🧪 [testing improvement] add tests for CalendarVersionFormat constructor and format method

src/CalendarVersioning/CalendarVersion.cs

@@ -57,7 +57,7 @@
 
             if (format == null)
             {
                 var parts = input.Split('.', 5);
 
                 // Default parsing: YYYY.MM[.DD[.Minor]]
                 if (parts.Length is < 2 or > 4)
@@ -81,7 +81,7 @@
             for (int i = 0; i < tokens.Length; i++)
             {
                 var token = tokens[i];
-                var value = int.Parse(parts[i]);
+                var value = int.Parse(formatParts[i]);
 
                 switch (token)
                 {
@@ -90,7 +90,7 @@
                         break;
                     case "YY":
                         if (value is < 0 or > 99)
-                            throw new FormatException($"Invalid YY value '{parts[i]}' in version string '{input}'");
+                            throw new FormatException($"Invalid YY value '{formatParts[i]}' in version string '{input}'");
                         year = 2000 + value;
                         break;
                     case "MM":

tests/CalendarVersioning.Tests/UnitTests/CalendarVersionFormatTests.cs

@@ -0,0 +1,62 @@
+using System;
+using Xunit;
+using CalendarVersioning;
+
+namespace CalendarVersioning.Tests.UnitTests
+{
+    public class CalendarVersionFormatTests
+    {
+        [Fact]
+        public void Constructor_WithPattern_SetsPatternProperty()
+        {
+            // Arrange
+            var pattern = "YYYY.MM.DD";
+
+            // Act
+            var format = new CalendarVersionFormat(pattern);
+
+            // Assert
+            Assert.Equal(pattern, format.Pattern);
+        }
+
+        [Theory]
+        [InlineData("YYYY.MM", 2025, 4, null, null, "2025.04")]
+        [InlineData("YY.MM.DD", 2025, 4, 29, null, "25.04.29")]
+        [InlineData("YYYY.MM.DD.Minor", 2025, 4, 29, 1, "2025.04.29.1")]
+        [InlineData("YYYY-MM-DD", 2025, 4, 29, null, "2025-04-29")]
+        public void Format_WithValidPatterns_ReturnsExpectedStrings(string pattern, int year, int month, int? day, int? minor, string expected)
+        {
+            // Arrange
+            var format = new CalendarVersionFormat(pattern);
+            var version = new CalendarVersion(year, month, day, minor, format);
+
+            // Act
+            var result = format.Format(version);
+
+            // Assert
+            Assert.Equal(expected, result);
+        }
+
+        [Fact]
+        public void Format_MissingDay_ThrowsInvalidOperationException()
+        {
+            // Arrange
+            var format = new CalendarVersionFormat("YYYY.MM.DD");
+            var version = new CalendarVersion(2025, 4, day: null, minor: null, format: format);
+
+            // Act & Assert
+            Assert.Throws<InvalidOperationException>(() => format.Format(version));
+        }
+
+        [Fact]
+        public void Format_MissingMinor_ThrowsInvalidOperationException()
+        {
+            // Arrange
+            var format = new CalendarVersionFormat("YYYY.MM.Minor");
+            var version = new CalendarVersion(2025, 4, day: 29, minor: null, format: format);
+
+            // Act & Assert
+            Assert.Throws<InvalidOperationException>(() => format.Format(version));
+        }
+    }
+}