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

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

manifest

@@ -1,5 +1,5 @@
-C Allow\slarge\shexadecimal\sliterals\sto\sbe\sused\sas\sDEFAULT\svalues.
+C Implement\sa\snew\salgorithm\sfor\scomputing\sISO\sweek\svalues\sin\sstrftime()\sbased\non\sthe\sidea\s(from\s[forum/forumpost/3681cb1bcd|Nuno\sCruces])\sof\sshifting\sthe\ndate\sbeing\stested\sto\sthe\sThursday\sof\sthe\ssame\sweek.
-D 2024-01-19T16:51:34.042
+D 2024-01-20T00:31:44.563
 F .fossil-settings/empty-dirs dbb81e8fc0401ac46a1491ab34a7f2c7c0452f2f06b54ebb845d024ca8283ef1
 F .fossil-settings/ignore-glob 35175cdfcf539b2318cb04a9901442804be81cd677d8b889fcc9149c21f239ea
 F LICENSE.md df5091916dbb40e6e9686186587125e1b2ff51f022cc334e886c19a0e9982724
@@ -684,7 +684,7 @@ F src/build.c e7d9044592eeeea8e78d8ae53ca8d31fd6e92ca0d4f53e2f2e8ccf7352e0b04b
 F src/callback.c db3a45e376deff6a16c0058163fe0ae2b73a2945f3f408ca32cf74960b28d490
 F src/complete.c a3634ab1e687055cd002e11b8f43eb75c17da23e
 F src/ctime.c 23331529e654be40ca97d171cbbffe9b3d4c71cc53b78fe5501230675952da8b
-F src/date.c 7ed194fee09bc3aa2129cfe74ebf5daca6fc24c0995dd28cf2a2feacf4d8d44f
+F src/date.c 35ea61eb7ab5b918db7a96725346234bf8fb5bc161c7b868c20a7f24f351a2da
 F src/dbpage.c 80e46e1df623ec40486da7a5086cb723b0275a6e2a7b01d9f9b5da0f04ba2782
 F src/dbstat.c 3b677254d512fcafd4d0b341bf267b38b235ccfddbef24f9154e19360fa22e43
 F src/delete.c cb766727c78e715f9fb7ec8a7d03658ed2a3016343ca687acfcec9083cdca500
@@ -2159,8 +2159,8 @@ F vsixtest/vsixtest.tcl 6a9a6ab600c25a91a7acc6293828957a386a8a93
 F vsixtest/vsixtest.vcxproj.data 2ed517e100c66dc455b492e1a33350c1b20fbcdc
 F vsixtest/vsixtest.vcxproj.filters 37e51ffedcdb064aad6ff33b6148725226cd608e
 F vsixtest/vsixtest_TemporaryKey.pfx e5b1b036facdb453873e7084e1cae9102ccc67a0
-P 1481baf3d55effcc117f2097e2d49a6f60f5f74b21190c3be943fd785b8a4c5b
+P 8cccc1f27d7470d3cdd3c9c6d74f6a5ac49ec6eaa7002bcf96f4842fb8c79e1a
-R 7a04f28ab4102a53adff1c3861de4c59
+R 6109198c01da554c8b8cc1d558fbd53e
-U dan
+U drh
-Z de8bd7b1420ad71ea0d7cb25c6eae88b
+Z ae5f14e34bf5701b2021ec4920aa096b
 # Remove this line to create a well-formed Fossil manifest.

manifest.uuid

@@ -1 +1 @@
-8cccc1f27d7470d3cdd3c9c6d74f6a5ac49ec6eaa7002bcf96f4842fb8c79e1a
+b06ab46a9ee98719159ed3e05cdfbf26281353d781206f56ed7cb12859210fed

src/date.c

@@ -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;
+        DateTime y = x;
-        if( x.M==12 && isoWeekNumber(&x)==1 ){
+        assert( y.validJD );
-          Y++;
+        /* Move y so that it is the Thursday in the same week as x */
-        }else if( x.M==1 && isoWeekNumber(&x)>=52 ){
+        y.iJD += (4 - dayOfWeek(&x))*86400000;
-          Y--;
+        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 */