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cal: explain magic values for day of week calculation 

Based on BERNDT E.SCHWERDTFEGER papers.

Signed-off-by: Karel Zak <kzak@redhat.com>
This commit is contained in:
Karel Zak 2017-12-13 19:09:56 +01:00
parent 616e75c02c
commit 3c49b23ad6
1 changed files with 16 additions and 8 deletions
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24
misc-utils/cal.c
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@ -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