Buffers: add write helpers writeLengthPrefixedBytes() and writeLengthPrefixedString()

Includes a test.

core/src/main/java/org/bitcoinj/base/internal/Buffers.java

@@ -18,6 +18,7 @@ package org.bitcoinj.base.internal;
 
 import org.bitcoinj.base.VarInt;
 
+import java.nio.BufferOverflowException;
 import java.nio.BufferUnderflowException;
 import java.nio.ByteBuffer;
 import java.nio.charset.StandardCharsets;
@@ -59,6 +60,18 @@ public class Buffers {
         return readBytes(buf, length);
     }
 
+    /**
+     * First write the length of the byte array as a {@link VarInt}. Then write the array contents.
+     *
+     * @param buf   buffer to write to
+     * @param bytes bytes to write
+     * @return the buffer
+     * @throws BufferOverflowException if the value doesn't fit the remaining buffer
+     */
+    public static ByteBuffer writeLengthPrefixedBytes(ByteBuffer buf, byte[] bytes) throws BufferOverflowException {
+        return buf.put(VarInt.of(bytes.length).serialize()).put(bytes);
+    }
+
     /**
      * First read a {@link VarInt} from the buffer and use it to determine the number of bytes to read. Then read
      * that many bytes and interpret it as an UTF-8 encoded string to be returned. This construct is frequently used
@@ -72,6 +85,20 @@ public class Buffers {
         return new String(readLengthPrefixedBytes(buf), StandardCharsets.UTF_8);
     }
 
+    /**
+     * Encode a given string using UTF-8. Then write the lnegth of the encoded bytes as a {@link VarInt}. Then write
+     * the bytes themselves.
+     *
+     * @param buf buffer to write to
+     * @param str string to write
+     * @return the buffer
+     * @throws BufferOverflowException if the value doesn't fit the remaining buffer
+     */
+    public static ByteBuffer writeLengthPrefixedString(ByteBuffer buf, String str) throws BufferOverflowException {
+        byte[] bytes = str.getBytes(StandardCharsets.UTF_8);
+        return writeLengthPrefixedBytes(buf, bytes);
+    }
+
     /**
      * Advance buffer position by a given number of bytes.
      *

core/src/test/java/org/bitcoinj/core/BuffersTest.java

@@ -0,0 +1,58 @@
+/*
+ * Copyright by the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.bitcoinj.core;
+
+import junitparams.JUnitParamsRunner;
+import junitparams.Parameters;
+import org.bitcoinj.base.VarInt;
+import org.bitcoinj.base.internal.Buffers;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+
+import java.nio.Buffer;
+import java.nio.ByteBuffer;
+import java.util.Iterator;
+import java.util.Random;
+import java.util.stream.Stream;
+
+import static org.junit.Assert.assertArrayEquals;
+import static org.junit.Assert.assertFalse;
+
+@RunWith(JUnitParamsRunner.class)
+public class BuffersTest {
+    @Test
+    @Parameters(method = "randomBytes")
+    public void readAndWrite(byte[] bytes) {
+        ByteBuffer buf = ByteBuffer.allocate(VarInt.sizeOf(bytes.length) + bytes.length);
+        Buffers.writeLengthPrefixedBytes(buf, bytes);
+        assertFalse(buf.hasRemaining());
+        ((Buffer) buf).rewind();
+        byte[] copy = Buffers.readLengthPrefixedBytes(buf);
+        assertFalse(buf.hasRemaining());
+        assertArrayEquals(bytes, copy);
+    }
+
+    private Iterator<byte[]> randomBytes() {
+        Random random = new Random();
+        return Stream.generate(() -> {
+            int length = random.nextInt(10);
+            byte[] bytes = new byte[length];
+            random.nextBytes(bytes);
+            return bytes;
+        }).limit(10).iterator();
+    }
+}