import dataclasses
from datetime import datetime
from decimal import Decimal


@dataclasses.dataclass(kw_only=True, frozen=True)
class Income:
    """Income models financial income paid on the first day of a month"""

    amount: Decimal

    def integrate(self, start: datetime, end: datetime) -> Decimal:
        """Integrate the income from a start to an end date"""

        retval = Decimal(0.0)

        current = datetime(start.year, start.month, 1)

        if start == current:
            retval += self.amount

        while True:
            if current.month == 12:
                current = datetime(current.year + 1, 1, 1)
            else:
                current = datetime(current.year, current.month + 1, 1)

            if current <= end:
                retval += self.amount
            else:
                break

        return retval