srfi-19: Fix ~V converter in date->string.

The ~V is supposed to print ISO week number, not a week number.  This
commit fixes that.

* module/srfi/srfi-19.scm (date-week-number-iso): New procedure taken
  from the reference implementation.
  (directives)<#\V>: Use it.
* test-suite/tests/srfi-19.test ("date->string ~V"): Add tests taken
  from the reference test suite.
* doc/ref/srfi-modules.texi (SRFI-19 Date to string): Mention ISO-8601
  in description for ~V.

Fixes <https://bugs.gnu.org/74841>.
Edited by lloda <lloda@sarc.name>.

doc/ref/srfi-modules.texi

@@ -2864,8 +2864,8 @@ with locale decimal point, eg.@: @samp{5.2}
 @item @nicode{~T} @tab time, 24 hour clock, @samp{~H:~M:~S}
 @item @nicode{~U} @tab week of year, Sunday first day of week,
 @samp{00} to @samp{52}
-@item @nicode{~V} @tab week of year, Monday first day of week,
-@samp{01} to @samp{53}
+@item @nicode{~V} @tab ISO 8601 week number of the year,
+Monday first day of week, @samp{01} to @samp{53}
 @item @nicode{~w} @tab day of week, 0 for Sunday, @samp{0} to @samp{6}
 @item @nicode{~W} @tab week of year, Monday first day of week,
 @samp{00} to @samp{52}

module/srfi/srfi-19.scm

@@ -753,6 +753,27 @@
                      (days-before-first-week  date day-of-week-starting-week))
                   7))
 
+;;; Adapted from the reference implementation.
+(define (date-week-number-iso date)
+  "Return a ISO-8601 week number for the @var{date}."
+  ;; The week with the year's first Thursday is week 01.
+  (let* ((first-day-of-the-week (week-day 1 1 (date-year date)))
+         (offset (if (> first-day-of-the-week 4) 0 1))
+         ;; -2: decrement one day to compensate 1-origin of date-year-day,
+         ;; and decrement one more day for Sunday belongs to the previous week.
+         (w (+ (floor-quotient (+ (date-year-day date) first-day-of-the-week -2)
+                               7)
+               offset)))
+    (cond ((zero? w)
+           ;; date belongs to the last week of the previous year
+           (date-week-number-iso (make-date 0 0 0 0 31 12
+                                            (- (date-year date) 1) 0)))
+          ((and (= w 53)
+                (<= (week-day 1 1 (+ (date-year date) 1)) 4))
+           ;; date belongs to the first week of the next year
+           1)
+          (else w))))
+
 (define (current-date . tz-offset)
   (let ((time (current-time time-utc)))
     (time-utc->date
@@ -1043,7 +1064,7 @@
                    (display (padding (date-week-number date 0)
                                      #\0 2) port))))
    (cons #\V (lambda (date pad-with port)
-               (display (padding (date-week-number date 1)
+               (display (padding (date-week-number-iso date)
                                  #\0 2) port)))
    (cons #\w (lambda (date pad-with port)
                (display (date-week-day date) port)))

test-suite/tests/srfi-19.test

@@ -26,7 +26,8 @@
   #:use-module (test-suite lib)
   #:use-module (srfi srfi-1)
   #:use-module (srfi srfi-19)
-  #:use-module (ice-9 format))
+  #:use-module (ice-9 format)
+  #:use-module (ice-9 match))
 
 ;; Make sure we use the default locale.
 (when (defined? 'setlocale)
@@ -412,7 +413,61 @@ incomplete numerical tower implementation.)"
   (with-test-prefix "date-week-number"
     (pass-if (= 0 (date-week-number (make-date 0 0 0 0 1 1 1984 0) 0)))
     (pass-if (= 0 (date-week-number (make-date 0 0 0 0 7 1 1984 0) 0)))
-    (pass-if (= 1 (date-week-number (make-date 0 0 0 0 8 1 1984 0) 0)))))
+    (pass-if (= 1 (date-week-number (make-date 0 0 0 0 8 1 1984 0) 0))))
+
+  (let ((convert (match-lambda
+                   ((y m d)
+                    (date->string (make-date 0 0 0 0 d m y 0)
+                                  "~V")))))
+    ;; Taken from the test suite for the reference implementation.
+    (with-test-prefix "date->string ~V"
+      (pass-if-equal "Thursday, week 53" "53"
+        (convert '(2020 12 31)))
+      (pass-if-equal "Friday, week 53 (previous year)" "53"
+        (convert '(2021 1 1)))
+      (pass-if-equal "Sunday, week 53 (previous year)" "53"
+        (convert '(2021 1 3)))
+      (pass-if-equal "Monday, week 1" "01"
+        (convert '(2021 1 4)))
+
+      (pass-if-equal "Sunday, week 52" "52"
+        (convert '(2019 12 29)))
+      (pass-if-equal "Monday, week 1 (next year)" "01"
+        (convert '(2019 12 30)))
+      (pass-if-equal "Tuesday, week 1 (next year)" "01"
+        (convert '(2019 12 31)))
+      (pass-if-equal "Wednesday, week 1" "01"
+        (convert '(2020 1 1)))
+
+      (pass-if-equal "Saturday, week 52" "52"
+        (convert '(2016 12 31)))
+      (pass-if-equal "Sunday, week 52 (previous year)" "52"
+        (convert '(2017 1 1)))
+      (pass-if-equal "Monday, week 1" "01"
+        (convert '(2017 1 2)))
+      (pass-if-equal "Sunday, week 1" "01"
+        (convert '(2017 1 8)))
+      (pass-if-equal "Monday, week 2" "02"
+        (convert '(2017 1 9)))
+
+      (pass-if-equal "Sunday, week 52" "52"
+        (convert '(2014 12 28)))
+      (pass-if-equal "Monday, week 1 (next year)" "01"
+        (convert '(2014 12 29)))
+      (pass-if-equal "Tuesday, week 1 (next year)" "01"
+        (convert '(2014 12 30)))
+      (pass-if-equal "Wednesday, week 1 (next year)" "01"
+        (convert '(2014 12 31)))
+      (pass-if-equal "Thursday, week 1" "01"
+        (convert '(2015 1 1)))
+      (pass-if-equal "Friday, week 1" "01"
+        (convert '(2015 1 2)))
+      (pass-if-equal "Saturday, week 1" "01"
+        (convert '(2015 1 3)))
+      (pass-if-equal "Sunday, week 1" "01"
+        (convert '(2015 1 4)))
+      (pass-if-equal "Monday, week 2" "02"
+        (convert '(2015 1 5))))))
 
 
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