Why is gmtime implemented this way? - c

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

Ran your code as y2 minix code as y1 Solaris 9 v245 & got this profiler data:
%Time Seconds Cumsecs #Calls msec/call Name
79.1 0.34 0.34 36966 0.0092 _write
7.0 0.03 0.37 1125566 0.0000 .rem
7.0 0.03 0.40 36966 0.0008 _doprnt
4.7 0.02 0.42 1817938 0.0000 _mcount
2.3 0.01 0.43 36966 0.0003 y2
0.0 0.00 0.43 4 0. atexit
0.0 0.00 0.43 1 0. _exithandle
0.0 0.00 0.43 1 0. main
0.0 0.00 0.43 1 0. _fpsetsticky
0.0 0.00 0.43 1 0. _profil
0.0 0.00 0.43 36966 0.0000 printf
0.0 0.00 0.43 147864 0.0000 .div
0.0 0.00 0.43 73932 0.0000 _ferror_unlocked
0.0 0.00 0.43 36966 0.0000 memchr
0.0 0.00 0.43 1 0. _findbuf
0.0 0.00 0.43 1 0. _ioctl
0.0 0.00 0.43 1 0. _isatty
0.0 0.00 0.43 73932 0.0000 _realbufend
0.0 0.00 0.43 36966 0.0000 _xflsbuf
0.0 0.00 0.43 1 0. _setbufend
0.0 0.00 0.43 1 0. _setorientation
0.0 0.00 0.43 137864 0.0000 _memcpy
0.0 0.00 0.43 3 0. ___errno
0.0 0.00 0.43 1 0. _fstat64
0.0 0.00 0.43 1 0. exit
0.0 0.00 0.43 36966 0.0000 y1
Maybe that is an answer

Answer 2

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.

Answer 3

Your method seems sound, but it's a little more difficult to get it to work for EPOCH_YR = 1970 because you are now mid-cycle on several cycles.
Can you see if you have an equivalent for that case and see whether it's still better?
You're certainly right that it's debatable whether that gmtime() implementation should be used in any high-performance code. That's a lot of busy work to be doing in any tight loops.

Answer 4

Correct approach. You definitely want to go for an O(1) algo. Would work in Mayan calendar without ado. Check the last line: dayno is limited to 0..364, although in leap years it needs to range 0..365 . The line before has a similar flaw.