src/training/losses.py

"""
Loss functions for signature verification training.
"""

import torch
import torch.nn as nn
import torch.nn.functional as F
from typing import Tuple, Optional


class ContrastiveLoss(nn.Module):
    """
    Contrastive loss for Siamese network training.
    """
    
    def __init__(self, margin: float = 1.0):
        """
        Initialize contrastive loss.
        
        Args:
            margin: Margin for dissimilar pairs
        """
        super(ContrastiveLoss, self).__init__()
        self.margin = margin
    
    def forward(self, 
                similarity: torch.Tensor, 
                labels: torch.Tensor) -> torch.Tensor:
        """
        Compute contrastive loss.
        
        Args:
            similarity: Similarity scores (B, 1)
            labels: Binary labels (1 for genuine, 0 for forged) (B,)
            
        Returns:
            Contrastive loss
        """
        # Convert labels to float
        labels = labels.float()
        
        # Compute loss for genuine pairs (similarity should be high)
        genuine_loss = labels * torch.pow(1 - similarity.squeeze(), 2)
        
        # Compute loss for forged pairs (similarity should be low)
        forged_loss = (1 - labels) * torch.pow(torch.clamp(similarity.squeeze() - self.margin, min=0), 2)
        
        # Total loss
        loss = torch.mean(genuine_loss + forged_loss)
        
        return loss


class TripletLoss(nn.Module):
    """
    Triplet loss for signature verification.
    """
    
    def __init__(self, margin: float = 1.0):
        """
        Initialize triplet loss.
        
        Args:
            margin: Margin between positive and negative distances
        """
        super(TripletLoss, self).__init__()
        self.margin = margin
    
    def forward(self, 
                anchor: torch.Tensor, 
                positive: torch.Tensor, 
                negative: torch.Tensor) -> torch.Tensor:
        """
        Compute triplet loss.
        
        Args:
            anchor: Anchor features (B, feature_dim)
            positive: Positive features (B, feature_dim)
            negative: Negative features (B, feature_dim)
            
        Returns:
            Triplet loss
        """
        # Compute distances
        pos_dist = F.pairwise_distance(anchor, positive, p=2)
        neg_dist = F.pairwise_distance(anchor, negative, p=2)
        
        # Compute triplet loss
        loss = F.relu(pos_dist - neg_dist + self.margin)
        
        return torch.mean(loss)


class CenterLoss(nn.Module):
    """
    Center loss for learning discriminative features.
    """
    
    def __init__(self, 
                 num_classes: int, 
                 feature_dim: int, 
                 lambda_c: float = 1.0):
        """
        Initialize center loss.
        
        Args:
            num_classes: Number of signature classes
            feature_dim: Dimension of feature vectors
            lambda_c: Weight for center loss
        """
        super(CenterLoss, self).__init__()
        self.num_classes = num_classes
        self.feature_dim = feature_dim
        self.lambda_c = lambda_c
        
        # Initialize centers
        self.centers = nn.Parameter(torch.randn(num_classes, feature_dim))
    
    def forward(self, 
                features: torch.Tensor, 
                labels: torch.Tensor) -> torch.Tensor:
        """
        Compute center loss.
        
        Args:
            features: Feature vectors (B, feature_dim)
            labels: Class labels (B,)
            
        Returns:
            Center loss
        """
        # Get centers for current batch
        batch_size = features.size(0)
        centers_batch = self.centers[labels]
        
        # Compute center loss
        loss = F.mse_loss(features, centers_batch)
        
        return self.lambda_c * loss


class CombinedLoss(nn.Module):
    """
    Combined loss function for signature verification.
    """
    
    def __init__(self, 
                 contrastive_weight: float = 1.0,
                 triplet_weight: float = 0.5,
                 center_weight: float = 0.1,
                 margin: float = 1.0,
                 num_classes: Optional[int] = None,
                 feature_dim: Optional[int] = None):
        """
        Initialize combined loss.
        
        Args:
            contrastive_weight: Weight for contrastive loss
            triplet_weight: Weight for triplet loss
            center_weight: Weight for center loss
            margin: Margin for contrastive and triplet losses
            num_classes: Number of classes for center loss
            feature_dim: Feature dimension for center loss
        """
        super(CombinedLoss, self).__init__()
        
        self.contrastive_weight = contrastive_weight
        self.triplet_weight = triplet_weight
        self.center_weight = center_weight
        
        # Initialize loss functions
        self.contrastive_loss = ContrastiveLoss(margin=margin)
        self.triplet_loss = TripletLoss(margin=margin)
        
        if num_classes is not None and feature_dim is not None:
            self.center_loss = CenterLoss(num_classes, feature_dim)
        else:
            self.center_loss = None
    
    def forward(self, 
                similarity: Optional[torch.Tensor] = None,
                labels: Optional[torch.Tensor] = None,
                anchor: Optional[torch.Tensor] = None,
                positive: Optional[torch.Tensor] = None,
                negative: Optional[torch.Tensor] = None,
                features: Optional[torch.Tensor] = None) -> torch.Tensor:
        """
        Compute combined loss.
        
        Args:
            similarity: Similarity scores for contrastive loss
            labels: Labels for contrastive and center loss
            anchor: Anchor features for triplet loss
            positive: Positive features for triplet loss
            negative: Negative features for triplet loss
            features: Features for center loss
            
        Returns:
            Combined loss
        """
        total_loss = 0.0
        
        # Contrastive loss
        if similarity is not None and labels is not None:
            contrastive_loss = self.contrastive_loss(similarity, labels)
            total_loss += self.contrastive_weight * contrastive_loss
        
        # Triplet loss
        if anchor is not None and positive is not None and negative is not None:
            triplet_loss = self.triplet_loss(anchor, positive, negative)
            total_loss += self.triplet_weight * triplet_loss
        
        # Center loss
        if self.center_loss is not None and features is not None and labels is not None:
            center_loss = self.center_loss(features, labels)
            total_loss += self.center_weight * center_loss
        
        return total_loss


class FocalLoss(nn.Module):
    """
    Focal loss for handling class imbalance in signature verification.
    """
    
    def __init__(self, 
                 alpha: float = 1.0, 
                 gamma: float = 2.0,
                 reduction: str = 'mean'):
        """
        Initialize focal loss.
        
        Args:
            alpha: Weighting factor for rare class
            gamma: Focusing parameter
            reduction: Reduction method ('mean', 'sum', 'none')
        """
        super(FocalLoss, self).__init__()
        self.alpha = alpha
        self.gamma = gamma
        self.reduction = reduction
    
    def forward(self, 
                inputs: torch.Tensor, 
                targets: torch.Tensor) -> torch.Tensor:
        """
        Compute focal loss.
        
        Args:
            inputs: Predicted probabilities (B, num_classes)
            targets: Target labels (B,)
            
        Returns:
            Focal loss
        """
        # Convert to one-hot encoding
        targets_one_hot = torch.zeros_like(inputs)
        targets_one_hot.scatter_(1, targets.unsqueeze(1), 1)
        
        # Compute cross entropy
        ce_loss = F.cross_entropy(inputs, targets, reduction='none')
        
        # Compute focal weight
        pt = torch.exp(-ce_loss)
        focal_weight = self.alpha * (1 - pt) ** self.gamma
        
        # Compute focal loss
        focal_loss = focal_weight * ce_loss
        
        if self.reduction == 'mean':
            return torch.mean(focal_loss)
        elif self.reduction == 'sum':
            return torch.sum(focal_loss)
        else:
            return focal_loss


class AdaptiveLoss(nn.Module):
    """
    Adaptive loss that adjusts weights based on training progress.
    """
    
    def __init__(self, 
                 initial_contrastive_weight: float = 1.0,
                 initial_triplet_weight: float = 0.5,
                 adaptation_rate: float = 0.01):
        """
        Initialize adaptive loss.
        
        Args:
            initial_contrastive_weight: Initial weight for contrastive loss
            initial_triplet_weight: Initial weight for triplet loss
            adaptation_rate: Rate of weight adaptation
        """
        super(AdaptiveLoss, self).__init__()
        
        self.contrastive_weight = nn.Parameter(torch.tensor(initial_contrastive_weight))
        self.triplet_weight = nn.Parameter(torch.tensor(initial_triplet_weight))
        self.adaptation_rate = adaptation_rate
        
        # Initialize loss functions
        self.contrastive_loss = ContrastiveLoss()
        self.triplet_loss = TripletLoss()
    
    def forward(self, 
                similarity: torch.Tensor,
                labels: torch.Tensor,
                anchor: torch.Tensor,
                positive: torch.Tensor,
                negative: torch.Tensor) -> Tuple[torch.Tensor, dict]:
        """
        Compute adaptive loss.
        
        Args:
            similarity: Similarity scores
            labels: Labels
            anchor: Anchor features
            positive: Positive features
            negative: Negative features
            
        Returns:
            Tuple of (total_loss, loss_info)
        """
        # Compute individual losses
        contrastive_loss = self.contrastive_loss(similarity, labels)
        triplet_loss = self.triplet_loss(anchor, positive, negative)
        
        # Compute total loss
        total_loss = (torch.sigmoid(self.contrastive_weight) * contrastive_loss + 
                     torch.sigmoid(self.triplet_weight) * triplet_loss)
        
        # Prepare loss info
        loss_info = {
            'contrastive_loss': contrastive_loss.item(),
            'triplet_loss': triplet_loss.item(),
            'contrastive_weight': torch.sigmoid(self.contrastive_weight).item(),
            'triplet_weight': torch.sigmoid(self.triplet_weight).item(),
            'total_loss': total_loss.item()
        }
        
        return total_loss, loss_info