import contextlib
import datetime
import sqlite3

class Cache:
    def __init__(self, db):
        self.db = db

    def get(self, key, ttl, f):
        now = datetime.datetime.now(datetime.timezone.utc)
        earliest_acceptable = now - ttl
        match self.db.execute(
            'SELECT value FROM cache WHERE key = ? AND timestamp >= ?',
            (key, earliest_acceptable),
        ).fetchall():
            case [(value,)]:
                return value

        value = f()
        self.db.execute(
            'INSERT INTO cache (key, timestamp, value) VALUES (?, ?, ?) ' +
            'ON CONFLICT(key) DO UPDATE SET timestamp = excluded.timestamp, value = excluded.value',
            (key, now, value),
        )
        return value

@contextlib.contextmanager
def open():
    with sqlite3.connect('cache.sqlite', isolation_level=None) as db:
        db.execute('CREATE TABLE IF NOT EXISTS cache (key TEXT PRIMARY KEY, timestamp TEXT NOT NULL, value TEXT NOT NULL)')
        yield Cache(db)