Dev: Add Zeller's Congruence utility class to calculate the day of the week (TheAlgorithms#6614)

* Add Zeller's Congruence utility class and unit tests

* fixed identified bugs

* fixed pmd failure

Author: BanulaKumarage

src/main/java/com/thealgorithms/maths/ZellersCongruence.java

@@ -0,0 +1,107 @@
+package com.thealgorithms.maths;
+
+import java.time.DateTimeException;
+import java.time.LocalDate;
+import java.util.Objects;
+
+/**
+ * A utility class for calculating the day of the week for a given date using Zeller's Congruence.
+ *
+ * <p>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".
+ *
+ * <p>This class is final and cannot be instantiated.
+ *
+ * @see <a href="https://en.wikipedia.org/wiki/Zeller%27s_congruence">Wikipedia: Zeller's Congruence</a>
+ */
+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.
+     *
+     * <p>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 '/'.");
+        }
+    }
+}

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<Arguments> validDates() {
+        return Stream.of(Arguments.of("01-01-2000", "Saturday"), Arguments.of("12-25-2021", "Saturday"), Arguments.of("07-04-1776", "Thursday"), Arguments.of("02-29-2020", "Saturday"), Arguments.of("03-01-1900", "Thursday"), Arguments.of("03/01/1900", "Thursday"));
+    }
+
+    static Stream<Arguments> invalidDates() {
+        return Stream.of(Arguments.of("13-01-2000", "Month must be between 1 and 12."), Arguments.of("02-30-2020", "Invalid date."), Arguments.of("00-15-2020", "Month must be between 1 and 12."), Arguments.of("01-01-0000", "Year must be between 46 and 8499."),
+            Arguments.of("01/01/200", "Input date must be 10 characters long in the format MM-DD-YYYY or MM/DD/YYYY."), Arguments.of("01@01>2000", "Date separator must be '-' or '/'."), Arguments.of("aa-01-1900", "Invalid numeric part: aa"),
+            Arguments.of(null, "Input date must be 10 characters long in the format MM-DD-YYYY or MM/DD/YYYY."));
+    }
+
+    @ParameterizedTest
+    @MethodSource("validDates")
+    void testValidDates(String inputDate, String expectedDay) {
+        String result = ZellersCongruence.calculateDay(inputDate);
+        assertEquals("The date " + inputDate + " falls on a " + expectedDay + ".", result);
+    }
+
+    @ParameterizedTest
+    @MethodSource("invalidDates")
+    void testInvalidDates(String inputDate, String expectedErrorMessage) {
+        Exception exception = assertThrows(IllegalArgumentException.class, () -> ZellersCongruence.calculateDay(inputDate));
+        assertEquals(expectedErrorMessage, exception.getMessage());
+    }
+}