# Note: The model and training settings do not follow the reference settings
# from the paper. The settings are chosen such that the example can easily be
# run on a small dataset with a single GPU.

import pytorch_lightning as pl
import torch
import torchvision
from torch import nn

from lightly.loss import NegativeCosineSimilarity
from lightly.models.modules import SimSiamPredictionHead, SimSiamProjectionHead
from lightly.transforms import SimSiamTransform


class SimSiam(pl.LightningModule):
    def __init__(self):
        super().__init__()
        resnet = torchvision.models.resnet18()
        self.backbone = nn.Sequential(*list(resnet.children())[:-1])
        self.projection_head = SimSiamProjectionHead(512, 512, 128)
        self.prediction_head = SimSiamPredictionHead(128, 64, 128)
        self.criterion = NegativeCosineSimilarity()

    def forward(self, x):
        f = self.backbone(x).flatten(start_dim=1)
        z = self.projection_head(f)
        p = self.prediction_head(z)
        z = z.detach()
        return z, p

    def training_step(self, batch, batch_idx):
        (x0, x1) = batch[0]
        z0, p0 = self.forward(x0)
        z1, p1 = self.forward(x1)
        loss = 0.5 * (self.criterion(z0, p1) + self.criterion(z1, p0))
        return loss

    def configure_optimizers(self):
        optim = torch.optim.SGD(self.parameters(), lr=0.06)
        return optim


resnet = torchvision.models.resnet18()
backbone = nn.Sequential(*list(resnet.children())[:-1])
model = SimSiam()

transform = SimSiamTransform(input_size=32)
dataset = torchvision.datasets.CIFAR10(
    "datasets/cifar10", download=True, transform=transform
)
# or create a dataset from a folder containing images or videos:
# dataset = LightlyDataset("path/to/folder", transform=transform)

dataloader = torch.utils.data.DataLoader(
    dataset,
    batch_size=256,
    shuffle=True,
    drop_last=True,
    num_workers=8,
)

# Train with DDP and use Synchronized Batch Norm for a more accurate batch norm
# calculation. Distributed sampling is also enabled with replace_sampler_ddp=True.
trainer = pl.Trainer(
    max_epochs=10,
    devices="auto",
    accelerator="gpu",
    strategy="ddp",
    sync_batchnorm=True,
    use_distributed_sampler=True,  # or replace_sampler_ddp=True for PyTorch Lightning <2.0
)
trainer.fit(model=model, train_dataloaders=dataloader)
