app.py

import gradio as gr
import torch
from transformers import AutoTokenizer, AutoModelForSequenceClassification

device = "cuda" if torch.cuda.is_available() else "cpu"

# ======================
# Load MODELS
# ======================

ROBERTA_MODEL = "roberta-base-openai-detector"
DISTIL_MODEL = "distilroberta-base"

# RoBERTa (AI Detector)
roberta_tokenizer = AutoTokenizer.from_pretrained(ROBERTA_MODEL)
roberta_model = AutoModelForSequenceClassification.from_pretrained(ROBERTA_MODEL).to(device)
roberta_model.eval()

# DistilRoBERTa (Auxiliary signal)
distil_tokenizer = AutoTokenizer.from_pretrained(DISTIL_MODEL)
distil_model = AutoModelForSequenceClassification.from_pretrained(
    DISTIL_MODEL,
    num_labels=2
).to(device)
distil_model.eval()


# ======================
# Prediction function
# ======================

def get_probs(tokenizer, model, text):
    inputs = tokenizer(
        text,
        return_tensors="pt",
        truncation=True,
        padding=True,
        max_length=512
    ).to(device)

    with torch.no_grad():
        outputs = model(**inputs)
        probs = torch.softmax(outputs.logits, dim=1)[0]

    return probs.cpu()


def detect_text(text):
    if not text.strip():
        return "Please enter some text.", None

    # Individual model predictions
    roberta_probs = get_probs(roberta_tokenizer, roberta_model, text)
    distil_probs = get_probs(distil_tokenizer, distil_model, text)

    # 🔥 ENSEMBLE (Soft Voting)
    ensemble_probs = (roberta_probs + distil_probs) / 2

    human_prob = ensemble_probs[0].item()
    ai_prob = ensemble_probs[1].item()

    if ai_prob > human_prob:
        label = "🤖 **AI Generated**"
        confidence = ai_prob
    else:
        label = "🧑 **Human Written**"
        confidence = human_prob

    message = f"{label}\n\nConfidence: **{confidence*100:.2f}%**"

    return message, {
        "Human": round(human_prob, 4),
        "AI": round(ai_prob, 4)
    }


# ======================
# Gradio UI
# ======================

demo = gr.Interface(
    fn=detect_text,
    inputs=gr.Textbox(lines=8, placeholder="Paste your text here..."),
    outputs=[
        gr.Markdown(label="Result"),
        gr.Label(label="Probabilities")
    ],
    title="AI Text Detector (Ensemble Model)",
    description="Ensemble of RoBERTa + DistilRoBERTa using soft voting."
)

demo.launch()