add unit tests for daysUntilNewYear

shared/src/commonMain/kotlin/com/tkhs0604/kmmsandbox/NewYear.kt

@@ -6,8 +6,7 @@ import kotlinx.datetime.TimeZone
 import kotlinx.datetime.daysUntil
 import kotlinx.datetime.todayIn
 
-fun daysUntilNewYear(): Int {
-    val today = Clock.System.todayIn(TimeZone.currentSystemDefault())
-    val closestNewYear = LocalDate(today.year + 1, 1, 1)
-    return today.daysUntil(closestNewYear)
+fun daysUntilNewYear(startingDate: LocalDate = Clock.System.todayIn(TimeZone.currentSystemDefault())): Int {
+    val closestNewYear = LocalDate(startingDate.year + 1, 1, 1)
+    return startingDate.daysUntil(closestNewYear)
 }

shared/src/commonTest/kotlin/com/tkhs0604/kmmsandbox/CommonNewYearTest.kt

@@ -0,0 +1,35 @@
+package com.tkhs0604.kmmsandbox
+
+import kotlinx.datetime.LocalDate
+import kotlin.test.Test
+import kotlin.test.assertEquals
+
+class CommonNewYearTest {
+
+    @Test
+    fun `Days remaining until the next year are 1 if the starting date is the end of the year`() {
+        val startingDate = LocalDate(2022, 12, 31)
+
+        val remainingDays = daysUntilNewYear(startingDate)
+
+        assertEquals(1, remainingDays)
+    }
+
+    @Test
+    fun `Days remaining until the next year are 365 if the starting date is not a leap year and the beginning of the year`() {
+        val startingDate = LocalDate(2023, 1, 1)
+
+        val remainingDays = daysUntilNewYear(startingDate)
+
+        assertEquals(365, remainingDays)
+    }
+
+    @Test
+    fun `Days remaining until the next year are 366 if the starting date is a leap year and the beginning of the year`() {
+        val startingDate = LocalDate(2024, 1, 1)
+
+        val remainingDays = daysUntilNewYear(startingDate)
+
+        assertEquals(366, remainingDays)
+    }
+}