text_detector.py

import math
import re
import numpy as np
import torch
from transformers import (
    AutoTokenizer,
    AutoModelForSequenceClassification,
    AutoModelForCausalLM,
)
from collections import Counter


class AITextDetector:
    """
    AI Text Detector

    - Transformer classifier for AI vs Human
    - Metrics: perplexity, burstiness, repetition, semantic smoothness
    - Returns AI-vs-Human probability + category distribution
    """

    def __init__(self, model_name="roberta-base-openai-detector", device=None):
        # Device setup
        self.device = device or ("cuda" if torch.cuda.is_available() else "cpu")

        # Classifier model & tokenizer
        self.classifier_tokenizer = AutoTokenizer.from_pretrained(model_name)
        self.model = AutoModelForSequenceClassification.from_pretrained(model_name).to(self.device)
        self.model.eval()

        # Language model for perplexity (lighter than full GPT-2 if needed)
        self.lm_tokenizer = AutoTokenizer.from_pretrained("gpt2")
        self.lm_model = AutoModelForCausalLM.from_pretrained("gpt2").to(self.device)
        self.lm_model.eval()

    # ------------------ Metrics ------------------
    def _compute_perplexity(self, text: str, max_length: int = 512):
        """Compute perplexity using GPT-2 LM."""
        encodings = self.lm_tokenizer(
            text,
            return_tensors="pt",
            truncation=True,
            max_length=max_length,
        ).to(self.device)

        with torch.no_grad():
            outputs = self.lm_model(**encodings, labels=encodings.input_ids)
            loss = outputs.loss.item()

        # Clamp to avoid overflow
        return float(min(math.exp(loss), 1e4))

    def _compute_burstiness(self, text: str):
        """Variance of sentence lengths (burstiness)."""
        sentences = [s.strip() for s in re.split(r"[.!?]", text) if s.strip()]
        if len(sentences) < 2:
            return 0.0
        lengths = [len(s.split()) for s in sentences]
        return float(np.var(lengths))

    def _compute_repetition_score(self, text: str):
        """Repetition = proportion of duplicate words."""
        words = [w.lower() for w in re.findall(r"\b\w+\b", text)]
        if not words:
            return 0.0
        counts = Counter(words)
        repeated = sum(c - 1 for c in counts.values() if c > 1)
        return repeated / len(words)

    def _compute_semantic_smoothness(self, text: str):
        """
        Semantic smoothness = avg cosine similarity between consecutive sentence embeddings.
        Uses last hidden states instead of raw embeddings.
        """
        sentences = [s.strip() for s in re.split(r"[.!?]", text) if s.strip()]
        if len(sentences) < 2:
            return 1.0

        embeddings = []
        for s in sentences:
            encodings = self.classifier_tokenizer(
                s,
                return_tensors="pt",
                truncation=True,
                padding=True,
                max_length=128,
            ).to(self.device)

            with torch.no_grad():
                outputs = self.model(
                    **encodings,
                    output_hidden_states=True,
                )
                hidden_states = outputs.hidden_states[-1]  # last layer
                sent_emb = hidden_states.mean(dim=1).cpu().numpy()
            embeddings.append(sent_emb)

        similarities = []
        for i in range(len(embeddings) - 1):
            a, b = embeddings[i], embeddings[i + 1]
            num = float(np.dot(a, b.T))
            denom = np.linalg.norm(a) * np.linalg.norm(b)
            if denom > 0:
                similarities.append(num / denom)
        return float(np.mean(similarities)) if similarities else 1.0

    # ------------------ Main detection ------------------
    def detect(self, text: str):
        """Run detection pipeline and return results."""
        # Empty text case
        if not text.strip():
            return {
                "ai_probability": 0.0,
                "metrics": {},
                "distribution": {},
                "final_label": "empty",
            }

        # Classifier prediction
        inputs = self.classifier_tokenizer(
            text,
            return_tensors="pt",
            truncation=True,
            padding=True,
            max_length=512,
        ).to(self.device)

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

        human_prob, ai_prob = float(probs[0]), float(probs[1])

        # Extra metrics
        perplexity = self._compute_perplexity(text)
        burstiness = self._compute_burstiness(text)
        repetition = self._compute_repetition_score(text)
        smoothness = self._compute_semantic_smoothness(text)

        # Normalize distribution
        distribution = {
            "Human-written": round(human_prob * 100, 2),
            "AI-generated": round(ai_prob * 100 * (1 - repetition), 2),
            "AI-generated & AI-refined": round(ai_prob * 100 * repetition, 2),
            "Mixed": round(ai_prob * 100 * (1 - smoothness), 2),
        }
        total = sum(distribution.values())
        if total > 0:
            for k in distribution:
                distribution[k] = round(distribution[k] / total * 100, 2)

        # Final label
        final_label = max(distribution, key=distribution.get)

        return {
            "summary": f"{distribution['AI-generated']}% of text is likely AI",
            "overall_ai_probability": overall_ai_probability,
            "category_distribution": distribution,
            "metrics": {
                "perplexity": round(perplexity, 2),
                "burstiness": round(burstiness, 3),
                "repetition_score": round(repetition, 3),
                "semantic_smoothness": round(smoothness, 3),
                "ai_probability": overall_ai_probability,
            },
            "interpretation": (
                "This detector uses structural patterns (perplexity, burstiness, repetition, semantic smoothness) "
                "to estimate the likelihood of AI authorship. Results are probabilistic, not definitive. "
                "Always apply judgment."
            ),
            "label": "AI-generated" if overall_ai_probability > 0.5 else "Human-written"
        }