Change project structure to a Maven Java project + Refactor (#2816)

src/main/java/com/thealgorithms/others/Luhn.java

@@ -0,0 +1,163 @@
+package com.thealgorithms.others;
+
+import java.util.Arrays;
+import java.util.Objects;
+
+/**
+ * The Luhn algorithm or Luhn formula, also known as the "modulus 10" or "mod
+ * 10" algorithm, named after its creator, IBM scientist Hans Peter Luhn, is a
+ * simple checksum formula used to validate a variety of identification numbers.
+ *
+ * <p>
+ * The algorithm is in the public domain and is in wide use today. It is
+ * specified in ISO/IEC 7812-1. It is not intended to be a cryptographically
+ * secure hash function; it was designed to protect against accidental errors,
+ * not malicious attacks. Most credit cards and many government identification
+ * numbers use the algorithm as a simple method of distinguishing valid numbers
+ * from mistyped or otherwise incorrect numbers.</p>
+ *
+ * <p>
+ * The Luhn algorithm will detect any single-digit error, as well as almost all
+ * transpositions of adjacent digits. It will not, however, detect transposition
+ * of the two-digit sequence 09 to 90 (or vice versa). It will detect most of
+ * the possible twin errors (it will not detect 22 ↔ 55, 33 ↔ 66 or 44 ↔
+ * 77).</p>
+ *
+ * <p>
+ * The check digit is computed as follows:</p>
+ * <ol>
+ * <li>Take the original number and starting from the rightmost digit moving
+ * left, double the value of every second digit (including the rightmost
+ * digit).</li>
+ * <li>Replace the resulting value at each position with the sum of the digits
+ * of this position's value or just subtract 9 from all numbers more or equal
+ * then 10.</li>
+ * <li>Sum up the resulting values from all positions (s).</li>
+ * <li>The calculated check digit is equal to {@code 10 - s % 10}.</li>
+ * </ol>
+ *
+ * @see <a href="https://en.wikipedia.org/wiki/Luhn_algorithm">Wiki</a>
+ */
+public class Luhn {
+
+    /**
+     * Check input digits array by Luhn algorithm. Initial array doesn't change
+     * while processing.
+     *
+     * @param digits array of digits from 0 to 9
+     * @return true if check was successful, false otherwise
+     */
+    public static boolean luhnCheck(int[] digits) {
+        int[] numbers = Arrays.copyOf(digits, digits.length);
+        int sum = 0;
+
+        for (int i = numbers.length - 1; i >= 0; i--) {
+            if (i % 2 == 0) {
+                int temp = numbers[i] * 2;
+                if (temp > 9) {
+                    temp = temp - 9;
+                }
+                numbers[i] = temp;
+            }
+            sum += numbers[i];
+        }
+
+        return sum % 10 == 0;
+    }
+
+    public static void main(String[] args) {
+        System.out.println("Luhn algorithm usage examples:");
+        int[] validInput = {4, 5, 6, 1, 2, 6, 1, 2, 1, 2, 3, 4, 5, 4, 6, 7};
+        int[] invalidInput = {4, 5, 6, 1, 2, 6, 1, 2, 1, 2, 3, 4, 5, 4, 6, 4}; //typo in last symbol
+        checkAndPrint(validInput);
+        checkAndPrint(invalidInput);
+
+        System.out.println("\nBusiness examples:");
+        String validCardNumber = "5265 9251 6151 1412";
+        String invalidCardNumber = "4929 3231 3088 1896";
+        String illegalCardNumber = "4F15 BC06 3A88 76D5";
+        businessExample(validCardNumber);
+        businessExample(invalidCardNumber);
+        businessExample(illegalCardNumber);
+    }
+
+    private static void checkAndPrint(int[] input) {
+        String validationResult = Luhn.luhnCheck(input)
+                ? "valid"
+                : "not valid";
+        System.out.println("Input " + Arrays.toString(input) + " is " + validationResult);
+    }
+
+
+    /*
+        ========================
+         Business usage example
+        ========================
+     */
+    /**
+     * Object representation of credit card.
+     */
+    private record CreditCard(int[] digits) {
+
+        private static final int DIGITS_COUNT = 16;
+
+        /**
+         * @param cardNumber string representation of credit card number - 16
+         * digits. Can have spaces for digits separation
+         * @return credit card object
+         * @throws IllegalArgumentException if input string is not 16 digits or
+         * if Luhn check was failed
+         */
+        public static CreditCard fromString(String cardNumber) {
+            Objects.requireNonNull(cardNumber);
+            String trimmedCardNumber = cardNumber.replaceAll(" ", "");
+            if (trimmedCardNumber.length() != DIGITS_COUNT || !trimmedCardNumber.matches("\\d+")) {
+                throw new IllegalArgumentException("{" + cardNumber + "} - is not a card number");
+            }
+
+            int[] cardNumbers = toIntArray(trimmedCardNumber);
+            boolean isValid = luhnCheck(cardNumbers);
+            if (!isValid) {
+                throw new IllegalArgumentException("Credit card number {" + cardNumber + "} - have a typo");
+            }
+
+            return new CreditCard(cardNumbers);
+        }
+
+        /**
+         * @return string representation separated by space every 4 digits.
+         * Example: "5265 9251 6151 1412"
+         */
+        public String number() {
+            StringBuilder result = new StringBuilder();
+            for (int i = 0; i < DIGITS_COUNT; i++) {
+                if (i % 4 == 0 && i != 0) {
+                    result.append(" ");
+                }
+                result.append(digits[i]);
+            }
+            return result.toString();
+        }
+
+        @Override
+        public String toString() {
+            return String.format("%s {%s}", CreditCard.class.getSimpleName(), number());
+        }
+
+        private static int[] toIntArray(String string) {
+            return string.chars()
+                    .map(i -> Character.digit(i, 10))
+                    .toArray();
+        }
+    }
+
+    private static void businessExample(String cardNumber) {
+        try {
+            System.out.println("Trying to create CreditCard object from valid card number: " + cardNumber);
+            CreditCard creditCard = CreditCard.fromString(cardNumber);
+            System.out.println("And business object is successfully created: " + creditCard + "\n");
+        } catch (IllegalArgumentException e) {
+            System.out.println("And fail with exception message: " + e.getMessage() + "\n");
+        }
+    }
+}