fix leap year calculations

Author: sami-eljabali

src/main/kotlin/YearUtil.kt

@@ -3,8 +3,7 @@ object YearUtil {
     fun isLeapYear(year: Int): Boolean =
         when {
             year % 4 != 0 -> false
-            year % 400 == 0 -> true
-            year % 100 == 0 -> false
+            year % 100 == 0 -> year % 400 == 0
             else -> true
         }
 }

src/main/kotlin/localdate/LocalDateUtil.kt

@@ -17,23 +17,13 @@ object LocalDateUtil {
      * @param day  Day of the month with range 1-31.
      * @return  LocalDate.
      */
-    fun new(
-        year: Int,
-        month: Int,
-        day: Int,
-    ): LocalDate =
-        LocalDate.of(
-            year,
-            Month.of(month),
-            day
-        )
+    fun new(year: Int, month: Int, day: Int): LocalDate = LocalDate.of(year, Month.of(month), day)
 
     /**
      * @param epochMilliseconds  Epoch time, aka Unix time, are seconds elapsed since January 1st 1970 at 00:00:00 UTC.
      * @return  LocalDate.
      */
-    fun new(epochMilliseconds: Long): LocalDate =
-        LocalDate.ofEpochDay(epochMilliseconds / 1000 / 60 / 60 / 24)
+    fun new(epochMilliseconds: Long): LocalDate = LocalDate.ofEpochDay(epochMilliseconds / 1000 / 60 / 60 / 24)
 
     /**
      * @param date  A wrapper of Epoch time in UTC.

src/main/kotlin/localdatetime/LocalDateTimeUtil.kt

@@ -30,16 +30,7 @@ object LocalDateTimeUtil {
         minute: Int = 0,
         second: Int = 0,
         nano: Int = 0
-    ): LocalDateTime =
-        LocalDateTime.of(
-            year,
-            Month.of(month),
-            day,
-            hourIn24,
-            minute,
-            second,
-            nano
-        )
+    ): LocalDateTime = LocalDateTime.of(year, Month.of(month), day, hourIn24, minute, second, nano)
 
     /**
      * @param year  Year, ie, 2020.
@@ -61,10 +52,7 @@ object LocalDateTimeUtil {
         second: Int = 0,
         nano: Int = 0,
         isAm: Boolean,
-    ): LocalDateTime =
-        new(
-            year, month, day, if (isAm) hour else hour + 12, minute, second, nano
-        )
+    ): LocalDateTime = new(year, month, day, if (isAm) hour else hour + 12, minute, second, nano)
 
     /**
      * @param epochMilliseconds  Epoch time, aka Unix time, are seconds elapsed since January 1st 1970 at 00:00:00 UTC.

src/main/kotlin/zoneddatetime/ZonedDateTimeUtil.kt

@@ -37,15 +37,7 @@ object ZonedDateTimeUtil {
         nano: Int = 0,
         useSystemTimeZone: Boolean = true
     ): ZonedDateTime {
-        val localDateTime = LocalDateTime.of(
-            year,
-            Month.of(month),
-            day,
-            hourIn24,
-            minute,
-            second,
-            nano
-        )
+        val localDateTime = LocalDateTime.of(year, Month.of(month), day, hourIn24, minute, second, nano)
         return ZonedDateTime.of(localDateTime, if (useSystemTimeZone) ZoneId.systemDefault() else UTC)
     }
 
@@ -71,10 +63,7 @@ object ZonedDateTimeUtil {
         nano: Int = 0,
         isAm: Boolean,
         useSystemTimeZone: Boolean = true
-    ): ZonedDateTime =
-        new(
-            year, month, day, if (isAm) hour else hour + 12, minute, second, nano, useSystemTimeZone
-        )
+    ): ZonedDateTime = new(year, month, day, if (isAm) hour else hour + 12, minute, second, nano, useSystemTimeZone)
 
     /**
      * @param epochMilliseconds  Epoch time, aka Unix time, are seconds elapsed since January 1st 1970 at 00:00:00 UTC.

src/test/kotlin/YearUtilTest.kt

@@ -0,0 +1,49 @@
+import org.junit.jupiter.api.Assertions.assertTrue
+import org.junit.jupiter.api.Test
+
+class YearUtilTest {
+
+    companion object {
+        val LEAP_YEAR_LIST = arrayOf(1804, 1808, 1812, 1816, 1820, 1824, 1828, 1832, 1836, 1840, 1844, 1848, 1852,
+            1856, 1860, 1864, 1868, 1872, 1876, 1880, 1884, 1888, 1892, 1896, 1904, 1908, 1912, 1916, 1920, 1924,
+            1928, 1932, 1936, 1940, 1944, 1948, 1952, 1956, 1960, 1964, 1968, 1972, 1976, 1980, 1984, 1988, 1992,
+            1996, 2000, 2004, 2008, 2012, 2016, 2020,2024, 2028, 2032, 2036, 2040, 2044, 2048, 2052, 2056, 2060,
+            2064, 2068, 2072, 2076, 2080, 2084, 2088, 2092, 2096
+        )
+    }
+
+    @Test
+    fun `given leap years, then should properly mark them as such`() {
+        // given
+        var areAllLeapYears = true
+
+        // when
+        LEAP_YEAR_LIST.forEach { year ->
+            areAllLeapYears = areAllLeapYears and YearUtil.isLeapYear(year)
+        }
+
+        // then
+        assertTrue(areAllLeapYears)
+    }
+
+    @Test
+    fun `given non-leap years, then should properly mark them as such`() {
+        // given
+        var areAllNonLeapYears = true
+
+        // when
+        LEAP_YEAR_LIST.forEachIndexed { index, currentLeapYear ->
+            if (index == LEAP_YEAR_LIST.size - 1) {
+                return@forEachIndexed
+            }
+            val nextLeapYear = LEAP_YEAR_LIST[index + 1]
+            for (nonLeapYear in (currentLeapYear + 1) until nextLeapYear) {
+                areAllNonLeapYears = areAllNonLeapYears and !YearUtil.isLeapYear(nonLeapYear)
+            }
+        }
+
+        // then
+        assertTrue(areAllNonLeapYears)
+    }
+
+}