strftime: ISO-8601 leap weeks

src/header/time/strftime.rs

@@ -208,6 +208,9 @@ pub unsafe fn strftime<W: WriteByte>(w: &mut W, format: *const c_char, t: *const
                 // Sunday-based week of year: %U
                 b'U' => w!("{}", ((*t).tm_yday + 7 - (*t).tm_wday) / 7),
 
+                // ISO-8601 week of year
+                b'V' => w!("{}", week_of_year(unsafe { &*t })),
+
                 // Weekday (0-6, Sunday=0): %w
                 b'w' => w!("{}", (*t).tm_wday),
 
@@ -246,3 +249,35 @@ pub unsafe fn strftime<W: WriteByte>(w: &mut W, format: *const c_char, t: *const
     }
     cw.written
 }
+
+/// Calculate number of weeks in a year as defined by ISO 8601
+///
+/// ## Source
+/// https://en.wikipedia.org/wiki/ISO_week_date
+fn weeks_per_year(year: c_int) -> c_int {
+    let year = year as f64;
+    // TODO: This function can be made const on newer Rust compilers but this line prevents it on
+    // our version in CI
+    let p_y = (year + (year / 4.) - (year / 100.) + (year / 400.)) as c_int % 7;
+    if p_y == 4 {
+        53
+    } else {
+        52
+    }
+}
+
+/// Calculate the week of the year accounting for leap weeks (ISO 8601)
+///
+/// ## Source
+/// https://en.wikipedia.org/wiki/ISO_week_date
+fn week_of_year(time: &tm) -> c_int {
+    let week = (10 + time.tm_yday - time.tm_wday) / 7;
+
+    if week <= 1 {
+        weeks_per_year(time.tm_year - 1)
+    } else if week > weeks_per_year(time.tm_year) {
+        1
+    } else {
+        week
+    }
+}

tests/expected/bins_dynamic/time/strftime.stdout

@@ -7,3 +7,8 @@
 6: 197 28
 28: Tue Jul 17 15:00:00 UTC 2018
 0: Tue Aug 07 19:17:11 UTC 2018Tue Aug 07 19:17:11 U
+53
+52
+52
+1
+53

tests/expected/bins_static/time/strftime.stdout

@@ -7,3 +7,8 @@
 6: 197 28
 28: Tue Jul 17 15:00:00 UTC 2018
 0: Tue Aug 07 19:17:11 UTC 2018Tue Aug 07 19:17:11 U
+53
+52
+52
+1
+53

tests/time/strftime.c

@@ -4,11 +4,53 @@
 
 #include "test_helpers.h"
 
+#define PRINT_BUFSZ 50
+#define V_BUFSZ 3
+
 void print(time_t timestamp, char* fmt) {
-    char* out = malloc(50);
-    size_t n = strftime(out, 50, fmt, localtime(&timestamp));
+    char out[PRINT_BUFSZ] = {0};
+    size_t n = strftime(out, PRINT_BUFSZ, fmt, localtime(&timestamp));
     printf("%zu: %s\n", n, out);
-    free(out);
+}
+
+void test_v(void) {
+    char buf[V_BUFSZ] = {0};
+    struct tm time = {0};
+
+    // Example dates copied from Wikipedia
+    // Saturday 2005-01-01
+    time.tm_yday = 0;
+    time.tm_wday = 6;
+    time.tm_year = 2005;
+    strftime(buf, V_BUFSZ, "%V", &time);
+    puts(buf);
+
+    // Saturday 2005-12-31
+    time.tm_yday = 365;
+    time.tm_wday = 6;
+    strftime(buf, V_BUFSZ, "%V", &time);
+    puts(buf);
+
+    // Sunday 2006-01-01
+    time.tm_yday = 0;
+    time.tm_wday = 0;
+    time.tm_year = 2006;
+    strftime(buf, V_BUFSZ, "%V", &time);
+    puts(buf);
+
+    // Sunday 2008-12-28
+    time.tm_yday = 362;
+    time.tm_wday = 0;
+    time.tm_year = 2008;
+    strftime(buf, V_BUFSZ, "%V", &time);
+    puts(buf);
+
+    // Friday 2010-01-01
+    time.tm_yday = 0;
+    time.tm_wday = 5;
+    time.tm_year = 2010;
+    strftime(buf, V_BUFSZ, "%V", &time);
+    puts(buf);
 }
 
 int main(void) {
@@ -21,4 +63,9 @@ int main(void) {
     print(1531839600, "%j %U");
     print(1531839600, "%+");
     print(1533669431, "%+%+%+%+%+"); // will overflow 50 characters
+
+    // ISO-8601 tests
+    test_v();
+
+    return EXIT_SUCCESS;
 }