from datetime import date
from typing import Optional

# 品种主力合约轮换规则。
# 每个品种维护:
#   exchange: tushare 合约后缀(交易所)
#   active: 当月 -> (主力合约月, 年份偏移)
#           例 FG 12 月用次年 5 月,故 12 -> (5, 1)
ROLLOVER_RULES: dict[str, dict] = {
    "FG": {
        "exchange": "ZCE",
        "active": {
            1: (5, 0), 2: (5, 0), 3: (5, 0),
            4: (9, 0), 5: (9, 0), 6: (9, 0), 7: (9, 0),
            8: (1, 1), 9: (1, 1), 10: (1, 1), 11: (1, 1),
            12: (5, 1),
        },
    },
}


def active_contract(symbol: str, today: Optional[date] = None) -> str:
    """按主力轮换规则,返回当日 ts_code(含交易所后缀)。"""
    if symbol not in ROLLOVER_RULES:
        raise ValueError(f"未配置 {symbol} 的主力轮换规则,可在 contracts.ROLLOVER_RULES 中追加")

    today = today or date.today()
    rule = ROLLOVER_RULES[symbol]
    contract_month, year_offset = rule["active"][today.month]
    year = today.year + year_offset
    return f"{symbol}{year % 100:02d}{contract_month:02d}.{rule['exchange']}"