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Implement a new algorithm for computing ISO week values in strftime() based

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on the idea (from [forum/forumpost/3681cb1bcd|Nuno Cruces]) of shifting the
date being tested to the Thursday of the same week.

FossilOrigin-Name: b06ab46a9ee98719159ed3e05cdfbf26281353d781206f56ed7cb12859210fed
This commit is contained in:
drh
2024-01-20 00:31:44 +00:00
parent a4802720a1
commit f08287cc12
3 changed files with 24 additions and 53 deletions
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61
src/date.c
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@@ -1249,7 +1249,7 @@ static int dayOfYear(DateTime *pDate){
jan01.M = 1;
jan01.D = 1;
computeJD(&jan01);
return (int)((pDate->iJD-jan01.iJD+45300000)/86400000) + 1;
return (int)((pDate->iJD-jan01.iJD+43200000)/86400000) + 1;
}
/*
@@ -1263,41 +1263,6 @@ static int dayOfWeek(DateTime *pDate){
return w;
}
/*
** Compute the day-of-week (0=Sunday, 1=Monday, ..., 6=Saturday) for
** the last day of the calendar year Y.
*/
static int lastDayOfYear(int Y){
return (Y + (Y/4) - (Y/100) + (Y/400))%7;
}
/*
** Return the number of ISO weeks in calendar year Y. The answer is
** either 52 or 53.
*/
static int weeksInYear(int Y){
if( lastDayOfYear(Y)==4 || lastDayOfYear(Y-1)==3 ){
return 53;
}else{
return 52;
}
}
/*
** Compute the number days since the start of the ISO-week year for pDate.
** The ISO-week year starts on the first day of the week (always a Monday)
** that contains the first Thursday on or after January 1.
*/
static int isoWeekNumber(DateTime *pDate){
int wn = (10 + dayOfYear(pDate) - dayOfWeek(pDate))/7;
if( wn<1 ){
wn = weeksInYear(pDate->Y-1);
}else if( wn>weeksInYear(pDate->Y) ){
wn = 1;
}
return wn;
}
/*
** strftime( FORMAT, TIMESTRING, MOD, MOD, ...)
**
@@ -1376,16 +1341,16 @@ static void strftimeFunc(
}
case 'G': /* Fall thru */
case 'g': {
int Y = x.Y;
if( x.M==12 && isoWeekNumber(&x)==1 ){
Y++;
}else if( x.M==1 && isoWeekNumber(&x)>=52 ){
Y--;
}
DateTime y = x;
assert( y.validJD );
/* Move y so that it is the Thursday in the same week as x */
y.iJD += (4 - dayOfWeek(&x))*86400000;
y.validYMD = 0;
computeYMD(&y);
if( cf=='g' ){
sqlite3_str_appendf(&sRes, "%02d", Y%100);
sqlite3_str_appendf(&sRes, "%02d", y.Y%100);
}else{
sqlite3_str_appendf(&sRes, "%04d", Y);
sqlite3_str_appendf(&sRes, "%04d", y.Y);
}
break;
}
@@ -1466,7 +1431,13 @@ static void strftimeFunc(
break;
}
case 'V': { /* Week num. 01-53. First week with a Thur is week 01 */
sqlite3_str_appendf(&sRes,"%02d", isoWeekNumber(&x));
DateTime y = x;
/* Adjust y so that is the Thursday in the same week as x */
assert( y.validJD );
y.iJD += (4 - dayOfWeek(&x))*86400000;
y.validYMD = 0;
computeYMD(&y);
sqlite3_str_appendf(&sRes,"%02d", (dayOfYear(&y)-1)/7+1);
break;
}
case 'W': { /* Week num. 00-53. First Mon of the year is week 01 */