app.py

import torch
import torch.nn.functional as F
from transformers import AutoTokenizer, AutoModelForSequenceClassification
import re
import pandas as pd
import gradio as gr

# -----------------------------
# MODEL INITIALIZATION
# -----------------------------
MODEL_NAME = "desklib/ai-text-detector-v1.01"
tokenizer = None
model = None
device = torch.device("cuda" if torch.cuda.is_available() else "cpu")

def get_model():
    global tokenizer, model
    if model is None:
        print(f"Loading Specialized Model: {MODEL_NAME} on {device}")
        tokenizer = AutoTokenizer.from_pretrained(MODEL_NAME)
        # Load with default labels; if the model has 2 (Human/AI), we handle it in analyze()
        model = AutoModelForSequenceClassification.from_pretrained(
            MODEL_NAME, 
            ignore_mismatched_sizes=True
        ).to(device).eval()
    return tokenizer, model

THRESHOLD = 0.59 

# -----------------------------
# UTILITIES
# -----------------------------
ABBR = ["e.g", "i.e", "mr", "mrs", "ms", "dr", "prof", "vs", "etc", "fig", "al", "jr", "sr", "st", "inc", "ltd", "u.s", "u.k"]
ABBR_REGEX = re.compile(r"\b(" + "|".join(map(re.escape, ABBR)) + r")\.", re.IGNORECASE)

def _protect(text):
    text = text.replace("...", "⟨ELLIPSIS⟩")
    text = re.sub(r"(?<=\d)\.(?=\d)", "⟨DECIMAL⟩", text)
    text = ABBR_REGEX.sub(r"\1⟨ABBRDOT⟩", text)
    return text

def _restore(text):
    return text.replace("⟨ABBRDOT⟩", ".").replace("⟨DECIMAL⟩", ".").replace("⟨ELLIPSIS⟩", "...")

def split_preserving_structure(text):
    blocks = re.split(r"(\n+)", text)
    final_blocks = []
    for block in blocks:
        if block.startswith("\n"):
            final_blocks.append(block)
        else:
            protected = _protect(block)
            parts = re.split(r"([.?!])(\s+)", protected)
            for i in range(0, len(parts), 3):
                sentence = parts[i]
                punct = parts[i+1] if i+1 < len(parts) else ""
                space = parts[i+2] if i+2 < len(parts) else ""
                if sentence.strip():
                    final_blocks.append(_restore(sentence + punct))
                if space:
                    final_blocks.append(space)
    return final_blocks

# -----------------------------
# ANALYSIS
# -----------------------------
@torch.inference_mode()
def analyze(text):
    text = text.strip()
    if not text:
        return "—", "—", "<em>Please enter text...</em>", None
        
    word_count = len(text.split())
    if word_count < 300: # Slightly lowered for testing flexibility
        warning_msg = f"⚠️ <b>Insufficient Text:</b> Your input has {word_count} words. Please enter at least 250-300 words for accurate results."
        return "Too Short", "N/A", f"<div style='color: #b80d0d; padding: 20px; border: 1px solid #b80d0d; border-radius: 8px;'>{warning_msg}</div>", None

    try:
        tok, mod = get_model()
    except Exception as e:
        return "ERROR", "0%", f"Failed to load model: {str(e)}", None

    blocks = split_preserving_structure(text)
    pure_sents_indices = [i for i, b in enumerate(blocks) if b.strip() and not b.startswith("\n")]
    pure_sents = [blocks[i] for i in pure_sents_indices]
    
    if not pure_sents:
        return "—", "—", "<em>No sentences detected.</em>", None

    # Sliding Window Generation (Context of 3 sentences)
    windows = []
    for i in range(len(pure_sents)):
        start = max(0, i - 1)
        end = min(len(pure_sents), i + 2)
        windows.append(" ".join(pure_sents[start:end]))

    # --- BATCHED INFERENCE (Prevents OOM) ---
    batch_size = 8
    probs = []
    for i in range(0, len(windows), batch_size):
        batch = windows[i : i + batch_size]
        inputs = tok(batch, return_tensors="pt", padding=True, truncation=True, max_length=512).to(device)
        output = mod(**inputs)
        
        # Check if model is binary classification (2 labels) or regression (1 label)
        if output.logits.shape[1] > 1:
            # Assumes Label 1 is 'AI'
            batch_probs = F.softmax(output.logits, dim=-1)[:, 1].cpu().numpy().tolist()
        else:
            batch_probs = torch.sigmoid(output.logits).cpu().numpy().flatten().tolist()
        probs.extend(batch_probs)

    # Calculation for Final Score
    lengths = [len(s.split()) for s in pure_sents]
    total_words = sum(lengths)
    weighted_avg = sum(p * l for p, l in zip(probs, lengths)) / total_words if total_words > 0 else 0

    # -----------------------------
    # HTML RECONSTRUCTION
    # -----------------------------
    highlighted_html = "<div style='font-family: -apple-system, BlinkMacSystemFont, \"Segoe UI\", Roboto, sans-serif; line-height: 1.8;'>"
    prob_map = {idx: probs[i] for i, idx in enumerate(pure_sents_indices)}
    
    for i, block in enumerate(blocks):
        if block.startswith("\n") or block.isspace():
            highlighted_html += block.replace("\n", "<br>")
            continue
        
        if i in prob_map:
            score = prob_map[i]
            # Color logic based on Threshold
            if score >= THRESHOLD:
                color, bg = "#d32f2f", "rgba(211, 47, 47, 0.12)" # Soft Red
                border = "2px solid #d32f2f"
            else:
                color, bg = "#2e7d32", "rgba(46, 125, 50, 0.08)" # Soft Green
                border = "1px solid transparent"
            
            highlighted_html += (
                f"<span style='background:{bg}; padding:1px 2px; border-radius:3px; border-bottom: {border}; cursor: help;' "
                f"title='AI Confidence: {score:.2%}'>"
                f"<span style='color:{color}; font-weight: bold; font-size: 0.75em; vertical-align: super; margin-right: 2px;'>{score:.0%}</span>"
                f"{block}</span>"
            )
        else:
            highlighted_html += block
    highlighted_html += "</div>"

    label = f"{weighted_avg:.1%} AI Written"
    display_score = f"{weighted_avg:.2%}"
    df = pd.DataFrame({"Sentence": pure_sents, "AI Confidence": [f"{p:.2%}" for p in probs]})
    
    return label, display_score, highlighted_html, df

# -----------------------------
# GRADIO INTERFACE
# -----------------------------
with gr.Blocks(theme=gr.themes.Soft(), title="AI Detector Pro") as demo:
    gr.Markdown("# 🕵️ AI Detector Pro")
    gr.Markdown(f"Utilizing **{MODEL_NAME}**. Values above **{THRESHOLD*100:.0f}%** are flagged as highly likely AI.")
    
    with gr.Row():
        with gr.Column(scale=3):
            text_input = gr.Textbox(label="Input Text", lines=15, placeholder="Paste your essay here (minimum 250 words for accuracy)...")
            with gr.Row():
                clear_btn = gr.Button("Clear")
                run_btn = gr.Button("Analyze Text", variant="primary")
        
        with gr.Column(scale=1):
            verdict_out = gr.Label(label="Global Verdict")
            score_out = gr.Label(label="Weighted Probability")
            gr.Markdown("---")
            gr.Markdown("### How to read:")
            gr.Markdown("- **Red Highlight:** High AI probability\n- **Green Highlight:** Likely Human\n- **Super-script:** Exact sentence-level AI score")
    
    with gr.Tabs():
        with gr.TabItem("Visual Heatmap"):
            html_out = gr.HTML(label="Heatmap")
        with gr.TabItem("Data Breakdown"):
            table_out = gr.Dataframe(headers=["Sentence", "AI Confidence"], wrap=True)
            
    run_btn.click(analyze, inputs=text_input, outputs=[verdict_out, score_out, html_out, table_out])
    clear_btn.click(lambda: ["", "", "", "", None], outputs=[text_input, verdict_out, score_out, html_out, table_out])

if __name__ == "__main__":
    demo.launch()