Preparations for file content encryption (no partial support yet)

main/filesystem-api/src/main/java/org/cryptomator/filesystem/WritableBytes.java

@@ -24,7 +24,7 @@ public interface WritableBytes {
 
 	/**
 	 * Writes the data in the given byte buffer to this readable bytes at the
-	 * given position.
+	 * given position, overwriting existing content (not inserting).
 	 * 
 	 * @param target
 	 *            the byte buffer to use

main/filesystem-api/src/main/java/org/cryptomator/io/ByteBuffers.java

@@ -0,0 +1,30 @@
+package org.cryptomator.io;
+
+import java.nio.ByteBuffer;
+
+/**
+ * TODO this probably doesn't belong into this maven module, but it is used by various filesystem layers.
+ */
+public final class ByteBuffers {
+
+	private ByteBuffers() {
+	}
+
+	/**
+	 * Copies as many bytes as possible from the given source to the destination buffer.
+	 * The position of both buffers will be incremented by as many bytes as have been copied.
+	 * 
+	 * @param source ByteBuffer from which bytes are read
+	 * @param destination ByteBuffer into which bytes are written
+	 * @return number of bytes copied, i.e. {@link ByteBuffer#remaining() source.remaining()} or {@link ByteBuffer#remaining() destination.remaining()}, whatever is less.
+	 */
+	public static int copy(ByteBuffer source, ByteBuffer destination) {
+		final int numBytes = Math.min(source.remaining(), destination.remaining());
+		final ByteBuffer tmp = source.asReadOnlyBuffer();
+		tmp.limit(tmp.position() + numBytes);
+		destination.put(tmp);
+		source.position(tmp.position());
+		return numBytes;
+	}
+
+}

main/filesystem-api/src/test/java/org/cryptomator/io/ByteBuffersTest.java

@@ -0,0 +1,73 @@
+package org.cryptomator.io;
+
+import java.nio.ByteBuffer;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+public class ByteBuffersTest {
+
+	@Test
+	public void testCopyOfEmptySource() {
+		final ByteBuffer src = ByteBuffer.allocate(0);
+		final ByteBuffer dst = ByteBuffer.allocate(5);
+		dst.put(new byte[3]);
+		Assert.assertEquals(0, src.position());
+		Assert.assertEquals(0, src.remaining());
+		Assert.assertEquals(3, dst.position());
+		Assert.assertEquals(2, dst.remaining());
+		ByteBuffers.copy(src, dst);
+		Assert.assertEquals(0, src.position());
+		Assert.assertEquals(0, src.remaining());
+		Assert.assertEquals(3, dst.position());
+		Assert.assertEquals(2, dst.remaining());
+	}
+
+	@Test
+	public void testCopyToEmptyDestination() {
+		final ByteBuffer src = ByteBuffer.wrap(new byte[4]);
+		final ByteBuffer dst = ByteBuffer.allocate(0);
+		src.put(new byte[2]);
+		Assert.assertEquals(2, src.position());
+		Assert.assertEquals(2, src.remaining());
+		Assert.assertEquals(0, dst.position());
+		Assert.assertEquals(0, dst.remaining());
+		ByteBuffers.copy(src, dst);
+		Assert.assertEquals(2, src.position());
+		Assert.assertEquals(2, src.remaining());
+		Assert.assertEquals(0, dst.position());
+		Assert.assertEquals(0, dst.remaining());
+	}
+
+	@Test
+	public void testCopyToBiggerDestination() {
+		final ByteBuffer src = ByteBuffer.wrap(new byte[2]);
+		final ByteBuffer dst = ByteBuffer.allocate(10);
+		dst.put(new byte[3]);
+		Assert.assertEquals(0, src.position());
+		Assert.assertEquals(2, src.remaining());
+		Assert.assertEquals(3, dst.position());
+		Assert.assertEquals(7, dst.remaining());
+		ByteBuffers.copy(src, dst);
+		Assert.assertEquals(2, src.position());
+		Assert.assertEquals(0, src.remaining());
+		Assert.assertEquals(5, dst.position());
+		Assert.assertEquals(5, dst.remaining());
+	}
+
+	@Test
+	public void testCopyToSmallerDestination() {
+		final ByteBuffer src = ByteBuffer.wrap(new byte[5]);
+		final ByteBuffer dst = ByteBuffer.allocate(2);
+		Assert.assertEquals(0, src.position());
+		Assert.assertEquals(5, src.remaining());
+		Assert.assertEquals(0, dst.position());
+		Assert.assertEquals(2, dst.remaining());
+		ByteBuffers.copy(src, dst);
+		Assert.assertEquals(2, src.position());
+		Assert.assertEquals(3, src.remaining());
+		Assert.assertEquals(2, dst.position());
+		Assert.assertEquals(0, dst.remaining());
+	}
+
+}