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| author | Karel Zak | 2017-12-13 19:09:56 +0100 |
|---|---|---|
| committer | Karel Zak | 2017-12-13 19:09:56 +0100 |
| commit | 3c49b23ad62a30ec5fa3864176cbfc21270ae7b2 (patch) | |
| tree | fcf01e5edcfcc8533b13ab77d48e807aaf591745 /misc-utils/cal.c | |
| parent | fincore: fix a typo in fincore.1 (diff) | |
| download | kernel-qcow2-util-linux-3c49b23ad62a30ec5fa3864176cbfc21270ae7b2.tar.gz kernel-qcow2-util-linux-3c49b23ad62a30ec5fa3864176cbfc21270ae7b2.tar.xz kernel-qcow2-util-linux-3c49b23ad62a30ec5fa3864176cbfc21270ae7b2.zip | |
cal: explain magic values for day of week calculation
Based on BERNDT E.SCHWERDTFEGER papers.
Signed-off-by: Karel Zak <kzak@redhat.com>
Diffstat (limited to 'misc-utils/cal.c')
| -rw-r--r-- | misc-utils/cal.c | 24 |
1 files changed, 16 insertions, 8 deletions
diff --git a/misc-utils/cal.c b/misc-utils/cal.c index 64847da3c..70a3d96c4 100644 --- a/misc-utils/cal.c +++ b/misc-utils/cal.c @@ -862,14 +862,22 @@ static int day_in_year(int day, int month, int32_t year) */ static int day_in_week(int day, int month, int32_t year) { - static const int reform[] = { - SUNDAY, WEDNESDAY, TUESDAY, FRIDAY, SUNDAY, WEDNESDAY, - FRIDAY, MONDAY, THURSDAY, SATURDAY, TUESDAY, THURSDAY - }; - static const int old[] = { - FRIDAY, MONDAY, SUNDAY, WEDNESDAY, FRIDAY, MONDAY, - WEDNESDAY, SATURDAY, TUESDAY, THURSDAY, SUNDAY, TUESDAY - }; + /* This leaves us to explain the ‘magic’ values for 'e'. Let us look at + * the following table with the number d(m) of days in a month, the sum + * e(m) of days for the previous months (number of days from begin of + * the year for non leap years only) and their values modulo 7: + * + * m 1 2 3 4 5 6 7 8 9 10 11 12 + * d(m) 31 28 31 30 31 30 31 31 30 31 30 31 + * e(m) 0 31 59 90 120 151 181 212 243 273 304 334 + * (mod 7) 0 3 3 6 1 4 6 2 5 0 3 5 + * + * The ‘magic’ value is e=e(m) for the months m=1,2 and e=e(m)−1 for + * the other months. + */ + static const int reform[] = { 0, 3, 2, 5, 0, 3, 5, 1, 4, 6, 2, 4 }; + static const int old[] = { 5, 1, 0, 3, 5, 1, 3, 6, 2, 4, 0, 2 }; + if (year != REFORMATION_YEAR + 1) year -= month < MARCH; else |