feat: add Difference Array algorithm implementation and tests (#7244)

src/test/java/com/thealgorithms/prefixsum/DifferenceArrayTest.java

@@ -0,0 +1,110 @@
+package com.thealgorithms.prefixsum;
+
+import static org.junit.jupiter.api.Assertions.assertArrayEquals;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import org.junit.jupiter.api.Test;
+
+class DifferenceArrayTest {
+
+    @Test
+    void testStandardRangeUpdate() {
+        int[] input = {10, 20, 30, 40, 50};
+        DifferenceArray da = new DifferenceArray(input);
+
+        da.update(1, 3, 5);
+
+        long[] expected = {10, 25, 35, 45, 50};
+        assertArrayEquals(expected, da.getResultArray());
+    }
+
+    @Test
+    void testMultipleOverlappingUpdates() {
+        int[] input = {10, 10, 10, 10, 10};
+        DifferenceArray da = new DifferenceArray(input);
+
+        da.update(0, 2, 10);
+        da.update(2, 4, 20);
+
+        long[] expected = {20, 20, 40, 30, 30};
+        assertArrayEquals(expected, da.getResultArray());
+    }
+
+    @Test
+    void testIntegerOverflowSafety() {
+        int[] input = {Integer.MAX_VALUE, 100};
+        DifferenceArray da = new DifferenceArray(input);
+
+        da.update(0, 0, 100);
+
+        long[] result = da.getResultArray();
+        long expectedVal = (long) Integer.MAX_VALUE + 100;
+
+        assertEquals(expectedVal, result[0]);
+    }
+
+    @Test
+    void testFullRangeUpdate() {
+        int[] input = {1, 2, 3};
+        DifferenceArray da = new DifferenceArray(input);
+
+        da.update(0, 2, 100);
+
+        long[] expected = {101, 102, 103};
+        assertArrayEquals(expected, da.getResultArray());
+    }
+
+    @Test
+    void testBoundaryWriteOptimization() {
+        int[] input = {5, 5};
+        DifferenceArray da = new DifferenceArray(input);
+
+        da.update(1, 1, 5);
+
+        long[] expected = {5, 10};
+
+        assertArrayEquals(expected, da.getResultArray());
+    }
+
+    @Test
+    void testLargeMassiveUpdate() {
+        int[] input = {0};
+        DifferenceArray da = new DifferenceArray(input);
+
+        int iterations = 100000;
+        for (int i = 0; i < iterations; i++) {
+            da.update(0, 0, 1);
+        }
+
+        assertEquals(100000L, da.getResultArray()[0]);
+    }
+
+    @Test
+    void testNullInputThrowsException() {
+        assertThrows(IllegalArgumentException.class, () -> new DifferenceArray(null));
+    }
+
+    @Test
+    void testEmptyInputThrowsException() {
+        assertThrows(IllegalArgumentException.class, () -> new DifferenceArray(new int[] {}));
+    }
+
+    @Test
+    void testInvalidRangeNegativeIndex() {
+        DifferenceArray da = new DifferenceArray(new int[] {1, 2, 3});
+        assertThrows(IllegalArgumentException.class, () -> da.update(-1, 1, 5));
+    }
+
+    @Test
+    void testInvalidRangeOutOfBounds() {
+        DifferenceArray da = new DifferenceArray(new int[] {1, 2, 3});
+        assertThrows(IllegalArgumentException.class, () -> da.update(0, 3, 5));
+    }
+
+    @Test
+    void testInvalidRangeStartGreaterThanEnd() {
+        DifferenceArray da = new DifferenceArray(new int[] {1, 2, 3});
+        assertThrows(IllegalArgumentException.class, () -> da.update(2, 1, 5));
+    }
+}