"""会话: 内存中的消息列表 + meta(cwd / model / created_at) + 落盘 json。

文件格式:
{
  "meta": {"id": "...", "created_at": "...", "cwd": "...", "model": "..."},
  "messages": [...]
}

兼容老格式: 如果文件根是 list,就当 messages 处理,meta 为空。
"""
from __future__ import annotations

import json
import os
from pathlib import Path
from typing import Any, Dict, List, Optional


def _to_dict(msg: Any) -> Any:
    if isinstance(msg, dict):
        return msg
    if hasattr(msg, "model_dump"):
        return msg.model_dump(exclude_none=True)
    if hasattr(msg, "dict"):
        return msg.dict(exclude_none=True)
    return msg


def atomic_write_text(path: Path, text: str, encoding: str = "utf-8") -> None:
    """原子写: 先写到 path.tmp 再 os.replace 到 path。

    防止写中途异常(磁盘满 / surrogate 编码错 / 进程被杀)留下 0 字节或半文件。
    单 REPL 单 task 假设下 .tmp 名固定;若上次写崩留下孤儿,本次写会覆盖它。
    `_cleanup_if_empty` 已配合放过 `*.tmp` 文件。
    """
    path.parent.mkdir(parents=True, exist_ok=True)
    tmp = path.with_suffix(path.suffix + ".tmp")
    with open(tmp, "w", encoding=encoding, newline="\n") as f:
        f.write(text)
        f.flush()
        os.fsync(f.fileno())
    os.replace(tmp, path)


class Session:
    def __init__(
        self,
        system_prompt: str = "",
        path: Optional[Path] = None,
        meta: Optional[dict] = None,
    ) -> None:
        self.messages: List[dict] = []
        self.path = path
        self.meta: Dict[str, Any] = dict(meta or {})
        if system_prompt:
            self.messages.append({"role": "system", "content": system_prompt})

    def append(self, msg: Any) -> None:
        self.messages.append(_to_dict(msg))
        if self.path is not None:
            self.save()

    def reset(self, keep_system: bool = True) -> None:
        if keep_system and self.messages and self.messages[0].get("role") == "system":
            self.messages = [self.messages[0]]
        else:
            self.messages = []
        if self.path is not None:
            self.save()

    def save(self) -> None:
        if self.path is None:
            return
        payload = {"meta": self.meta, "messages": self.messages}
        atomic_write_text(
            self.path,
            json.dumps(payload, ensure_ascii=False, indent=2),
        )

    @classmethod
    def load(cls, path: Path) -> "Session":
        s = cls(path=path)
        if not path.exists():
            return s
        data = json.loads(path.read_text(encoding="utf-8"))
        if isinstance(data, list):
            # 老格式: 纯消息列表
            s.messages = data
            s.meta = {}
        elif isinstance(data, dict):
            s.messages = data.get("messages", []) or []
            s.meta = data.get("meta", {}) or {}
        return s