Zeller's Congruence is an algorithm devised by Christian Zeller in the 19th century to calculate + * the day of the week for any Julian or Gregorian calendar date. The input date must be in the format + * "MM-DD-YYYY" or "MM/DD/YYYY". + * + *
This class is final and cannot be instantiated. + * + * @see Wikipedia: Zeller's Congruence + */ +public final class ZellersCongruence { + + private static final String[] DAYS = {"Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"}; + + // Private constructor to prevent instantiation + private ZellersCongruence() { + } + + /** + * Calculates the day of the week for a given date using Zeller's Congruence. + * + *
The algorithm works for both Julian and Gregorian calendar dates. The input date must be
+ * in the format "MM-DD-YYYY" or "MM/DD/YYYY".
+ *
+ * @param input the date in the format "MM-DD-YYYY" or "MM/DD/YYYY"
+ * @return a string indicating the day of the week for the given date
+ * @throws IllegalArgumentException if the input format is invalid, the date is invalid,
+ * or the year is out of range
+ */
+ public static String calculateDay(String input) {
+ if (input == null || input.length() != 10) {
+ throw new IllegalArgumentException("Input date must be 10 characters long in the format MM-DD-YYYY or MM/DD/YYYY.");
+ }
+
+ int month = parsePart(input.substring(0, 2), 1, 12, "Month must be between 1 and 12.");
+ char sep1 = input.charAt(2);
+ validateSeparator(sep1);
+
+ int day = parsePart(input.substring(3, 5), 1, 31, "Day must be between 1 and 31.");
+ char sep2 = input.charAt(5);
+ validateSeparator(sep2);
+
+ int year = parsePart(input.substring(6, 10), 46, 8499, "Year must be between 46 and 8499.");
+
+ try {
+ Objects.requireNonNull(LocalDate.of(year, month, day));
+ } catch (DateTimeException e) {
+ throw new IllegalArgumentException("Invalid date.", e);
+ }
+ if (month <= 2) {
+ year -= 1;
+ month += 12;
+ }
+
+ int century = year / 100;
+ int yearOfCentury = year % 100;
+ int t = (int) (2.6 * month - 5.39);
+ int u = century / 4;
+ int v = yearOfCentury / 4;
+ int f = (int) Math.round((day + yearOfCentury + t + u + v - 2 * century) % 7.0);
+
+ int correctedDay = (f + 7) % 7;
+
+ return "The date " + input + " falls on a " + DAYS[correctedDay] + ".";
+ }
+
+ /**
+ * Parses a part of the date string and validates its range.
+ *
+ * @param part the substring to parse
+ * @param min the minimum valid value
+ * @param max the maximum valid value
+ * @param error the error message to throw if validation fails
+ * @return the parsed integer value
+ * @throws IllegalArgumentException if the part is not a valid number or is out of range
+ */
+ private static int parsePart(String part, int min, int max, String error) {
+ try {
+ int value = Integer.parseInt(part);
+ if (value < min || value > max) {
+ throw new IllegalArgumentException(error);
+ }
+ return value;
+ } catch (NumberFormatException e) {
+ throw new IllegalArgumentException("Invalid numeric part: " + part, e);
+ }
+ }
+
+ /**
+ * Validates the separator character in the date string.
+ *
+ * @param sep the separator character
+ * @throws IllegalArgumentException if the separator is not '-' or '/'
+ */
+ private static void validateSeparator(char sep) {
+ if (sep != '-' && sep != '/') {
+ throw new IllegalArgumentException("Date separator must be '-' or '/'.");
+ }
+ }
+}
diff --git a/src/test/java/com/thealgorithms/maths/ZellersCongruenceTest.java b/src/test/java/com/thealgorithms/maths/ZellersCongruenceTest.java
new file mode 100644
index 000000000000..71bcbdc5ed9f
--- /dev/null
+++ b/src/test/java/com/thealgorithms/maths/ZellersCongruenceTest.java
@@ -0,0 +1,36 @@
+package com.thealgorithms.maths;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import java.util.stream.Stream;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.Arguments;
+import org.junit.jupiter.params.provider.MethodSource;
+
+class ZellersCongruenceTest {
+
+ static Stream