通过右移>>和左移<<按位运算进行日期编码

Question

I am trying to solve a C programming problem, which is date encoding in 2 bytes using bit shifting operations and decoding back to date, month and year. While I could also use bitwise & and | operation, I want to use just bit shifting.

The issue is: even though everything seems to be right the output of date and month has no effect of dual bit shifting operation. The year is okay because there is only one operation. Meaning that the dual operation of bit shift is somehow not behaving as required.

I have used unsigned char so last sign bit is definitely not the issue. The Showbits function is just to show the bits. I am using gcc g++ compiler with VSCode in Windows 10.

//  Date encoding in 2-byte number

#include <stdio.h>
#include <conio.h>

typedef unsigned int uint;

Showbits(int n)
{
    int i, k, mask;

    for (i = 15; i >= 0; i--)
    {
        mask = 1 << i;
        k = n & mask;
        k == 0 ? printf("0") : printf("1");
    }
}

int main()
{

    uint encd_date, date, month, year;

    year = 2022;
    month = 9;
    date = 15;
    encd_date = 512 * (year - 1980) + 32 * month + date;

    printf("\nEncoded date:%u\n", encd_date);
    printf("\nencd_date: ");
    Showbits(encd_date);

    year = (1980 + (encd_date >> 9));
    month = (encd_date << 7);
    month = (month >> 12);
    date = (encd_date << 11);
    date = (date >> 11);

    printf("\ndate: ");
    Showbits(date);

    printf("\nmonth: ");
    Showbits(month);

    printf("\nyear: ");
    Showbits(year);

    printf("\nDecoded date %u month %u year %u", date, month, year);

    return 0;
}

Answer 1

I change the data type suitable for 16-bit as suggested by Fe2O3 and it works as expected.

#include <stdio.h>
#include <stdint.h>
#include <inttypes.h>

typedef uint16_t uint;

void Showbits(int n)
{
    int i, k, mask;

    for (i = 15; i >= 0; i--)
    {
        mask = 1 << i;
        k = n & mask;
        k == 0 ? printf("0") : printf("1");
    }
}

int main()
{

    uint encd_date, date, month, year;

    year = 2022;
    month = 9;
    date = 15;
    encd_date = 512 * (year - 1980) + 32 * month + date;

    printf("\nEncoded date:%u\n", encd_date);
    printf("\nencd_date: ");
    Showbits(encd_date);

    year = (1980 + (encd_date >> 9));
    month = (encd_date << 7);
    month = (month >> 12);
    date = (encd_date << 11);
    date = (date >> 11);

    printf("\ndate: ");
    Showbits(date);

    printf("\nmonth: ");
    Showbits(month);

    printf("\nyear: ");
    Showbits(year);

    printf("\nDecoded date %u month %u year %u", date, month, year);

    return 0;
}

Answer 2

Please don't ask me to explain it. It's late here.

It's almost if the carry-out of the left shift is being dragged back in on the right shift operations... (or something...)

A bridging 'fix' is to use an intermediary variable that seems to "clear up" the problem. This seems to work(around) for whatever reason.

int main() {
    uint16_t y = 2022;
    uint16_t m = 9;
    uint16_t d = 15;
    uint16_t u;

    uint16_t encd_date = (y-1980)*512 + m*32 + d;

                            y = (encd_date >> 9) + 1980;
    u = encd_date <<  7;    m = u >> (7+5);
    u = encd_date << 11;    d = u >> 11;

    printf( "\nDecoded year %d month %d date %d", y, m, d );

    return 0;
}

/* Output
Decoded year 2022 month 9 date 15
*/