/* File:    dos-date.c
   Purpose: Illustrates low-level bit manipulation.  Converts dates, as used
            in the Microsoft 16-bit format (e.g., Microsoft FAT)
   Author:  Bill Slough
   Notes:   See King, Ch. 20 for more details
*/

#include <stdio.h>

#define EPOCH 1980   /* beginning of time according to Microsoft */
/*
 *     +--------------+----------+-----------+
 *     |    year      |   month  |   day     |
 *     +--------------+----------+-----------+
 *          7 bits       4 bits     5 bits
 */
struct filedate_t {
    unsigned int day: 5;
    unsigned int month: 4;
    unsigned int year: 7;
};

union int_date {
    unsigned short i;
    struct filedate_t fd;
};

void display_packed(int m, int d, int y);
void display_mdy(unsigned short packed_date);

int main(void) {
    int m, d, y;
    unsigned short ms_date;

    printf("Enter date (mm/dd/yyyy): ");
    scanf("%d/%d/%d", &m, &d, &y);
    display_packed(m, d, y);

    printf("Enter date (4-digit packed hexadecimal): ");
    scanf("%hx", &ms_date);
    display_mdy(ms_date);

    return 0;
}

/* Given a month, day, and year, show the MS-DOS packed form */
void display_packed(int m, int d, int y) {
    struct filedate_t mydate;
    union int_date u;

    mydate.month = m;
    mydate.day = d;
    mydate.year = y - EPOCH;

    u.fd = mydate;

    /* Display the date in its 16 bit, 3-field form */
    printf("DOS date as 16 bit quantity: %4x\n\n", u.i);
}

/* Given the MS-DOS packed form, show the month, day and year */
void display_mdy(unsigned short packed_date) {
    union int_date u;

    u.i = packed_date;

    /* Display each field of the date */
    printf("Month: %2d Day: %2d  Year: %4d\n",
           u.fd.month,
           u.fd.day,
           u.fd.year + EPOCH);
}