import torch
import torch.nn.functional as F


def invariance_loss(z1: torch.Tensor, z2: torch.Tensor) -> torch.Tensor:
    """Computes mse loss given batch of projected features z1 from view 1 and
    projected features z2 from view 2.

    Args:
        z1 (torch.Tensor): NxD Tensor containing projected features from view 1.
        z2 (torch.Tensor): NxD Tensor containing projected features from view 2.

    Returns:
        torch.Tensor: invariance loss (mean squared error).
    """

    return F.mse_loss(z1, z2)


def variance_loss(z1: torch.Tensor, z2: torch.Tensor) -> torch.Tensor:
    """Computes variance loss given batch of projected features z1 from view 1 and
    projected features z2 from view 2.

    Args:
        z1 (torch.Tensor): NxD Tensor containing projected features from view 1.
        z2 (torch.Tensor): NxD Tensor containing projected features from view 2.

    Returns:
        torch.Tensor: variance regularization loss.
    """

    eps = 1e-4
    std_z1 = torch.sqrt(z1.var(dim=0) + eps)
    std_z2 = torch.sqrt(z2.var(dim=0) + eps)
    std_loss = torch.mean(F.relu(1 - std_z1)) + torch.mean(F.relu(1 - std_z2))
    return std_loss


def covariance_loss(z1: torch.Tensor, z2: torch.Tensor) -> torch.Tensor:
    """Computes covariance loss given batch of projected features z1 from view 1 and
    projected features z2 from view 2.

    Args:
        z1 (torch.Tensor): NxD Tensor containing projected features from view 1.
        z2 (torch.Tensor): NxD Tensor containing projected features from view 2.

    Returns:
        torch.Tensor: covariance regularization loss.
    """

    N, D = z1.size()

    z1 = z1 - z1.mean(dim=0)
    z2 = z2 - z2.mean(dim=0)
    cov_z1 = (z1.T @ z1) / (N - 1)
    cov_z2 = (z2.T @ z2) / (N - 1)

    diag = torch.eye(D, device=z1.device)
    cov_loss = cov_z1[~diag.bool()].pow_(2).sum() / D + cov_z2[~diag.bool()].pow_(2).sum() / D
    return cov_loss


def vicreg_loss_func(
    z1: torch.Tensor,
    z2: torch.Tensor,
    sim_loss_weight: float = 25.0,
    var_loss_weight: float = 25.0,
    cov_loss_weight: float = 1.0,
) -> torch.Tensor:
    """Computes VICReg's loss given batch of projected features z1 from view 1 and
    projected features z2 from view 2.

    Args:
        z1 (torch.Tensor): NxD Tensor containing projected features from view 1.
        z2 (torch.Tensor): NxD Tensor containing projected features from view 2.
        sim_loss_weight (float): invariance loss weight.
        var_loss_weight (float): variance loss weight.
        cov_loss_weight (float): covariance loss weight.

    Returns:
        torch.Tensor: VICReg loss.
    """

    sim_loss = invariance_loss(z1, z2)
    var_loss = variance_loss(z1, z2)
    cov_loss = covariance_loss(z1, z2)

    loss = sim_loss_weight * sim_loss + var_loss_weight * var_loss + cov_loss_weight * cov_loss
    return loss
