// 示例代码：如何实现会员有效期叠加逻辑
function updateMembership(user, newGroup, newDuration) {
    if (user.currentGroup !== newGroup) {
        switch (siteSettings.membershipHandling) {
            case 'noOverlap':
                return '无法叠加有效期';
            case 'overwrite':
                user.expiryDate = calculateExpiryDate(newDuration);
                break;
            case 'stackable':
                user.expiryDate = extendExpiryDate(user.expiryDate, newDuration);
                break;
        }
    }
    return '会员有效期已成功更新';
}

function calculateExpiryDate(duration) {
    return new Date(Date.now() + duration * 24 * 60 * 60 * 1000);
}

function extendExpiryDate(currentExpiry, additionalDuration) {
    return new Date(currentExpiry.getTime() + additionalDuration * 24 * 60 * 60 * 1000);
}