partial
from megatron import get_args, get_timers, mpu, print_rank_0
from megatron.data.vit_dataset import build_train_valid_datasets
from megatron.model.vit_model import VitModel
from megatron.training import pretrain
from megatron.utils import average_losses_across_data_parallel_group

def model_provider(pre_process=True, post_process=True):
    """Build the model."""

    print_rank_0("building VIT model ...")
    args = get_args()

    model = VitModel(num_classes=args.num_classes,
                     pre_process=pre_process,
                     post_process=post_process)
    return model

def get_batch(data_iterator):
    """Build the batch."""
    data = next(data_iterator)

    # only data parallelism; no need for broadcast
    images = data[0].cuda()
    labels = data[1].cuda()

    return images, labels

def loss_func(labels, output_tensor):
    logits = output_tensor.contiguous().float()
    loss = F.cross_entropy(logits, labels)

    outputs = torch.argmax(logits, -1)
    correct = (outputs == labels).float()
    accuracy = torch.mean(correct)

    averaged_loss = average_losses_across_data_parallel_group([loss, accuracy])

    return loss, {"loss": averaged_loss[0], "accuracy": averaged_loss[1]}

def forward_step(data_iterator, model):
    """Forward step."""
    timers = get_timers()

    # Get the batch.
    timers("batch-generator").start()
    (
        images,
        labels,
    ) = get_batch(data_iterator)
    timers("batch-generator").stop()

    # Forward model. lm_labels
    output_tensor = model(images)

    return output_tensor, partial(loss_func, labels)

def train_valid_test_datasets_provider(train_val_test_num_samples):
    """Build train, valid, and test datasets."""
    args = get_args()

    print_rank_0(
        "> building train, validation, and test datasets " "for VIT ..."
    )
    train_ds, valid_ds = build_train_valid_datasets(data_path=args.data_path)
    print_rank_0("> finished creating VIT datasets ...")

    return train_ds, valid_ds, None


if __name__ == "__main__":

    pretrain(
        train_valid_test_datasets_provider,
        model_provider,
        forward_step,
        args_defaults={'dataloader_type': 'cyclic'}
    )