"""Backend interface used by compactor.py."""
from __future__ import annotations
from typing import Protocol


class CompressionBackend(Protocol):
    def summarise(self, system_prompt: str, conversation_text: str) -> str: ...


def build_backend(cfg: dict) -> CompressionBackend:
    backend_cfg = cfg.get("backend") or {}
    btype = backend_cfg.get("type", "openai_compat")
    if btype == "openai_compat":
        from .openai_compat import OpenAICompatibleBackend
        return OpenAICompatibleBackend(backend_cfg)
    if btype == "anthropic_native":
        from .anthropic_native import AnthropicNativeBackend
        return AnthropicNativeBackend(backend_cfg)
    raise ValueError(f"Unsupported backend.type: {btype!r}")