Add ObjectIdSerializer

This is based on the ObjectIdSerialization class written by Shawn Pearce
for the Gerrit Code Review project in 2009 [1]. As mentioned in the
commit message there, it should be part of core JGit.

This implementation is slightly different to Shawn's version. Rather
than having separate methods for null/non-null ids, single methods are
implemented with @Nullable annotations.

[1] https://gerrit-review.googlesource.com/c/gerrit/+/9792

Change-Id: I7599cf8bd1ecd546e2252783d6d672eb76804060
Signed-off-by: David Pursehouse <david.pursehouse@gmail.com>
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

org.eclipse.jgit.test/tst/org/eclipse/jgit/lib/ObjectIdSerializerTest.java

@@ -0,0 +1,88 @@
+/*
+ * Copyright (C) 2018, David Pursehouse <david.pursehouse@gmail.com>
+ * and other copyright owners as documented in the project's IP log.
+ *
+ * This program and the accompanying materials are made available
+ * under the terms of the Eclipse Distribution License v1.0 which
+ * accompanies this distribution, is reproduced below, and is
+ * available at http://www.eclipse.org/org/documents/edl-v10.php
+ *
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or
+ * without modification, are permitted provided that the following
+ * conditions are met:
+ *
+ * - Redistributions of source code must retain the above copyright
+ *   notice, this list of conditions and the following disclaimer.
+ *
+ * - Redistributions in binary form must reproduce the above
+ *   copyright notice, this list of conditions and the following
+ *   disclaimer in the documentation and/or other materials provided
+ *   with the distribution.
+ *
+ * - Neither the name of the Eclipse Foundation, Inc. nor the
+ *   names of its contributors may be used to endorse or promote
+ *   products derived from this software without specific prior
+ *   written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND
+ * CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES,
+ * INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+ * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
+ * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+ * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+ * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
+ * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+package org.eclipse.jgit.lib;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNull;
+
+import java.io.File;
+import java.io.FileInputStream;
+import java.io.FileOutputStream;
+import java.io.InputStream;
+import java.io.OutputStream;
+
+import org.junit.Test;
+
+public class ObjectIdSerializerTest {
+	@Test
+	public void serialize() throws Exception {
+		ObjectId original = new ObjectId(1, 2, 3, 4, 5);
+		ObjectId deserialized = writeAndReadBackFromTempFile(original);
+		assertEquals(original, deserialized);
+	}
+
+	@Test
+	public void serializeZeroId() throws Exception {
+		ObjectId original = ObjectId.zeroId();
+		ObjectId deserialized = writeAndReadBackFromTempFile(original);
+		assertEquals(original, deserialized);
+	}
+
+	@Test
+	public void serializeNull() throws Exception {
+		ObjectId deserialized = writeAndReadBackFromTempFile(null);
+		assertNull(deserialized);
+	}
+
+	private ObjectId writeAndReadBackFromTempFile(ObjectId objectId)
+			throws Exception {
+		File file = File.createTempFile("ObjectIdSerializerTest_", "");
+		try (OutputStream out = new FileOutputStream(file)) {
+			ObjectIdSerializer.write(out, objectId);
+		}
+		try (InputStream in = new FileInputStream(file)) {
+			return ObjectIdSerializer.read(in);
+		}
+	}
+}

org.eclipse.jgit/src/org/eclipse/jgit/lib/ObjectIdSerializer.java

@@ -0,0 +1,107 @@
+/*
+ * Copyright (C) 2009, The Android Open Source Project
+ * Copyright (C) 2009, Shawn O. Pearce <spearce@spearce.org>
+ * Copyright (C) 2018, David Pursehouse <david.pursehouse@gmail.com>
+ * and other copyright owners as documented in the project's IP log.
+ *
+ * This program and the accompanying materials are made available
+ * under the terms of the Eclipse Distribution License v1.0 which
+ * accompanies this distribution, is reproduced below, and is
+ * available at http://www.eclipse.org/org/documents/edl-v10.php
+ *
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or
+ * without modification, are permitted provided that the following
+ * conditions are met:
+ *
+ * - Redistributions of source code must retain the above copyright
+ *   notice, this list of conditions and the following disclaimer.
+ *
+ * - Redistributions in binary form must reproduce the above
+ *   copyright notice, this list of conditions and the following
+ *   disclaimer in the documentation and/or other materials provided
+ *   with the distribution.
+ *
+ * - Neither the name of the Eclipse Foundation, Inc. nor the
+ *   names of its contributors may be used to endorse or promote
+ *   products derived from this software without specific prior
+ *   written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND
+ * CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES,
+ * INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+ * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
+ * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+ * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+ * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
+ * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+package org.eclipse.jgit.lib;
+
+import static org.eclipse.jgit.lib.Constants.OBJECT_ID_LENGTH;
+
+import java.io.IOException;
+import java.io.InputStream;
+import java.io.OutputStream;
+
+import org.eclipse.jgit.annotations.Nullable;
+import org.eclipse.jgit.util.IO;
+
+/**
+ * Helper to serialize {@link ObjectId} instances. {@link ObjectId} is already
+ * serializable, but this class provides a more optimal implementation. It
+ * writes out a flag (0 or 1) followed by the object's words, or nothing if it
+ * was a null id.
+ *
+ * @since 4.11
+ */
+public class ObjectIdSerializer {
+	/**
+	 * @param out
+	 *            the output stream
+	 * @param id
+	 *            the object id to serialize
+	 * @throws IOException
+	 *             the stream writing failed
+	 */
+	public static void write(OutputStream out, @Nullable AnyObjectId id)
+			throws IOException {
+		if (id != null) {
+			out.write((byte) 1);
+			id.copyRawTo(out);
+		} else {
+			out.write((byte) 0);
+		}
+	}
+
+	/**
+	 * @param in
+	 *            the input stream
+	 * @return the object id
+	 * @throws IOException
+	 *             there was an error reading the stream
+	 */
+	@Nullable
+	public static ObjectId read(InputStream in) throws IOException {
+		switch (in.read()) {
+		case 0:
+			return null;
+		case 1:
+			final byte[] b = new byte[OBJECT_ID_LENGTH];
+			IO.readFully(in, b, 0, OBJECT_ID_LENGTH);
+			return ObjectId.fromRaw(b);
+		default:
+			throw new IOException("Invalid flag before ObjectId"); //$NON-NLS-1$
+		}
+	}
+
+	private ObjectIdSerializer() {
+	}
+}