from datetime import datetime
from decimal import Decimal
from typing import Generator

from finance.model import Flow


def integrate(flow: Flow, start: datetime, end: datetime) -> Decimal:
    """Integrate the flow between two dates to an amount of money"""

    payments: int = 0

    if flow.since is not None:
        if start < flow.since:
            start = flow.since

    if flow.until is not None:
        if end > flow.until:
            end = flow.until

    for candidate in monthly_candidates(start):
        if start <= candidate <= end:
            payments += 1
        if candidate > end:
            break

    return flow.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)


def simulate(
    start: datetime, end: datetime, flows: tuple[Flow, ...]
) -> list[tuple[datetime, Decimal]]:
    dates: list[datetime] = []
    values: list[Decimal] = []

    for candidate in monthly_candidates(start):
        if start <= candidate <= end:
            dates.append(candidate)

        if candidate > end:
            break

    for date in dates:
        value = Decimal(0.0)
        for flow in flows:
            value += integrate(flow, start, date)
        values.append(value)

    return [(date, values[index]) for index, date in enumerate(dates)]