From 1007eacf88aa31a827b674b9ab002889b2524874 Mon Sep 17 00:00:00 2001
From: Maximilian Muecke <muecke.maximilian@gmail.com>
Date: Sun, 18 Oct 2026 18:03:29 +0200
Subject: [PATCH] isoweek()/isoyear(): compute in C via convertDate()

---
 R/IDateTime.R   | 13 +++----------
 src/idatetime.c | 20 +++++++++++++++++++-
 2 files changed, 22 insertions(+), 11 deletions(-)

diff --git a/R/IDateTime.R b/R/IDateTime.R
index c061de3ca4..471e197f02 100644
--- a/R/IDateTime.R
+++ b/R/IDateTime.R
@@ -393,15 +393,8 @@ yday    = function(x) convertDate(as.IDate(x), "yday")
 wday    = function(x) convertDate(as.IDate(x), "wday")
 mday    = function(x) convertDate(as.IDate(x), "mday")
 week    = function(x) convertDate(as.IDate(x), "week")
-# ISO 8601-conformant week, as described at
-#   https://en.wikipedia.org/wiki/ISO_week_date
-# Approach:
-# * Find nearest Thursday to each element of x
-# * Find the number of weeks having passed between
-#   January 1st of the year of the nearest Thursdays and x
-nearest_thurs = function(x) 7L * ((as.integer(as.IDate(x)) + 3L) %/% 7L)
-isoweek = function(x) (convertDate(nearest_thurs(x), "yday") - 1L) %/% 7L + 1L
-isoyear = function(x) convertDate(nearest_thurs(x), "year")
+isoweek = function(x) convertDate(as.IDate(x), "isoweek")
+isoyear = function(x) convertDate(as.IDate(x), "isoyear")
 
 month   = function(x) convertDate(as.IDate(x), "month")
 quarter = function(x) convertDate(as.IDate(x), "quarter")
@@ -432,6 +425,6 @@ yearqtr = function(x, format = c("numeric", "character")) {
 }
 
 convertDate = function(x, type) {
-  type = match.arg(type, c("yday", "wday", "mday", "week", "month", "quarter", "year", "yearmon", "yearqtr"))
+  type = match.arg(type, c("yday", "wday", "mday", "week", "isoweek", "isoyear", "month", "quarter", "year", "yearmon", "yearqtr"))
   .Call(CconvertDate, x, type)
 }
diff --git a/src/idatetime.c b/src/idatetime.c
index 026b0458b7..2634d849a6 100644
--- a/src/idatetime.c
+++ b/src/idatetime.c
@@ -5,7 +5,7 @@ static const int YEARS100 = 36524;
 static const int YEARS4 = 1461;
 static const int YEARS1 = 365;
 
-typedef enum { YDAY, WDAY, MDAY, WEEK, MONTH, QUARTER, YEAR, YEARMON, YEARQTR} datetype;
+typedef enum { YDAY, WDAY, MDAY, WEEK, ISOWEEK, ISOYEAR, MONTH, QUARTER, YEAR, YEARMON, YEARQTR} datetype;
 
 static inline bool isLeapYear(int year) {
     return (year % 100 != 0 || year % 400 == 0) && year % 4 == 0;
@@ -29,6 +29,22 @@ void convertSingleDate(int x, datetype type, void *out)
         return;
     }
 
+    if (type == ISOWEEK || type == ISOYEAR) {
+        // ISO 8601-conformant week, as described at https://en.wikipedia.org/wiki/ISO_week_date
+        // The ISO week and year are those of the nearest Thursday
+        // Thursdays are multiples of 7, since x=0 (1970-01-01) was a Thursday
+        int thu = x + (3 - (x % 7 + 10) % 7);
+        int res;
+        if (type == ISOWEEK) {
+            convertSingleDate(thu, YDAY, &res);
+            res = (res - 1) / 7 + 1;
+        } else {
+            convertSingleDate(thu, YEAR, &res);
+        }
+        *(int *)out = res;
+        return;
+    }
+
     int days = x - 11017;
 
     int years400 = days / YEARS400;
@@ -134,6 +150,8 @@ SEXP convertDate(SEXP x, SEXP type)
     else if (!strcmp(ctype_str, "wday")) ctype = WDAY;
     else if (!strcmp(ctype_str, "mday")) ctype = MDAY;
     else if (!strcmp(ctype_str, "week")) ctype = WEEK;
+    else if (!strcmp(ctype_str, "isoweek")) ctype = ISOWEEK;
+    else if (!strcmp(ctype_str, "isoyear")) ctype = ISOYEAR;
     else if (!strcmp(ctype_str, "month")) ctype = MONTH;
     else if (!strcmp(ctype_str, "quarter")) ctype = QUARTER;
     else if (!strcmp(ctype_str, "year")) ctype = YEAR;