"""Abstract base classes for surrogate models."""

from abc import ABC, abstractmethod
from typing import Optional, Tuple

import torch
from torch import Tensor


class SurrogateModel(ABC):
    """Abstract base class for all surrogate models in the platform.

    A surrogate model provides predictions (mean + uncertainty) and can be
    updated with new observations.
    """

    @abstractmethod
    def predict(self, X: Tensor) -> Tuple[Tensor, Tensor]:
        """Return posterior mean and variance at input locations X.

        Args:
            X: Input tensor of shape (n, d).

        Returns:
            mean: Predicted mean of shape (n, 1).
            variance: Predicted variance of shape (n, 1).
        """

    @abstractmethod
    def fit(self, X: Tensor, y: Tensor) -> None:
        """Fit/update the surrogate model with observed data.

        Args:
            X: Training inputs of shape (n, d).
            y: Training targets of shape (n, 1).
        """

    @abstractmethod
    def posterior(self, X: Tensor):
        """Return the full posterior distribution at X (for BoTorch compatibility).

        Args:
            X: Input tensor of shape (batch, n, d).
        """

    def condition_on_observations(self, X: Tensor, y: Tensor) -> "SurrogateModel":
        """Return a new model conditioned on additional observations.

        Default implementation refits the model. Subclasses can override
        for fantasy-based conditioning.
        """
        raise NotImplementedError(
            "Fantasy conditioning not implemented for this model. Use fit() instead."
        )