export type SalaryShiftSettings = {
  enabled: boolean;
  categoryNames: string[];
  dayThreshold: number;
};

export function bookingMonth(bookingDate?: string): string | undefined {
  if (!bookingDate) return undefined;
  return bookingDate.slice(0, 7);
}

export function computeEffectiveMonth(
  bookingDate: string | undefined,
  assignedMonth: string | undefined,
): string | undefined {
  if (assignedMonth) return assignedMonth;
  return bookingMonth(bookingDate);
}

export function addMonthsToMonthKey(monthKey: string, months: number): string {
  const [yearStr, monthStr] = monthKey.split("-");
  const date = new Date(Number(yearStr), Number(monthStr) - 1 + months, 1);
  const y = date.getFullYear();
  const m = String(date.getMonth() + 1).padStart(2, "0");
  return `${y}-${m}`;
}

export function getDayFromDate(dateStr: string): number {
  return Number(dateStr.slice(8, 10));
}

export function applySalaryShiftRule(
  bookingDate: string | undefined,
  amount: number,
  categoryName: string | undefined,
  salaryShift: SalaryShiftSettings,
  existingAssignedMonth?: string,
): string | undefined {
  if (existingAssignedMonth) return existingAssignedMonth;
  if (!salaryShift.enabled || !bookingDate || amount <= 0 || !categoryName) {
    return undefined;
  }
  if (!salaryShift.categoryNames.includes(categoryName)) {
    return undefined;
  }
  const day = getDayFromDate(bookingDate);
  if (day < salaryShift.dayThreshold) {
    return undefined;
  }
  const booking = bookingMonth(bookingDate);
  if (!booking) return undefined;
  return addMonthsToMonthKey(booking, 1);
}

export function resolveAssignedAndEffective(
  bookingDate: string | undefined,
  amount: number,
  categoryName: string | undefined,
  salaryShift: SalaryShiftSettings,
  manualAssignedMonth?: string,
): { assignedMonth?: string; effectiveMonth?: string } {
  const assignedMonth =
    manualAssignedMonth ??
    applySalaryShiftRule(bookingDate, amount, categoryName, salaryShift);
  const effectiveMonth = computeEffectiveMonth(bookingDate, assignedMonth);
  return { assignedMonth, effectiveMonth };
}

export function monthKeyFromBasis(
  tx: { bookingDate?: string; effectiveMonth?: string },
  basis: "effective" | "booking",
): string | undefined {
  if (basis === "effective") {
    return tx.effectiveMonth ?? bookingMonth(tx.bookingDate);
  }
  return bookingMonth(tx.bookingDate);
}