from datetime import datetime

import asyncpg

DEFAULT_TZ = "UTC"
MAX_DAYS = 365

_timezones: set[str] = set()


async def known_timezone(pool: asyncpg.Pool, tz: str) -> bool:
    if not _timezones:
        rows = await pool.fetch("SELECT name FROM pg_timezone_names")
        _timezones.update(row["name"] for row in rows)
    return tz in _timezones


async def period_bounds(
    pool: asyncpg.Pool, days: int, tz: str
) -> tuple[datetime, datetime]:
    row = await pool.fetchrow(
        "SELECT (date_trunc('day', now() AT TIME ZONE $1) "
        "- make_interval(days => $2 - 1)) AT TIME ZONE $1 AS since, now() AS until",
        tz,
        days,
    )
    return row["since"], row["until"]