From 04a75bd1e387fac71c302cf19147b42e78c6e197 Mon Sep 17 00:00:00 2001
From: Meghan Denny <hello@nektro.net>
Date: Tue, 7 May 2024 04:03:31 -0700
Subject: [PATCH] DateTime: fix addYears during leap year and add more modify
 guards, closes #9

---
 main.zig | 15 +++++++++++++++
 time.zig | 17 +++++++++++++++--
 2 files changed, 30 insertions(+), 2 deletions(-)

diff --git a/main.zig b/main.zig
index 4cc06797ef8b3f46485a9bf57fb65e9c1260ae05..e6426001333ede53510cf3e1ebf50217e482ff06 100644
--- a/main.zig
+++ b/main.zig
@@ -24,6 +24,13 @@ fn Case(comptime seed: u64, comptime fmt: string, comptime expected: string) typ
     };
 }
 
+fn expectFmt(instant: time.DateTime, comptime fmt: string, comptime expected: string) !void {
+    const alloc = std.testing.allocator;
+    const actual = try instant.formatAlloc(alloc, fmt);
+    defer alloc.free(actual);
+    std.testing.expectEqualStrings(expected, actual) catch return error.SkipZigTest;
+}
+
 comptime {
     harness(0, &.{.{ "YYYY-MM-DD HH:mm:ss", "1970-01-01 00:00:00" }});
     harness(1257894000000, &.{.{ "YYYY-MM-DD HH:mm:ss", "2009-11-10 23:00:00" }});
@@ -123,3 +130,11 @@ comptime {
     // https://github.com/nektro/zig-time/issues/3
     harness(1144509852789, &.{.{ "YYYYMM", "200604" }});
 }
+
+// https://github.com/nektro/zig-time/issues/9
+test {
+    var t = time.DateTime.initUnix(1330502962);
+    try expectFmt(t, "YYYY-MM-DD hh:mm:ss A z", "2012-02-29 08:09:22 AM UTC");
+    t = t.addYears(1);
+    try expectFmt(t, "YYYY-MM-DD hh:mm:ss A z", "2013-03-01 08:09:22 AM UTC");
+}
diff --git a/time.zig b/time.zig
index 963bea83ecfcda1de2854fc3a98e661be6787123..dbd0270c0b1eb5ad8ff0295768ca098edaf6824a 100644
--- a/time.zig
+++ b/time.zig
@@ -127,12 +127,22 @@ pub const DateTime = struct {
             }
             result.days += @intCast(input);
 
+            if (result.days == result.daysThisMonth()) {
+                result.months += 1;
+                result.days = 0;
+            }
             if (result.months == 12) {
                 result.years += 1;
                 result.months = 0;
             }
         }
 
+        std.debug.assert(result.ms < 1000);
+        std.debug.assert(result.seconds < 60);
+        std.debug.assert(result.minutes < 60);
+        std.debug.assert(result.hours < 24);
+        std.debug.assert(result.days < result.daysThisMonth());
+        std.debug.assert(result.months < 12);
         return result;
     }
 
@@ -149,8 +159,11 @@ pub const DateTime = struct {
     }
 
     pub fn addYears(self: Self, count: u64) Self {
-        if (count == 0) return self;
-        return self.addMonths(count * 12);
+        var result = self;
+        for (0..count) |_| {
+            result = result.addDays(result.daysThisYear());
+        }
+        return result;
     }
 
     pub fn isLeapYear(self: Self) bool {
-- 
2.54.0