algs4: 1.2.12 使用 蔡勒公式 计算今天星期几

SmartDate.java

package life.yanfeng.study.algorithm;

import edu.princeton.cs.algs4.StdOut;

public class SmartDate {
    private final int month;
    private final int day;
    private final int year;
    public SmartDate(int m, int d, int y) throws RuntimeException
    {
        // 校验规则, 基于 P56 页首描述的简易规则.
        if (d < 0 || d > 31) {
            throw new RuntimeException("illegal day");
        }

        if (m< 0 || m > 15) {
            throw new RuntimeException("illegal month");
        }

        if (y <= 0) {
            throw new RuntimeException("illegal year");
        }

        month = m;
        day = d;
        year = y;
    }

    public int month() {
        return month;
    }

    public int day() {
        return day;
    }

    public int year() {
        return year;
    }

    public String toString() {
        return month() + "/" + day() + "/" + year();
    }

    public String dayOfTheWeek() {
//        // "简单" 估算.
//        int days = month * 31 + year * 365 + day;
//        switch (days % 7) {
//            case 1:
//                return "Monday";
//            case 2:
//                return "Tuesday";
//            case 3:
//                return "Wednesday";
//            case 4:
//                return "Thursday";
//            case 5:
//                return "Friday";
//            case 6:
//                return "Saturday";
//            case 7:
//                return "Sunday";
//        }
//
//        return "";
        // ref: https://zhangjia.io/927.html  https://zh.wikipedia.org/wiki/%E8%94%A1%E5%8B%92%E5%85%AC%E5%BC%8F
        String[] week = {"星期日", "星期一", "星期二", "星期三", "星期四", "星期五", "星期六"};
        int y = year;
        int m = month;
        int d = day;
        //如果月份是1月份或者2月份，则看成去一年的13月份和14月份
        if (month == 1 || month == 2) {
            m += 12;
            y--;
        }
        int c = y / 100; //获取年份后两位
        y = y % 100;   //获取年份后两位
        int w = (y + (y / 4) + (c / 4) - (2 * c) + ((26 * (m + 1)) / 10) + d - 1) % 7;
        w = (w + 7) % 7;
        return year + "/" + month + "/" + day +": " +  week[w];
    }

    public static void main(String[] args) {
        SmartDate date = new SmartDate(10, 15, 2021);
        StdOut.println(date.dayOfTheWeek());
    }
}

开卷有益.
一直是 借助于 系统方法来判定. 没想到 已经有前辈 总结好 直接计算的公式了.