Question

I happened across the source for Minix’s gmtime function. I was interested in the bit that calculated the year number from days since epoch. Here are the guts of that bit:

http://www.raspberryginger.com/jbailey/minix/html/gmtime_8c-source.html

http://www.raspberryginger.com/jbailey/minix/html/loc__time_8h-source.html

#define EPOCH_YR 1970
#define LEAPYEAR(year) (!((year) % 4) && (((year) % 100) || !((year) % 400)))
#define YEARSIZE(year) (LEAPYEAR(year) ? 366 : 365)

int year = EPOCH_YR;

while (dayno >= YEARSIZE(year)) {
    dayno -= YEARSIZE(year);
    year++;
}

It looks like the algorithm is O(n), where n is the distance from the epoch. Additionally, it seems that LEAPYEAR must be calculated separately for each year – dozens of times for current dates and many more for dates far in the future. I had the following algorithm for doing the same thing (in this case from the ISO-9601 epoch (Year 0 = 1 BC) rather than UNIX epoch):

#define CYCLE_1   365
#define CYCLE_4   (CYCLE_1   *  4 + 1)
#define CYCLE_100 (CYCLE_4   * 25 - 1)
#define CYCLE_400 (CYCLE_100 *  4 + 1)

year += 400 * (dayno / CYCLE_400)
dayno = dayno % CYCLE_400

year += 100 * (dayno / CYCLE_100)
dayno = dayno % CYCLE_100

year +=   4 * (dayno / CYCLE_4)
dayno = dayno % CYCLE_4

year +=   1 * (dayno / CYCLE_1)
dayno = dayno % CYCLE_1

This runs in O(1) for any date, and looks like it should be faster even for dates reasonably close to 1970.

So, assuming that the Minix developers are Smart People who did it their way for a Reason, and probably know a bit more about C than I do, why?

Answer 1

This is pure speculation, but perhaps MINIX had requirements that were more important than execution speed, such as simplicity, ease of understanding, and conciseness? Some of the code was printed in a textbook, after all.