Util: New zero-copy BAOS

WIP, to be hooked in

core/java/src/net/i2p/util/ByteArrayStream.java

@@ -0,0 +1,52 @@
+package net.i2p.util;
+
+import java.io.ByteArrayOutputStream;
+import java.io.ByteArrayInputStream;
+import java.util.Arrays;
+
+/**
+ *  OutputStream to InputStream adapter.
+ *  Zero-copy where possible. Unsynchronized.
+ *  This is NOT a Pipe.
+ *  Do NOT reset after writing.
+ *
+ *  @since 0.9.48
+ */
+public class ByteArrayStream extends ByteArrayOutputStream {
+
+    public ByteArrayStream() {
+        super();
+    }
+
+    public ByteArrayStream(int size) {
+        super(size);
+    }
+
+    /**
+     *  @throws IllegalStateException if previously written
+     */
+    @Override
+    public void reset() {
+        if (count > 0)
+            throw new IllegalStateException();
+    }
+
+    /**
+     *  Zero-copy only if the data fills the buffer.
+     *  Use asInputStream() for guaranteed zero-copy.
+     */
+    @Override
+    public byte[] toByteArray() {
+        if (count == buf.length)
+            return buf;
+        return Arrays.copyOfRange(buf, 0, count);
+    }
+
+    /**
+     *  All data previously written. Zero-copy. Not a Pipe.
+     *  Data written after this call will not appear.
+     */
+    public ByteArrayInputStream asInputStream() {
+        return new ByteArrayInputStream(buf, 0, count);
+    }
+}