feature: Added LuhnModnSnippet class to Algorithm package

README.md

@@ -279,6 +279,97 @@ public class LinearSearchSnippet {
   }
 }
 ```
+### Luhn Mod N
+
+
+```java
+public class LuhnModnSnippet {
+
+  private static final String CODE_POINTS = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";
+
+  /**
+   * Generates a check character using the Luhn mod N algorithm.
+   *
+   * @param character the input string consisting of valid alphanumeric characters
+   * @return the generated check character
+   * @throws IllegalArgumentException if the input contains invalid characters
+   */
+  public static int codePointFromCharacter(char character) {
+    if (CODE_POINTS.indexOf(character) == -1) {
+      throw new IllegalArgumentException("Invalid character: " + character);
+    }
+    return CODE_POINTS.indexOf(character);
+  }
+
+  /**
+   * Converts a code point to its corresponding character.
+   *
+   * @param codePoint the code point to be converted
+   * @return the character representation of the code point
+   * @throws IllegalArgumentException if the code point is out of range.
+   */
+  public static char characterFromCodePoint(int codePoint) {
+    if (codePoint < 0 || codePoint >= CODE_POINTS.length()) {
+      throw new IllegalArgumentException("Invalid code point: " + codePoint);
+    }
+    return CODE_POINTS.charAt(codePoint);
+  }
+
+  public static int numberOfValidInputCharacters() {
+    return CODE_POINTS.length();
+  }
+
+  /**
+   * Helper method to calculate the sum for both check character generation and validation.
+   *
+   * @param input the input string
+   * @param factorStart the initial factor to start with (1 or 2)
+   * @return the calculated sum, reminder, and the numberOfValidInputCharacters
+   */
+  private static int[] calculateSum(String input, int factorStart) {
+    if (input == null || input.isEmpty()) {
+      throw new IllegalArgumentException("Input cannot be empty");
+    }
+
+    int factor = factorStart;
+    int sum = 0;
+    int n = numberOfValidInputCharacters();
+
+    for (int i = input.length() - 1; i >= 0; i--) {
+      int codePoint = codePointFromCharacter(input.charAt(i));
+      int addend = factor * codePoint;
+      factor = (factor == 2) ? 1 : 2;
+      addend = (addend / n) + (addend % n);
+      sum += addend;
+    }
+    return new int[]{sum, sum % n, n};
+  }
+
+  /**
+   * Generates a check character for the given input string using the Luhn mod N algorithm.
+   *
+   * @param input the input string (non-empty)
+   * @return the generated check character
+   * @throws IllegalArgumentException if the input is null or empty
+   */
+  public static char generateCheckCharacter(String input) {
+    int[] result = calculateSum(input, 2);
+    return characterFromCodePoint((result[2] - result[1]) % result[2]);
+  }
+
+  /**
+   * Validates a check character by applying the Luhn mod N algorithm.
+   *
+   * @param input the input string (including the check character)
+   * @return true if the input passes validation, false otherwise
+   * @throws IllegalArgumentException if the input is null or empty
+   */
+  public static boolean validateCheckCharacter(String input) {
+    int[] result = calculateSum(input, 1);
+    return (result[1] == 0);
+  }
+}
+```
 
 ### Merge Sort
 

src/main/java/algorithm/LuhnModnSnippet.java

@@ -0,0 +1,115 @@
+/*
+ * MIT License
+ *
+ * Copyright (c) 2017-2022 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+package algorithm;
+
+/**
+ * LuhnModnSnippet.
+ */
+public class LuhnModnSnippet {
+
+  private static final String CODE_POINTS = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";
+
+  /**
+   * Generates a check character using the Luhn mod N algorithm.
+   *
+   * @param character the input string consisting of valid alphanumeric characters
+   * @return the generated check character
+   * @throws IllegalArgumentException if the input contains invalid characters
+   */
+  public static int codePointFromCharacter(char character) {
+    if (CODE_POINTS.indexOf(character) == -1) {
+      throw new IllegalArgumentException("Invalid character: " + character);
+    }
+    return CODE_POINTS.indexOf(character);
+  }
+
+  /**
+   * Converts a code point to its corresponding character.
+   *
+   * @param codePoint the code point to be converted
+   * @return the character representation of the code point
+   * @throws IllegalArgumentException if the code point is out of range.
+   */
+  public static char characterFromCodePoint(int codePoint) {
+    if (codePoint < 0 || codePoint >= CODE_POINTS.length()) {
+      throw new IllegalArgumentException("Invalid code point: " + codePoint);
+    }
+    return CODE_POINTS.charAt(codePoint);
+  }
+
+  public static int numberOfValidInputCharacters() {
+    return CODE_POINTS.length();
+  }
+
+  /**
+   * Helper method to calculate the sum for both check character generation and validation.
+   *
+   * @param input the input string
+   * @param factorStart the initial factor to start with (1 or 2)
+   * @return the calculated sum, reminder, and the numberOfValidInputCharacters
+   */
+  private static int[] calculateSum(String input, int factorStart) {
+    if (input == null || input.isEmpty()) {
+      throw new IllegalArgumentException("Input cannot be empty");
+    }
+
+    int factor = factorStart;
+    int sum = 0;
+    int n = numberOfValidInputCharacters();
+
+    for (int i = input.length() - 1; i >= 0; i--) {
+      int codePoint = codePointFromCharacter(input.charAt(i));
+      int addend = factor * codePoint;
+      factor = (factor == 2) ? 1 : 2;
+      addend = (addend / n) + (addend % n);
+      sum += addend;
+    }
+    return new int[]{sum, sum % n, n};
+  }
+
+  /**
+   * Generates a check character for the given input string using the Luhn mod N algorithm.
+   *
+   * @param input the input string (non-empty)
+   * @return the generated check character
+   * @throws IllegalArgumentException if the input is null or empty
+   */
+  public static char generateCheckCharacter(String input) {
+    int[] result = calculateSum(input, 2);
+    return characterFromCodePoint((result[2] - result[1]) % result[2]);
+  }
+
+  /**
+   * Validates a check character by applying the Luhn mod N algorithm.
+   *
+   * @param input the input string (including the check character)
+   * @return true if the input passes validation, false otherwise
+   * @throws IllegalArgumentException if the input is null or empty
+   */
+  public static boolean validateCheckCharacter(String input) {
+    int[] result = calculateSum(input, 1);
+    return (result[1] == 0);
+  }
+}

src/test/java/algorithm/LuhnModnSnippetTest.java

@@ -0,0 +1,85 @@
+/*
+ * MIT License
+ *
+ * Copyright (c) 2017-2022 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+package algorithm;
+
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+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;
+
+/**
+ * Tests for {@link LuhnModnSnippet#generateCheckCharacter(String)} and
+ * {@link LuhnModnSnippet#validateCheckCharacter(String)}.
+ */
+class LuhnModnSnippetTest {
+
+  /**
+   * Tests the full process of generating and validating a check character.
+   */
+  @ParameterizedTest
+  @MethodSource("validInputProvider")
+  public void testGenerateCheckCharacter(String input) {
+    char checkCharacter = LuhnModnSnippet.generateCheckCharacter(input);
+    String fullInput = input + checkCharacter;
+    assertTrue(LuhnModnSnippet.validateCheckCharacter(fullInput),
+            "Validation should pass for the generated check character.");
+  }
+
+  @ParameterizedTest
+  @MethodSource("invalidInputProvider")
+  public void testInvalidInputs(String input, Character checkCharacter, boolean throwException) {
+    if (throwException) {
+      assertThrows(IllegalArgumentException.class, () ->
+                      LuhnModnSnippet.generateCheckCharacter(input),
+              "Exception should be thrown for invalid input.");
+      assertThrows(IllegalArgumentException.class, () ->
+                      LuhnModnSnippet.validateCheckCharacter(input),
+              "Exception should be thrown for invalid input.");
+    } else {
+      String fullInput = input + checkCharacter;
+      assertFalse(LuhnModnSnippet.validateCheckCharacter(fullInput),
+              "Validation should fail for a mismatched check character.");
+    }
+  }
+
+  private static Stream<String> validInputProvider() {
+    return Stream.of(
+            "HELLO",
+            "12345",
+            "A1B2C3"
+    );
+  }
+
+  private static Stream<Arguments> invalidInputProvider() {
+    return Stream.of(
+            Arguments.of("", null, true),
+            Arguments.of("WORLD", 'A', false)
+    );
+  }
+}