from typing import Optional
from transformers import AutoTokenizer, AutoModelForSeq2SeqLM
import torch
import difflib


class GrammarAgent:
    def __init__(
        self,
        model_name: str = "vennify/t5-base-grammar-correction",
        device: Optional[str] = None,
    ):
        """
        Grammar Agent
        - model_name: HF model id
        - device: "cuda" or "cpu" (auto-detect if None)
        """
        self.model_name = model_name
        self.tokenizer = AutoTokenizer.from_pretrained(model_name)
        self.model = AutoModelForSeq2SeqLM.from_pretrained(model_name)

        if device is None:
            self.device = "cuda" if torch.cuda.is_available() else "cpu"
        else:
            self.device = device

        self.model.to(self.device)

    def _generate(self, text: str, max_length: int = 256, num_beams: int = 5) -> str:
        """
        Internal helper to call the model and get corrected text.
        """
        prefixed = "grammar: " + text  # T5-style task prefix

        inputs = self.tokenizer(
            prefixed,
            return_tensors="pt",
            truncation=True,
            max_length=max_length,
        ).to(self.device)

        with torch.no_grad():
            outputs = self.model.generate(
                **inputs,
                max_length=max_length,
                num_beams=num_beams,
                early_stopping=True,
            )

        corrected = self.tokenizer.decode(outputs[0], skip_special_tokens=True)
        return corrected.strip()

    def _diff_explanation(self, original: str, corrected: str):
        """
        Create a simple, human-readable explanation of changes.
        Returns a list of {type, from, to}.
        """
        diff = list(difflib.ndiff(original.split(), corrected.split()))
        changes = []
        current_del = []
        current_add = []

        for token in diff:
            if token.startswith("- "):
                current_del.append(token[2:])
            elif token.startswith("+ "):
                current_add.append(token[2:])
            elif token.startswith("  "):
                if current_del or current_add:
                    changes.append(
                        {
                            "from": " ".join(current_del) if current_del else None,
                            "to": " ".join(current_add) if current_add else None,
                            "type": self._infer_change_type(current_del, current_add),
                        }
                    )
                    current_del, current_add = [], []

        if current_del or current_add:
            changes.append(
                {
                    "from": " ".join(current_del) if current_del else None,
                    "to": " ".join(current_add) if current_add else None,
                    "type": self._infer_change_type(current_del, current_add),
                }
            )

        changes = [c for c in changes if c["from"] or c["to"]]
        return changes

    @staticmethod
    def _infer_change_type(deleted_tokens, added_tokens):
        """
        Very simple heuristic for change type.
        You can later improve this with more logic.
        """
        if deleted_tokens and not added_tokens:
            return "deletion"
        if added_tokens and not deleted_tokens:
            return "insertion"
        return "replacement"

    def correct(self, text: str) -> dict:
        """
        Main method your system will call.

        Returns a dict:
        {
            "original": ...,
            "corrected": ...,
            "changes": [ {type, from, to}, ... ],
            "confidence": float,
            "agent": "grammar"
        }
        """
        corrected = self._generate(text)
        changes = self._diff_explanation(text, corrected)

        # simple heuristic confidence based on how much was changed
        change_ratio = len(changes) / max(len(text.split()), 1)
        confidence = max(0.3, 1.0 - change_ratio)

        return {
            "original": text,
            "corrected": corrected,
            "changes": changes,
            "confidence": round(confidence, 2),
            "agent": "grammar",
        }