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import dataclasses
from datetime import datetime
from decimal import Decimal
from typing import Generator
@dataclasses.dataclass(kw_only=True, frozen=True)
class Flow:
"""Time-discrete flow of money paid on the first day of a month"""
amount: Decimal
def integrate(self, start: datetime, end: datetime) -> Decimal:
"""Integrate the flow between two dates to an amount of money"""
payments: int = 0
for candidate in monthly_candidates(start):
if start <= candidate <= end:
payments += 1
if candidate > end:
break
return self.amount * Decimal(payments)
def monthly_candidates(start: datetime) -> Generator[datetime, None, None]:
current = datetime(start.year, start.month, 1)
while True:
yield current
if current.month == 12:
current = datetime(current.year + 1, 1, 1)
else:
current = datetime(current.year, current.month + 1, 1)
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