# State-of-the-Art Document Understanding Methods

## 🎯 You're Right - This IS Document Understanding!

Contract analysis = **Document Understanding** + Legal Domain Knowledge

---

## 🏆 Current SOTA Methods (2024-2025)

### **1. Long-Context Transformers** ⭐ BEST FOR YOUR USE CASE

#### **Longformer** (Allen AI, 2020)
- **Max Length**: 4,096+ tokens (vs BERT's 512)
- **Innovation**: Sliding window + global attention
- **Use Case**: Full contract documents without chunking

```python
from transformers import LongformerModel, LongformerTokenizer

model = LongformerModel.from_pretrained('allenai/longformer-base-4096')
tokenizer = LongformerTokenizer.from_pretrained('allenai/longformer-base-4096')

# Can process entire contract at once!
inputs = tokenizer(full_contract, return_tensors='pt', max_length=4096)
outputs = model(**inputs)
```

**Legal Variant**: [**Longformer-Legal**](https://huggingface.co/lexlms/legal-longformer-base)
```python
# Pre-trained on legal documents!
model = LongformerModel.from_pretrained('lexlms/legal-longformer-base')
```

---

#### **LED (Longformer Encoder-Decoder)** (Allen AI, 2020)
- **Max Length**: 16,384 tokens
- **Use Case**: Document summarization + Q&A

```python
from transformers import LEDForConditionalGeneration, LEDTokenizer

model = LEDForConditionalGeneration.from_pretrained('allenai/led-large-16384')

# Summarize entire contract
summary = model.generate(contract_tokens, max_length=512)
```

---

#### **BigBird** (Google, 2020)
- **Max Length**: 4,096 tokens
- **Innovation**: Sparse attention (random + global + sliding)
- **Efficiency**: O(n) instead of O(n²)

```python
from transformers import BigBirdModel

model = BigBirdModel.from_pretrained('google/bigbird-roberta-base')
```

---

### **2. Hierarchical Document Models** 🔥 RECOMMENDED

#### **Hierarchical Attention Networks (HAN)** (Yang et al., 2016)
- **Structure**: Word → Sentence → Document
- **Perfect for**: Legal contracts with clause hierarchy

```
Document
├── Section 1: SERVICES
│   ├── Sentence 1 (attention weights)
│   ├── Sentence 2 (attention weights)
│   └── Sentence 3 (attention weights)
├── Section 2: PAYMENT
│   └── ...
└── Section 3: TERMINATION
```

**Implementation**:
```python
class HierarchicalContractModel(nn.Module):
    def __init__(self):
        super().__init__()
        # Word-level encoder
        self.word_encoder = nn.GRU(embedding_dim, hidden_dim, bidirectional=True)
        self.word_attention = nn.Linear(hidden_dim*2, 1)
        
        # Sentence-level encoder
        self.sentence_encoder = nn.GRU(hidden_dim*2, hidden_dim, bidirectional=True)
        self.sentence_attention = nn.Linear(hidden_dim*2, 1)
        
        # Document-level classifier
        self.classifier = nn.Linear(hidden_dim*2, num_classes)
    
    def forward(self, document):
        # document shape: [batch, num_sentences, num_words, embedding_dim]
        
        # 1. Encode words in each sentence
        sentence_vectors = []
        for sentence in document:
            word_hidden = self.word_encoder(sentence)
            word_attn = F.softmax(self.word_attention(word_hidden), dim=0)
            sentence_vec = (word_hidden * word_attn).sum(dim=0)
            sentence_vectors.append(sentence_vec)
        
        # 2. Encode sentences in document
        doc_hidden = self.sentence_encoder(sentence_vectors)
        sent_attn = F.softmax(self.sentence_attention(doc_hidden), dim=0)
        doc_vec = (doc_hidden * sent_attn).sum(dim=0)
        
        # 3. Classify
        return self.classifier(doc_vec)
```

---

#### **BERT-HAN** (Modern variant)
Combine BERT with hierarchical structure:

```python
class BERTHierarchical(nn.Module):
    def __init__(self):
        super().__init__()
        # Use BERT for sentence encoding
        self.bert = AutoModel.from_pretrained('nlpaueb/legal-bert-base-uncased')
        
        # Hierarchical aggregation
        self.clause_encoder = nn.LSTM(768, 256, bidirectional=True)
        self.section_encoder = nn.LSTM(512, 256, bidirectional=True)
        
        # Attention mechanisms
        self.clause_attention = nn.Linear(512, 1)
        self.section_attention = nn.Linear(512, 1)
        
    def forward(self, sections):
        # sections: List[List[clause_text]]
        section_vectors = []
        
        for section in sections:
            # Encode each clause with BERT
            clause_embeddings = []
            for clause in section:
                bert_output = self.bert(**tokenizer(clause, return_tensors='pt'))
                clause_embeddings.append(bert_output.last_hidden_state[:, 0, :])
            
            # Aggregate clauses -> section
            clause_hidden, _ = self.clause_encoder(torch.stack(clause_embeddings))
            clause_attn = F.softmax(self.clause_attention(clause_hidden), dim=0)
            section_vec = (clause_hidden * clause_attn).sum(dim=0)
            section_vectors.append(section_vec)
        
        # Aggregate sections -> document
        section_hidden, _ = self.section_encoder(torch.stack(section_vectors))
        section_attn = F.softmax(self.section_attention(section_hidden), dim=0)
        document_vec = (section_hidden * section_attn).sum(dim=0)
        
        return document_vec
```

---

### **3. Document Graph Neural Networks** 🌐

#### **Graph Transformer** (Microsoft, 2022)
Model document as graph: clauses = nodes, references = edges

```
[Clause 1: "Services in Exhibit A"] ──references──> [Exhibit A]
         │
    mentions
         │
         ↓
[Clause 2: "Such Services..."] ──references──> [Section 5]
```

```python
import torch_geometric as pyg

class DocumentGraphNN(nn.Module):
    def __init__(self):
        super().__init__()
        # Node encoder (BERT for each clause)
        self.node_encoder = AutoModel.from_pretrained('legal-bert')
        
        # Graph convolution layers
        self.conv1 = pyg.nn.GCNConv(768, 256)
        self.conv2 = pyg.nn.GCNConv(256, 128)
        
        # Classifier
        self.classifier = nn.Linear(128, num_classes)
    
    def forward(self, clauses, edges):
        # 1. Encode nodes (clauses)
        node_features = []
        for clause in clauses:
            bert_out = self.node_encoder(**tokenizer(clause, return_tensors='pt'))
            node_features.append(bert_out.last_hidden_state[:, 0, :])
        
        x = torch.stack(node_features)
        
        # 2. Graph convolution (propagate context)
        x = F.relu(self.conv1(x, edges))
        x = self.conv2(x, edges)
        
        # 3. Classify
        return self.classifier(x)
```

**Edge Types**:
- Sequential (Clause N → Clause N+1)
- Reference ("Section 5" → actual Section 5)
- Semantic similarity (cosine > threshold)

---

### **4. Retrieval-Augmented Models** 🔍

#### **RAG (Retrieval-Augmented Generation)** (Facebook, 2020)
Retrieve relevant clauses before classification

```python
from transformers import RagTokenizer, RagRetriever, RagModel

# Index all contract clauses
retriever = RagRetriever.from_pretrained('facebook/rag-token-base')
model = RagModel.from_pretrained('facebook/rag-token-base')

# For each clause, retrieve similar clauses
def predict_with_retrieval(clause):
    # Retrieve top-k similar clauses
    retrieved = retriever.retrieve(clause, top_k=5)
    
    # Generate prediction with context
    output = model.generate(
        context_input_ids=retrieved['input_ids'],
        context_attention_mask=retrieved['attention_mask'],
        decoder_input_ids=tokenizer(clause, return_tensors='pt')['input_ids']
    )
    
    return output
```

---

### **5. Large Language Models (2023-2025)** 🤖

#### **GPT-4 / Claude** (OpenAI / Anthropic)
- **Context**: 128k tokens (GPT-4 Turbo), 200k (Claude 3)
- **Approach**: Few-shot learning + prompting

```python
import openai

def analyze_contract_llm(contract):
    prompt = f"""
    Analyze this contract for risk clauses. For each clause, provide:
    1. Risk severity (0-10)
    2. Risk category
    3. Explanation
    
    Contract:
    {contract}
    
    Format as JSON.
    """
    
    response = openai.ChatCompletion.create(
        model="gpt-4-turbo",
        messages=[{"role": "user", "content": prompt}],
        temperature=0.1  # Low for consistency
    )
    
    return json.loads(response.choices[0].message.content)
```

**Pros**: No training needed, incredible understanding
**Cons**: Expensive, slower, privacy concerns

---

#### **Legal-Specific LLMs**
- **LegalBench** (Stanford, 2023)
- **ChatLaw** (Peking University, 2023)
- **LawGPT** (2024)

---

### **6. Multi-Modal Document Understanding** 📄

#### **LayoutLM** (Microsoft, 2020-2023)
Understands document **layout** + text

```python
from transformers import LayoutLMv3Model

# Processes:
# 1. Text content
# 2. Bounding boxes (where text appears)
# 3. Images (if PDF has visual elements)

model = LayoutLMv3Model.from_pretrained('microsoft/layoutlmv3-base')

# Input includes position information
inputs = {
    'input_ids': text_tokens,
    'bbox': bounding_boxes,  # [x0, y0, x1, y1] for each token
    'pixel_values': document_image
}

outputs = model(**inputs)
```

**Why this matters**: Contracts have structure (headers, indentation, tables)

---

## 📊 Comparison for Your Use Case

| Method | Context Length | Structure Aware | Training Cost | Inference Speed | Best For |
|--------|---------------|-----------------|---------------|-----------------|----------|
| **BERT** (current) | 512 tokens | ❌ | Low | Fast | Clause-level |
| **Longformer** | 4,096 tokens | ❌ | Medium | Medium | Full documents |
| **Hierarchical BERT** | Unlimited* | ✅✅✅ | Medium | Medium | **RECOMMENDED** |
| **Graph NN** | Unlimited* | ✅✅ | High | Slow | Complex references |
| **LLM (GPT-4)** | 128k tokens | ✅ | Zero! | Slow | No training data |
| **LayoutLM** | 512 tokens | ✅ (visual) | High | Medium | Scanned PDFs |

*Processes document in chunks with aggregation

---

## 🎯 RECOMMENDATION: Hierarchical BERT

### Why?
1. ✅ **Respects document structure** (clauses → sections → document)
2. ✅ **Handles any document length** (processes hierarchically)
3. ✅ **Better context modeling** than your current sliding window
4. ✅ **Interpretable** (attention weights show important sections)
5. ✅ **Moderate complexity** (not too hard to implement)

### Implementation Plan

```python
# hierarchical_bert.py

import torch
import torch.nn as nn
from transformers import AutoModel, AutoTokenizer

class HierarchicalContractBERT(nn.Module):
    """
    Hierarchical document understanding for legal contracts
    
    Structure:
        Word → Clause → Section → Document
    """
    
    def __init__(self, model_name='nlpaueb/legal-bert-base-uncased', num_labels=3):
        super().__init__()
        
        # Clause encoder (BERT)
        self.bert = AutoModel.from_pretrained(model_name)
        self.tokenizer = AutoTokenizer.from_pretrained(model_name)
        
        # Hierarchical aggregation
        self.clause_to_section = nn.LSTM(
            input_size=768,  # BERT hidden size
            hidden_size=256,
            num_layers=2,
            bidirectional=True,
            dropout=0.1,
            batch_first=True
        )
        
        self.section_to_document = nn.LSTM(
            input_size=512,  # bidirectional * 256
            hidden_size=256,
            num_layers=2,
            bidirectional=True,
            dropout=0.1,
            batch_first=True
        )
        
        # Attention mechanisms
        self.clause_attention = nn.Sequential(
            nn.Linear(512, 128),
            nn.Tanh(),
            nn.Linear(128, 1)
        )
        
        self.section_attention = nn.Sequential(
            nn.Linear(512, 128),
            nn.Tanh(),
            nn.Linear(128, 1)
        )
        
        # Task heads (same as your current model)
        self.severity_head = nn.Linear(512, 1)
        self.category_head = nn.Linear(512, num_labels)
    
    def encode_clause(self, clause_text):
        """Encode single clause with BERT"""
        inputs = self.tokenizer(
            clause_text,
            return_tensors='pt',
            padding='max_length',
            truncation=True,
            max_length=128  # Shorter for clauses
        )
        
        outputs = self.bert(**inputs)
        return outputs.last_hidden_state[:, 0, :]  # [CLS] token
    
    def aggregate_with_attention(self, hidden_states, attention_module):
        """Apply attention-based aggregation"""
        # hidden_states: [batch, seq_len, hidden_dim]
        
        # Compute attention weights
        attention_logits = attention_module(hidden_states)  # [batch, seq_len, 1]
        attention_weights = torch.softmax(attention_logits, dim=1)
        
        # Weighted sum
        context_vector = torch.sum(hidden_states * attention_weights, dim=1)
        
        return context_vector, attention_weights
    
    def forward(self, document_structure):
        """
        Args:
            document_structure: List of sections
                Each section is a list of clause texts
                Example: [
                    ['clause 1', 'clause 2'],  # Section 1
                    ['clause 3', 'clause 4', 'clause 5'],  # Section 2
                ]
        
        Returns:
            document_embedding, clause_predictions, attention_weights
        """
        section_vectors = []
        all_clause_predictions = []
        attention_weights = {'clause': [], 'section': None}
        
        # Process each section
        for section_clauses in document_structure:
            clause_vectors = []
            
            # 1. Encode each clause
            for clause in section_clauses:
                clause_vec = self.encode_clause(clause)
                clause_vectors.append(clause_vec)
            
            # Stack: [num_clauses, 768]
            clause_hidden = torch.stack(clause_vectors).squeeze(1)
            
            # 2. LSTM over clauses (captures sequential context)
            clause_lstm_out, _ = self.clause_to_section(clause_hidden.unsqueeze(0))
            # clause_lstm_out: [1, num_clauses, 512]
            
            # 3. Attention over clauses
            section_vec, clause_attn = self.aggregate_with_attention(
                clause_lstm_out, 
                self.clause_attention
            )
            
            section_vectors.append(section_vec)
            attention_weights['clause'].append(clause_attn)
            
            # 4. Predict for each clause (using context-aware representation)
            for i in range(len(section_clauses)):
                clause_repr = clause_lstm_out[0, i, :]  # Context-aware!
                
                severity = self.severity_head(clause_repr)
                category = self.category_head(clause_repr)
                
                all_clause_predictions.append({
                    'severity': severity,
                    'category': category,
                    'text': section_clauses[i]
                })
        
        # 5. Stack sections: [num_sections, 512]
        section_hidden = torch.stack(section_vectors)
        
        # 6. LSTM over sections
        section_lstm_out, _ = self.section_to_document(section_hidden.unsqueeze(0))
        
        # 7. Attention over sections
        document_vec, section_attn = self.aggregate_with_attention(
            section_lstm_out,
            self.section_attention
        )
        
        attention_weights['section'] = section_attn
        
        return {
            'document_embedding': document_vec,
            'clause_predictions': all_clause_predictions,
            'attention_weights': attention_weights
        }
    
    def predict_document(self, document_structure):
        """Convenience method for inference"""
        outputs = self.forward(document_structure)
        
        # Extract predictions
        predictions = []
        for clause_pred in outputs['clause_predictions']:
            predictions.append({
                'text': clause_pred['text'],
                'severity': torch.sigmoid(clause_pred['severity']).item() * 10,
                'category': torch.softmax(clause_pred['category'], dim=-1).tolist()
            })
        
        return {
            'clauses': predictions,
            'attention_weights': outputs['attention_weights']
        }


# Usage example
if __name__ == '__main__':
    model = HierarchicalContractBERT()
    
    # Parse document into hierarchical structure
    document = [
        # Section 1: Services
        [
            "Provider shall deliver software services as described in Exhibit A.",
            "Such Services shall be performed in a professional manner.",
            "Services include maintenance and support."
        ],
        # Section 2: Payment
        [
            "Client shall pay within 30 days of invoice.",
            "Late payments incur 1.5% monthly interest.",
        ],
        # Section 3: Termination
        [
            "Either party may terminate with 30 days written notice.",
            "Upon termination, all obligations under Section 2 remain in effect."
        ]
    ]
    
    # Predict
    results = model.predict_document(document)
    
    print("Clause Predictions:")
    for i, clause in enumerate(results['clauses']):
        print(f"\n{i+1}. {clause['text']}")
        print(f"   Severity: {clause['severity']:.2f}/10")
        print(f"   Category: {clause['category']}")
    
    print("\n\nAttention Weights:")
    print("Clause-level attention shows which clauses are most important in each section")
    print("Section-level attention shows which sections are most critical overall")
```

---

## 🚀 Migration Path

### Phase 1: Quick Win (Current + Context)
```python
# Use your current model + sliding window context
# Already implemented! ✅
analyze_full_document(contract, model, use_context=True, context_window=2)
```

### Phase 2: Upgrade to Longformer (Easy)
```python
# Just swap BERT for Longformer
# Can process 8x longer context
from transformers import LongformerModel

model = LongformerModel.from_pretrained('lexlms/legal-longformer-base')
```

### Phase 3: Hierarchical BERT (Recommended)
```python
# Implement hierarchical model (code above)
# Better document understanding + interpretability
model = HierarchicalContractBERT()
```

### Phase 4: Graph NN (Advanced)
```python
# Add graph connections for references
# Build clause dependency graph
model = DocumentGraphNN()
```

---

## 📚 Key Papers

1. **Longformer**: "Longformer: The Long-Document Transformer" (Beltagy et al., 2020)
2. **BigBird**: "Big Bird: Transformers for Longer Sequences" (Zaheer et al., 2020)
3. **HAN**: "Hierarchical Attention Networks for Document Classification" (Yang et al., 2016)
4. **LayoutLM**: "LayoutLMv3: Pre-training for Document AI" (Huang et al., 2022)
5. **Legal-BERT**: "LEGAL-BERT: The Muppets straight out of Law School" (Chalkidis et al., 2020)
6. **Document Understanding**: "A Survey on Document-level Neural Machine Translation" (2023)

---

## 💡 My Suggestion

**Short term** (Today):
- ✅ Keep using context-aware analysis (already done!)
- Test with `use_context=True, context_window=2`

**Medium term** (This week):
- 🔄 Implement Hierarchical BERT (code provided above)
- Train on your CUAD dataset with section structure
- Compare performance: BERT vs Hierarchical BERT

**Long term** (If needed):
- Consider Longformer for very long contracts (>2000 words)
- Experiment with Graph NN if many cross-references
- Try GPT-4 for zero-shot (if budget allows)

---

## 🎯 Bottom Line

**You're correct**: This is document understanding, not just text classification!

**Current approach**: Clause-by-clause is limiting
**SOTA approach**: Hierarchical models that understand document structure

**Best ROI**: Implement Hierarchical BERT (code above)
- Moderate complexity
- Big performance gain
- Interpretable (attention weights)
- Handles full documents

Would you like me to integrate the Hierarchical BERT into your pipeline? 🚀