使用按位运算从int日期中提取月份(yyyyMMdd) [英] using bitwise operation to extract month from int date (yyyyMMdd)

问题描述

是否可以使用按位运算符从以 int (格式为YYYYMMDD，例如20110401)表示的日期中提取月份?

Is it possible to extract the month from date represented as int (format YYYYMMDD, e.g. 20110401) using some bitwise operators?

如果是这样，怎么办?

我目前正在使用20110401％10000/100.我认为按位操作可能会更快.DateTime.Parse等对于我想做的事情来说太慢了.

edit: I am currently using 20110401 % 10000 / 100. I thought bit-wise could be faster. DateTime.Parse etc. are too slow for what I am trying to do.

推荐答案

如果您以二进制格式表示日期，则可以按位操作有效地提取月份，例如，每天5位月份中的4位，月份号中的4位，年份中的其余部分，而不是十进制数字.对于您的示例，日期将为(2011<< 9)+(4<< 5)+1(当然不等于20110401).要使用按位运算从此类表示中提取字段，请执行以下操作:

You could efficiently extract the month with bitwise operations if you represented the date in a binary format, e.g., 5 bits for the day of the month, 4 bits for the month number, and the rest for the year, rather than as decimal digits. For your example, the date would be (2011 << 9) + (4 << 5) + 1 (which of course is not equal to 20110401). To use bitwise operations to extract the fields from such a representation:

int year = date >> 9;
int month = (date >> 5) & 0xF;
int day = date & 0x1F;

马克·拜尔斯(Mark Byers)提到的另一种方法是使用结构，例如，

Another approach, as mentioned by Mark Byers, is to use a struct, e.g.,

typedef struct {
    short year;
    char  month;
    char  day;
} Date;

您可以将它们传递到堆栈上，按名称提取字段，并将其初始化为

You can pass these around on the stack, extract the fields by name, and initialize them as

Date d = { 2011, 4, 1};

或者在C99中，

Date d = { .year = 2011, .month = 4, .day = 1 };

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