function addMonthsToDate(dateString, monthsToAdd) {
    const date = new Date(dateString);
    date.setMonth(date.getMonth() + monthsToAdd);
    date.setDate(date.getDate() - 1);
    return formatDate(date);
}

function dateFormatter(dateString) {
    if (!dateString || !/^\d{4}-\d{2}-\d{2}$/.test(dateString)) return "날짜 형식 오류";
    const [year, month, day] = dateString.split('-');
    return `${year}년 ${month}월 ${day}일`;
}

function calculateDaysFromNow(dateString, now = new Date()) {
    const today = new Date(now.getFullYear(), now.getMonth(), now.getDate());
    today.setHours(0,0,0,0);
    const inputDate = new Date(dateString);
    inputDate.setHours(0,0,0,0);
    return Math.ceil((inputDate.getTime() - today.getTime()) / 86400000);
}

function calculateDateDifference(date1Str, date2Str) {
    const d1 = new Date(date1Str);
    d1.setHours(0, 0, 0, 0);
    const d2 = new Date(date2Str);
    d2.setHours(0, 0, 0, 0);
    return Math.ceil(Math.abs(d2.getTime() - d1.getTime()) / 86400000);
}

function calculateProgress(startDateStr, endDateStr, now = new Date()) {
    const start = new Date(startDateStr);
    start.setHours(0, 0, 0, 0);
    const end = new Date(endDateStr);
    end.setHours(0, 0, 0, 0);
    const currentTime = now.getTime();
    if (start.getTime() >= end.getTime() || currentTime >= end.getTime()) return '100.0000000';
    if (currentTime <= start.getTime()) return 0;
    return Math.max(0, Math.min(100, ((currentTime - start.getTime()) / (end.getTime() - start.getTime())) * 100));
}

function calculateMonthDifference(sYear, sMonth, eYear, eMonth) {
    return (Number(eYear) - Number(sYear)) * 12 + (Number(eMonth) - Number(sMonth));
}

function formatDate(date) {
    const year = date.getFullYear();
    const month = String(date.getMonth() + 1).padStart(2, '0');
    const day = String(date.getDate()).padStart(2, '0');
    return `${year}-${month}-${day}`;
}

module.exports = {
    addMonthsToDate,
    dateFormatter,
    calculateDaysFromNow,
    calculateDateDifference,
    calculateProgress,
    calculateMonthDifference,
    formatDate
};