add() and sub() byte-based methods introduced

Author: bosik

portable/comp-transfer/src/main/java/org/bosik/merklesync/HashUtils.java

@@ -17,6 +17,7 @@
 package org.bosik.merklesync;
 
 import java.util.ArrayList;
+import java.util.Arrays;
 import java.util.List;
 import java.util.Locale;
 import java.util.Map.Entry;
@@ -196,6 +197,81 @@ public static String sum(Iterable<String> hashes)
 		return result;
 	}
 
+	public static byte[] add(byte[] a, byte[] b)
+	{
+		if (b == null)
+		{
+			return a;
+		}
+
+		if (a == null)
+		{
+			return Arrays.copyOf(b, b.length);
+		}
+
+		if (a.length != b.length)
+		{
+			throw new IllegalArgumentException(String.format(
+					Locale.US,
+					"Mismatched array size: %d != %d",
+					a.length,
+					b.length
+			));
+		}
+
+		for (int i = 0; i < a.length; i++)
+		{
+			int highA = (a[i] + 128) / 16;
+			int lowA = (a[i] + 128) % 16;
+
+			int highB = (b[i] + 128) / 16;
+			int lowB = (b[i] + 128) % 16;
+
+			final int highC = (highA + highB) % 16;
+			final int lowC = (lowA + lowB) % 16;
+
+			a[i] = (byte) ((highC * 16 + lowC + 256) % 256);
+		}
+
+		return a;
+	}
+
+	public static byte[] sub(byte[] a, byte[] b)
+	{
+		if (b == null) // a - 0
+		{
+			return a;
+		}
+
+		if (a != null && a.length != b.length)
+		{
+			throw new IllegalArgumentException(String.format(
+					Locale.US,
+					"Mismatched array size: %d != %d",
+					a.length,
+					b.length
+			));
+		}
+
+		final byte[] c = new byte[b.length];
+
+		for (int i = 0; i < b.length; i++)
+		{
+			final int highA = a == null ? 0 : ((a[i] + 256) % 256) / 16;
+			final int lowA = a == null ? 0 : ((a[i] + 256) % 256) % 16;
+
+			final int highB = ((b[i] + 256) % 256) / 16;
+			final int lowB = ((b[i] + 256) % 256) % 16;
+
+			final int highC = (16 + highA - highB) % 16;
+			final int lowC = (16 + lowA - lowB) % 16;
+
+			c[i] = (byte) ((highC * 16 + lowC + 256) % 256);
+		}
+
+		return c;
+	}
+
 	// --------------------------------------------------------------------------------------
 	// MERKLE TREE OPERATIONS
 	// --------------------------------------------------------------------------------------

portable/comp-transfer/src/test/java/org/bosik/merklesync/TestHashUtils.java

@@ -27,8 +27,8 @@
 import java.util.SortedMap;
 import java.util.TreeMap;
 
-import static junit.framework.Assert.fail;
-import static junit.framework.TestCase.assertEquals;
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.fail;
 
 @SuppressWarnings("static-method")
 public class TestHashUtils
@@ -176,6 +176,71 @@ public void test_sumStrings()
 				HashUtils.sum("88888888888888888888888888888888", "88888888888888888888888888888888"));
 	}
 
+	@Test
+	public void test_subHash()
+	{
+		// corner cases
+		assertArraysEquals(new byte[] {}, HashUtils.sub(new byte[] {}, new byte[] {}));
+		assertArraysEquals(new byte[] { 1, 2, 3 }, HashUtils.sub(new byte[] { 1, 2, 3 }, null));
+
+		// "000000" - "010203" = "0f0e0d"
+		assertArraysEquals(new byte[] { 15, 14, 13 }, HashUtils.sub(new byte[] { 0, 0, 0 }, new byte[] { 1, 2, 3 }));
+		assertArraysEquals(new byte[] { 15, 14, 13 }, HashUtils.sub(null, new byte[] { 1, 2, 3 }));
+
+		// "0f0e0d" - "000000" = "0f0e0d"
+		assertArraysEquals(new byte[] { 15, 14, 13 }, HashUtils.sub(new byte[] { 15, 14, 13 }, new byte[] { 0, 0, 0 }));
+
+		// normal cases
+		assertArraysEquals(new byte[] { 0, 0, 0 }, HashUtils.sub(new byte[] { 1, 2, 3 }, new byte[] { 1, 2, 3 }));
+		// "7f" - "ff" = "80"
+		assertArraysEquals(new byte[] { -128 }, HashUtils.sub(new byte[] { 127 }, new byte[] { -1 }));
+
+		// "7f" = 127
+		// "02" = 2
+		// "01" = 1
+		// "00" = 0
+		// "ff" = -1
+		// "fe" = -2
+		// ..
+		// "82" = -126
+		// "81" = -127
+		// "80" = -128
+
+		assertArraysEquals(HashUtils.strToByte("8f"), HashUtils.sub(
+				HashUtils.strToByte("80"),
+				HashUtils.strToByte("01")
+		));
+	}
+
+	@Test
+	public void test_addSubHash()
+	{
+		// given
+		final byte[] a = { 127, -128, 0 };
+		final byte[] b = { 4, 5, 6 };
+
+		// when / then
+		assertArraysEquals(new byte[] { 127, -128, 0 }, HashUtils.sub(HashUtils.add(a, b), b));
+	}
+
+	@Test
+	public void test_addHash()
+	{
+		// given
+		final byte[] data = new byte[] { 0, 0, 0, 0 };
+
+		// when
+		HashUtils.add(data, null);
+		HashUtils.add(data, new byte[] { 0, 0, 0, 0 });
+		HashUtils.add(data, new byte[] { 1, 2, 3, 4 });
+
+		// then
+		assertEquals(
+				HashUtils.byteToStr(new byte[] { 1, 2, 3, 4 }),
+				HashUtils.byteToStr(data)
+		);
+	}
+
 	@Test
 	public void test_sumIterable()
 	{