Adding check digit utilities

Author: RobFaustLZ

build.gradle

@@ -6,7 +6,7 @@ plugins {
 }
 
 group = 'com.labelzoom.api'
-version = '1.3.0'
+version = '1.3.1'
 
 java {
     sourceCompatibility = JavaVersion.VERSION_1_8

src/main/java/com/labelzoom/api/util/CheckDigitUtils.java

@@ -0,0 +1,85 @@
+package com.labelzoom.api.util;
+
+public class CheckDigitUtils
+{
+    public enum CheckDigitType
+    {
+        MOD10,
+    }
+
+    public static int getCheckDigit(final String barcode, final CheckDigitType type)
+    {
+        switch (type)
+        {
+            case MOD10: return getMod10CheckDigit(barcode);
+            default: throw new IllegalArgumentException("Invalid check digit type");
+        }
+    }
+
+    /**
+     * <p>
+     * <a href="https://www.axicon.com/checkdigitcalculator.html">MOD10 Check Digit Calculator</a>
+     * </p>
+     *
+     * <p>
+     * How to calculate your check digit yourself
+     * </p>
+     *
+     * <p>
+     * Example barcode number: 501234576421
+     * </p>
+     *
+     * <p>
+     * Step 1: add together all alternate numbers starting from the right<br>
+     * <code>5 0 1 2 3 4 5 7 6 4 2 1</code><br>
+     * <code>0 + 2 + 4 + 7 + 4 + 1 = 18</code>
+     * </p>
+     *
+     * <p>
+     * Step 2: multiply the answer by 3<br>
+     * <code>18 x 3 = 54</code>
+     * </p>
+     *
+     * <p>
+     * Step 3: now add together the remaining numbers<br>
+     * <code>5 0 1 2 3 4 5 7 6 4 2 1</code><br>
+     * <code>5 + 1 + 3 + 5 + 6 + 2 = 22</code>
+     * </p>
+     *
+     * <p>
+     * Step 4: add step 2 and 3 together<br>
+     * <code>54 + 22 = 76</code>
+     * </p>
+     *
+     * <p>
+     * Step 5: the difference between step 4 and the next 10th number:<br>
+     * <code>76 + 4 = 80</code><br>
+     * Check digit = <code>4</code>
+     * </p>
+     *
+     * @param barcode the barcode data
+     * @return the check digit
+     */
+    private static int getMod10CheckDigit(final String barcode)
+    {
+        char[] digits = barcode.toCharArray();
+        /* Sum odds and evens separately so that we only perform one multiplication. In practice, one large
+         * multiplication was faster than several small multiplications
+         */
+        int evens = 0;
+        int odds = 0;
+        boolean isEven = false; // Use alternating boolean variable rather than modular division (e.g., i % 2)
+        for (int i = digits.length - 1; i >= 0; i--)
+        {
+            if (isEven = !isEven) // Invert and update value as we read it. Ignore IntelliJ, it doesn't appreciate my l33tness
+            {
+                evens += Character.getNumericValue(digits[i]);
+            }
+            else
+            {
+                odds += Character.getNumericValue(digits[i]);
+            }
+        }
+        return 10 - (((evens * 3) + odds) % 10);
+    }
+}

src/test/java/com/labelzoom/api/util/CheckDigitUtilsTests.java

@@ -0,0 +1,17 @@
+package com.labelzoom.api.util;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+class CheckDigitUtilsTests
+{
+    @Test
+    void testAxiconExample() { assertEquals(4, CheckDigitUtils.getCheckDigit("501234576421", CheckDigitUtils.CheckDigitType.MOD10)); }
+
+    @Test
+    void testSsccExample() { assertEquals(8, CheckDigitUtils.getCheckDigit("0000123456000000001", CheckDigitUtils.CheckDigitType.MOD10)); }
+
+    @Test
+    void testCode128Example() { assertEquals(8, CheckDigitUtils.getCheckDigit("0008100887950411637", CheckDigitUtils.CheckDigitType.MOD10)); }
+}