"""统一响应体 {code, message, data, request_id} + 游标分页。"""

from __future__ import annotations

import base64
import binascii
import json
from datetime import datetime
from typing import Any, Generic, TypeVar
from uuid import UUID

from pydantic import BaseModel, Field

from .errors import AppError, ErrorCode
from .logging_setup import request_id_ctx

T = TypeVar("T")

DEFAULT_LIMIT = 20
MAX_LIMIT = 100


class Envelope(BaseModel, Generic[T]):
    code: int = 0
    message: str = "ok"
    data: T | None = None
    request_id: str = "-"


class Page(BaseModel, Generic[T]):
    items: list[T]
    next_cursor: str | None = None
    has_more: bool = False


class Cursor(BaseModel):
    """(created_at, id) 复合游标：单用 created_at 会在同毫秒记录上漏数据。"""

    created_at: datetime
    id: UUID


def ok(data: Any = None, message: str = "ok") -> dict[str, Any]:
    return {
        "code": int(ErrorCode.OK),
        "message": message,
        "data": data,
        "request_id": request_id_ctx.get(),
    }


def encode_cursor(created_at: datetime, row_id: UUID) -> str:
    raw = json.dumps({"t": created_at.isoformat(), "i": str(row_id)}, separators=(",", ":"))
    return base64.urlsafe_b64encode(raw.encode()).decode().rstrip("=")


def decode_cursor(cursor: str) -> Cursor:
    try:
        padded = cursor + "=" * (-len(cursor) % 4)
        payload = json.loads(base64.urlsafe_b64decode(padded))
        return Cursor(created_at=datetime.fromisoformat(payload["t"]), id=UUID(payload["i"]))
    except (binascii.Error, KeyError, ValueError, json.JSONDecodeError) as exc:
        raise AppError(ErrorCode.BAD_REQUEST, "分页游标不合法") from exc


def clamp_limit(limit: int) -> int:
    if limit <= 0:
        return DEFAULT_LIMIT
    return min(limit, MAX_LIMIT)


class Pagination(BaseModel):
    limit: int = Field(default=DEFAULT_LIMIT, ge=1, le=MAX_LIMIT)
    cursor: str | None = None

    def parsed(self) -> tuple[int, Cursor | None]:
        return clamp_limit(self.limit), (decode_cursor(self.cursor) if self.cursor else None)