from __future__ import annotations

import functools
from typing import Literal

from pydantic import Field
from pydantic_settings import BaseSettings, SettingsConfigDict


class Settings(BaseSettings):
    """Application settings loaded from environment variables.

    Required variables must be present at startup.
    Optional variables have sensible defaults.
    No .env files are used — all values come from os.environ.
    """

    model_config = SettingsConfigDict(
        env_file="",
        env_file_encoding="utf-8",
        case_sensitive=True,
        extra="ignore",
    )

    BOT_TOKEN: str | None = None
    OPENAI_API_KEY: str | None = None
    ADMIN_ID: str | None = None

    ADMIN_USERNAME: str = ""
    PORT: int = Field(default=7860, ge=1, le=65535)
    SUPPORT_API_KEY: str = ""
    YOOKASSA_SECRET_KEY: str = ""
    YOOKASSA_SHOP_ID: str = ""
    DATABASE_URL: str = ""
    TELEGRAM_API_SERVER: str = "https://api.telegram.org"
    WEBHOOK_URL: str = ""
    DEFAULT_PROVIDER: Literal["openai", "claude", "gemini", "glm", "qwen"] = "openai"
    LOG_LEVEL: Literal["DEBUG", "INFO", "WARNING", "ERROR", "CRITICAL"] = "INFO"
    LOG_FORMAT: Literal["json", "console"] = "json"

    @property
    def required_fields(self) -> dict[str, str | None]:
        """Return mapping of required field names to their current values."""
        return {
            "BOT_TOKEN": self.BOT_TOKEN,
            "OPENAI_API_KEY": self.OPENAI_API_KEY,
            "ADMIN_ID": self.ADMIN_ID,
        }


@functools.lru_cache(maxsize=1)
def get_settings() -> Settings:
    """Return the cached Settings singleton.

    Lazy initialization — Settings() is created only on the first call,
    not at import time. This avoids import-time side effects and allows
    test fixtures to override environment variables before the first call.

    To reset the cache in tests, call:
        get_settings.cache_clear()
    """
    return Settings()