From 8386b5dee41454b596f705061617a36a9648fdcd Mon Sep 17 00:00:00 2001
From: Gekctek <gekctek@gmail.com>
Date: Sun, 19 Nov 2023 16:15:18 -0800
Subject: [PATCH] Fixes `DateComponents.dayOfWeek` fails with `arithmetic
 overflow` on Saturdays #4

---
 internal/Components.mo  |  6 +++++-
 test/Components.test.mo | 16 ++++++++++++++++
 2 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/internal/Components.mo b/internal/Components.mo
index 2989f6c..caaf058 100644
--- a/internal/Components.mo
+++ b/internal/Components.mo
@@ -1010,7 +1010,11 @@ module Module {
         let yearWithoutCentury = year % 100;
         let century = year / 100;
         let h = day + 13 * (month + 1) / 5 + yearWithoutCentury + yearWithoutCentury / 4 + century / 4 + 5 * century;
-        Int.abs(h) % 7 - 1;
+        let position = Int.abs(h) % 7;
+        switch (position) {
+            case (0) 6; // Wrap around -1 (Saturday)
+            case (i) i - 1;
+        };
     };
 
     public func daysInYear(year : Int) : Nat {
diff --git a/test/Components.test.mo b/test/Components.test.mo
index 9bc1102..c97a7b6 100644
--- a/test/Components.test.mo
+++ b/test/Components.test.mo
@@ -638,6 +638,22 @@ test(
                     dayOfYear = 114;
                 };
             },
+            {
+                components = {
+                    year = 2_023;
+                    month = 11;
+                    day = 18;
+                    hour = 0;
+                    minute = 0;
+                    nanosecond = 0;
+                };
+                expected = {
+                    weekOfYear = 47;
+                    weekYear = 2023;
+                    dayOfWeek = #saturday;
+                    dayOfYear = 322;
+                };
+            },
         ];
         for (testCase in Iter.fromArray(testCases)) {
             let actual : Components.DayOfWeek = Components.dayOfWeek(testCase.components);