#include <iostream>
#include <iomanip>
#include <ctime>
#include <string>
#include <sstream>
#include <cstdlib>

// 判断闰年
bool isLeapYear(int year) {
    return (year % 4 == 0 && year % 100 != 0) || (year % 400 == 0);
}

// 获取某个月的天数
int daysInMonth(int year, int month) {
    if (month == 2) {
        return isLeapYear(year) ? 29 : 28;
    }
    if (month == 4 || month == 6 || month == 9 || month == 11) {
        return 30;
    }
    return 31;
}

// 获取某年某月1号是星期几 (0 = Sunday, 1 = Monday, ..., 6 = Saturday)
int getFirstDayOfWeek(int year, int month) {
    std::tm timeInfo = {0};
    timeInfo.tm_year = year - 1900;
    timeInfo.tm_mon = month - 1;
    timeInfo.tm_mday = 1;
    std::mktime(&timeInfo);
    return timeInfo.tm_wday; // 0=周日, 6=周六
}

// 打印某年某月的日历
void printCalendar(int year, int month) {
    // 月份名称数组
    const char* monthNames[] = {
        "January", "February", "March", "April", "May", "June",
        "July", "August", "September", "October", "November", "December"
    };

    // 获取当月天数及第一天星期几
    int days = daysInMonth(year, month);
    int startWeekday = getFirstDayOfWeek(year, month); // 0=周日

    // 打印标题（居中显示，总宽度约20个字符）
    std::cout << "\n     " << monthNames[month - 1] << " " << year << "\n";
    std::cout << "Su Mo Tu We Th Fr Sa\n";

    // 打印日历网格（6行×7列 = 42格，保证完整）
    int dayCounter = 1;
    for (int i = 0; i < 42; ++i) {
        if (i < startWeekday) {
            // 月初前的空白格子
            std::cout << "   ";
        } else if (dayCounter <= days) {
            // 正常日期：右对齐占2位，后跟一个空格
            std::cout << std::setw(2) << dayCounter << " ";
            ++dayCounter;
        } else {
            // 月末后的空白格子
            std::cout << "   ";
        }

        // 每7个格子换行
        if (i % 7 == 6) {
            std::cout << "\n";
        }
    }
    std::cout << std::endl;
}

// 从字符串解析 "yyyy-mm-dd" 格式，成功返回true，并将year, month, day填入
bool parseDate(const std::string& dateStr, int& year, int& month, int& day) {
    std::istringstream iss(dateStr);
    char dash1, dash2;
    if (iss >> year >> dash1 >> month >> dash2 >> day && dash1 == '-' && dash2 == '-') {
        // 基础范围检查
        if (year >= 1 && month >= 1 && month <= 12 && day >= 1 && day <= daysInMonth(year, month)) {
            return true;
        }
    }
    return false;
}

// 获取当前系统时间的年份和月份
void getCurrentYearMonth(int& year, int& month) {
    std::time_t t = std::time(nullptr);
    std::tm* now = std::localtime(&t);
    year = now->tm_year + 1900;
    month = now->tm_mon + 1;
}

int main(int argc, char* argv[]) {
    int year, month;

    if (argc == 2) {
        // 提供了日期参数，尝试解析
        std::string dateArg = argv[1];
        int day; // 解析出的日无需使用，仅用于校验
        if (parseDate(dateArg, year, month, day)) {
            printCalendar(year, month);
        } else {
            std::cerr << "错误：无效的日期格式或非法日期。请使用 yyyy-mm-dd 格式。\n";
            std::cerr << "示例: " << argv[0] << " 2024-12-25\n";
            return 1;
        }
    } else if (argc == 1) {
        // 无参数，显示当前月份的日历
        getCurrentYearMonth(year, month);
        printCalendar(year, month);
    } else {
        std::cerr << "用法: " << argv[0] << " [yyyy-mm-dd]\n";
        std::cerr << "若无参数，则显示当前月份的日历。\n";
        return 1;
    }

    return 0;
}