src/streamlit_app.py

"""
Snippet Generator - Recreates Google Vertex AI/Gemini grounding snippets
Uses MS MARCO Cross-Encoder for search relevance ranking.
"""

import re
import numpy as np
import streamlit as st
import torch
from sentence_transformers import CrossEncoder

# --- Configuration ---
MODEL_NAME = "cross-encoder/ms-marco-electra-base"
MAX_SNIPPET_CHARS = 450
MAX_SENTENCES = 5

st.set_page_config(
    page_title="Snippet Generator",
    page_icon="✂️",
    layout="centered"
)


@st.cache_resource
def load_model():
    """Load CrossEncoder model."""
    device = "cuda" if torch.cuda.is_available() else "cpu"
    model = CrossEncoder(MODEL_NAME, device=device)
    return model


def segment_sentences(text: str) -> list[str]:
    """Sentence segmentation with deduplication and filtering."""
    # Split on sentence boundaries AND newlines
    pattern = r'(?<=[.!?])\s+|\n+'
    raw_sentences = re.split(pattern, text)
    
    seen = set()
    sentences = []
    
    for s in raw_sentences:
        s = s.strip()
        
        if not s or len(s) < 20:
            continue
        
        if s.startswith('http') or s.startswith('URL:'):
            continue
        
        # Skip low-alpha content (metadata, tables, prices)
        alpha_ratio = sum(c.isalpha() for c in s) / max(len(s), 1)
        if alpha_ratio < 0.5:
            continue
        
        # Skip questions
        if s.endswith('?'):
            continue
            
        normalized = ' '.join(s.lower().split())
        if normalized in seen:
            continue
        seen.add(normalized)
        
        sentences.append(s)
    
    return sentences


def generate_snippet(query: str, document: str, model, max_chars: int, max_sents: int) -> tuple[str, list]:
    """Generate snippet using Cross-Encoder scoring."""
    sentences = segment_sentences(document)
    
    if not sentences:
        return "", []
    
    # Cross-encoder: score query-sentence pairs
    pairs = [[query, sent] for sent in sentences]
    scores = model.predict(pairs)
    
    ranked_indices = np.argsort(scores)[::-1]
    
    # Select with budget
    selected = []
    total_length = 0
    
    for idx in ranked_indices:
        sent = sentences[idx]
        if total_length + len(sent) <= max_chars and len(selected) < max_sents:
            selected.append((idx, sent, scores[idx]))
            total_length += len(sent)
    
    if not selected:
        best_idx = ranked_indices[0]
        return sentences[best_idx][:max_chars] + "...", []
    
    # Sort by document order
    selected.sort(key=lambda x: x[0])
    
    # Stitch with ellipsis for gaps
    snippet_parts = []
    prev_idx = -1
    
    for idx, sent, _ in selected:
        if prev_idx >= 0 and idx > prev_idx + 1:
            snippet_parts.append("...")
        snippet_parts.append(sent)
        prev_idx = idx
    
    if prev_idx < len(sentences) - 1:
        snippet_parts.append("...")
    
    # Debug info
    debug_info = [(scores[ranked_indices[i]], sentences[ranked_indices[i]]) 
                  for i in range(min(5, len(ranked_indices)))]
    
    return " ".join(snippet_parts), debug_info


# --- Streamlit UI ---
st.title("✂️ Snippet Generator")
st.caption("Recreates Google Vertex AI / Gemini grounding-style snippets")

st.markdown("""
This tool generates extractive snippets from documents using a Cross-Encoder model trained on MS MARCO search relevance data.

**How it works:**
1. Segments document into sentences
2. Scores each sentence against your query using `cross-encoder/ms-marco-electra-base`
3. Selects top-scoring sentences within budget
4. Stitches them in document order with `...` for gaps
""")

st.markdown("---")

query = st.text_input("🔍 Query", value="best prostate cancer treatment in the world")

document = st.text_area(
    "📄 Document", 
    height=250,
    placeholder="Paste document content here..."
)

with st.expander("⚙️ Settings"):
    max_chars = st.slider("Max snippet characters", 200, 1500, MAX_SNIPPET_CHARS, 50)
    max_sents = st.slider("Max sentences", 2, 15, MAX_SENTENCES)
    show_debug = st.checkbox("Show debug info", value=True)

if st.button("Generate Snippet", type="primary"):
    if query and document:
        with st.spinner("Loading model & scoring sentences..."):
            model = load_model()
            snippet, debug = generate_snippet(query, document, model, max_chars, max_sents)
        
        st.subheader("Generated Snippet")
        st.code(snippet, language=None)
        
        if show_debug and debug:
            st.markdown("---")
            st.write("**Top sentences by score:**")
            for score, sent in debug:
                st.text(f"{score:.4f}: {sent[:80]}...")
    else:
        st.warning("Please enter both a query and document.")

st.markdown("---")
st.caption("Model: `cross-encoder/ms-marco-electra-base` | [GitHub](https://github.com/UKPLab/sentence-transformers)")