BAEL-4920 - How BigInteger works (#11016)

Co-authored-by: Mateusz Szablak <mateusz.szablak@pekao.com.pl>

java-numbers-4/src/test/java/com/baeldung/biginteger/BigIntegerUnitTest.java

@@ -0,0 +1,76 @@
+package com.baeldung.biginteger;
+
+import org.junit.jupiter.api.Test;
+
+import java.math.BigInteger;
+import java.nio.ByteBuffer;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+public class BigIntegerUnitTest {
+
+    @Test
+    void givenPositiveAndNegativeAndZeroBigInteger_whenGetSigNumValue_shouldReturnOneAndMinusOneAndZero() {
+        assertEquals(1, BigInteger.TEN.signum());
+        assertEquals(-1, BigInteger.TEN.negate().signum());
+        assertEquals(0, BigInteger.ZERO.signum());
+    }
+
+    @Test
+    void givenByteArrays_whenCreateBigInteger_shouldTranslateToTwosComplementBinary() {
+        assertEquals(new BigInteger("1"), new BigInteger(new byte[]{0b1}));
+        assertEquals(new BigInteger("2"), new BigInteger(new byte[]{0b10}));
+        assertEquals(new BigInteger("4"), new BigInteger(new byte[]{0b100}));
+    }
+
+    @Test
+    void givenSingleByte_whenCreateBigIntegerAndDifferentSigNum_thenOppositeValues() {
+        byte[] bytes = { (byte) -128 }; // 0b1000_0000
+
+        BigInteger positive = new BigInteger(1, bytes);
+        BigInteger negative = new BigInteger(-1, bytes);
+
+        assertEquals(new BigInteger("128"), positive);
+        assertEquals("10000000", positive.toString(2));
+
+        assertEquals(new BigInteger("-128"), negative);
+        assertEquals("-10000000", negative.toString(2));
+    }
+
+    @Test
+    void givenZeroBigInteger_whenCheckMagnitude_thenIsEmpty() {
+        assertEquals(0, BigInteger.ZERO.bitCount());
+        assertEquals(BigInteger.ZERO, new BigInteger(0, new byte[]{}));
+    }
+
+    @Test
+    void given63rdBitSet_whenCreateBigInteger_thenIsLargerThanLongByOne() {
+        // first
+        BigInteger bi1 = BigInteger.ZERO.setBit(63);
+        String str1 = bi1.toString(2);
+
+        // second
+        byte[] bytes = ByteBuffer.allocate(Long.BYTES).putLong(Long.MIN_VALUE).array();
+        BigInteger bi2 = new BigInteger(1, bytes);
+        String str2 = bi2.toString(2);
+
+        largerThanLongAssertionSet(bi1, str1);
+
+        assertEquals(bi1, bi2);
+
+        largerThanLongAssertionSet(bi2, str2);
+
+    }
+
+    private static void largerThanLongAssertionSet(BigInteger bi, String str)
+    {
+        assertEquals(64, bi.bitLength());
+        assertEquals(1, bi.signum());
+        assertEquals("9223372036854775808", bi.toString());
+        assertEquals(BigInteger.ONE,  bi.subtract(BigInteger.valueOf(Long.MAX_VALUE)));
+
+        assertEquals(64, str.length());
+        assertTrue(str.matches("^10{63}$")); // 1000 0000
+    }
+}