"""Exchange-difference calculation helper.

Pure-Python FX gain/loss computation. The engine uses this for rapid
pre-checks; Odoo's account.move._create_exchange_difference_move() is
invoked separately for the actual GL posting.
"""

from dataclasses import dataclass


@dataclass
class ExchangeDiffResult:
    needs_diff_move: bool
    diff_amount: float           # in company currency; positive = gain, negative = loss
    line_company_amount: float
    against_company_amount: float


def compute_exchange_diff(*, line_amount, line_currency_code, against_amount,
                            against_currency_code, line_rate, against_rate) -> ExchangeDiffResult:
    """Compute whether an exchange-diff move is needed and its magnitude.

    Args:
        line_amount: Bank line amount in its currency
        line_currency_code: e.g. 'USD'
        against_amount: Matched journal item amount in its currency
        against_currency_code: e.g. 'USD' (or different)
        line_rate: FX rate (foreign per company currency) at line date
        against_rate: FX rate at journal item posting date

    Returns:
        ExchangeDiffResult with needs_diff_move flag and computed diff
        in company currency (positive = gain, negative = loss).
    """
    line_company = line_amount * line_rate
    against_company = against_amount * against_rate

    diff = line_company - against_company
    needs_diff = abs(diff) > 0.005  # rounding tolerance

    return ExchangeDiffResult(
        needs_diff_move=needs_diff,
        diff_amount=round(diff, 2),
        line_company_amount=round(line_company, 2),
        against_company_amount=round(against_company, 2),
    )