Add basic hash tree API and implementation.

bramble-api/src/main/java/org/briarproject/bramble/api/sync/MessageId.java

@@ -24,6 +24,12 @@ public class MessageId extends UniqueId {
 	public static final String BLOCK_LABEL =
 			"org.briarproject.bramble/MESSAGE_BLOCK";
 
+	/**
+	 * Label for hashing two tree hashes to produce a parent.
+	 */
+	public static final String TREE_LABEL =
+			"org.briarproject.bramble/MESSAGE_TREE";
+
 	public MessageId(byte[] id) {
 		super(id);
 	}

bramble-api/src/main/java/org/briarproject/bramble/api/sync/tree/LeafNode.java

@@ -0,0 +1,8 @@
+package org.briarproject.bramble.api.sync.tree;
+
+public class LeafNode extends TreeNode {
+
+	public LeafNode(TreeHash hash, int blockNumber) {
+		super(hash, 0, blockNumber, blockNumber);
+	}
+}

bramble-api/src/main/java/org/briarproject/bramble/api/sync/tree/ParentNode.java

@@ -0,0 +1,9 @@
+package org.briarproject.bramble.api.sync.tree;
+
+public class ParentNode extends TreeNode {
+
+	public ParentNode(TreeHash hash, TreeNode left, TreeNode right) {
+		super(hash, Math.max(left.getHeight(), right.getHeight()) + 1,
+				left.getFirstBlockNumber(), right.getLastBlockNumber());
+	}
+}

bramble-api/src/main/java/org/briarproject/bramble/api/sync/tree/TreeHash.java

@@ -0,0 +1,24 @@
+package org.briarproject.bramble.api.sync.tree;
+
+import org.briarproject.bramble.api.UniqueId;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import javax.annotation.concurrent.ThreadSafe;
+
+/**
+ * Type-safe wrapper for a byte array that uniquely identifies a sequence of
+ * one or more message blocks.
+ */
+@ThreadSafe
+@NotNullByDefault
+public class TreeHash extends UniqueId {
+
+	public TreeHash(byte[] id) {
+		super(id);
+	}
+
+	@Override
+	public boolean equals(Object o) {
+		return o instanceof TreeHash && super.equals(o);
+	}
+}

bramble-api/src/main/java/org/briarproject/bramble/api/sync/tree/TreeHasher.java

@@ -0,0 +1,8 @@
+package org.briarproject.bramble.api.sync.tree;
+
+public interface TreeHasher {
+
+	LeafNode hashBlock(int blockNumber, byte[] data);
+
+	ParentNode mergeTrees(TreeNode left, TreeNode right);
+}

bramble-api/src/main/java/org/briarproject/bramble/api/sync/tree/TreeNode.java

@@ -0,0 +1,31 @@
+package org.briarproject.bramble.api.sync.tree;
+
+public class TreeNode {
+
+	private final TreeHash hash;
+	private final int height, firstBlockNumber, lastBlockNumber;
+
+	TreeNode(TreeHash hash, int height, int firstBlockNumber,
+			int lastBlockNumber) {
+		this.hash = hash;
+		this.height = height;
+		this.firstBlockNumber = firstBlockNumber;
+		this.lastBlockNumber = lastBlockNumber;
+	}
+
+	public TreeHash getHash() {
+		return hash;
+	}
+
+	public int getHeight() {
+		return height;
+	}
+
+	public int getFirstBlockNumber() {
+		return firstBlockNumber;
+	}
+
+	public int getLastBlockNumber() {
+		return lastBlockNumber;
+	}
+}

bramble-core/src/main/java/org/briarproject/bramble/sync/tree/TreeHasherImpl.java

@@ -0,0 +1,40 @@
+package org.briarproject.bramble.sync.tree;
+
+import org.briarproject.bramble.api.crypto.CryptoComponent;
+import org.briarproject.bramble.api.sync.tree.LeafNode;
+import org.briarproject.bramble.api.sync.tree.ParentNode;
+import org.briarproject.bramble.api.sync.tree.TreeHash;
+import org.briarproject.bramble.api.sync.tree.TreeHasher;
+import org.briarproject.bramble.api.sync.tree.TreeNode;
+
+import javax.inject.Inject;
+
+import static org.briarproject.bramble.api.sync.Message.FORMAT_VERSION;
+import static org.briarproject.bramble.api.sync.MessageId.BLOCK_LABEL;
+import static org.briarproject.bramble.api.sync.MessageId.TREE_LABEL;
+
+class TreeHasherImpl implements TreeHasher {
+
+	private static final byte[] FORMAT_VERSION_BYTES =
+			new byte[] {FORMAT_VERSION};
+
+	private final CryptoComponent crypto;
+
+	@Inject
+	TreeHasherImpl(CryptoComponent crypto) {
+		this.crypto = crypto;
+	}
+
+	@Override
+	public LeafNode hashBlock(int blockNumber, byte[] data) {
+		byte[] hash = crypto.hash(BLOCK_LABEL, FORMAT_VERSION_BYTES, data);
+		return new LeafNode(new TreeHash(hash), blockNumber);
+	}
+
+	@Override
+	public ParentNode mergeTrees(TreeNode left, TreeNode right) {
+		byte[] hash = crypto.hash(TREE_LABEL, FORMAT_VERSION_BYTES,
+				left.getHash().getBytes(), right.getHash().getBytes());
+		return new ParentNode(new TreeHash(hash), left, right);
+	}
+}