1 files changed, 106 insertions, 0 deletions
diff --git a/base/src/main/java/bjc/utils/ioutils/blocks/PushbackBlockReader.java b/base/src/main/java/bjc/utils/ioutils/blocks/PushbackBlockReader.java
new file mode 100644
index 0000000..0cc9dea
--- /dev/null
+++ b/base/src/main/java/bjc/utils/ioutils/blocks/PushbackBlockReader.java
@@ -0,0 +1,106 @@
+package bjc.utils.ioutils.blocks;
+
+import java.io.IOException;
+import java.util.Deque;
+import java.util.LinkedList;
+
+/**
+ * A block reader that supports pushing blocks onto the input queue so that they
+ * are provided before blocks read from an input source.
+ *
+ * @author bjculkin
+ *
+ */
+public class PushbackBlockReader implements BlockReader {
+	private final BlockReader source;
+
+	/*
+	 * The queue of pushed-back blocks.
+	 */
+	private final Deque<Block> waiting;
+
+	private Block curBlock;
+
+	private int blockNo;
+
+	/**
+	 * Create a new pushback block reader.
+	 *
+	 * @param src
+	 *                The block reader to use when no blocks are queued.
+	 */
+	public PushbackBlockReader(final BlockReader src) {
+		source = src;
+
+		waiting = new LinkedList<>();
+	}
+
+	@Override
+	public boolean hasNextBlock() {
+		return !waiting.isEmpty() || source.hasNextBlock();
+	}
+
+	@Override
+	public Block getBlock() {
+		return curBlock;
+	}
+
+	@Override
+	public boolean nextBlock() {
+		/*
+		 * Drain pushed-back blocks first.
+		 */
+		if (!waiting.isEmpty()) {
+			curBlock = waiting.pop();
+
+			blockNo += 1;
+
+			return true;
+		} else {
+			final boolean succ = source.nextBlock();
+			curBlock = source.getBlock();
+
+			if (succ) {
+				blockNo += 1;
+			}
+
+			return succ;
+		}
+	}
+
+	@Override
+	public int getBlockCount() {
+		return blockNo;
+	}
+
+	@Override
+	public void close() throws IOException {
+		source.close();
+	}
+
+	/**
+	 * Insert a block at the back of the queue of pending blocks.
+	 *
+	 * @param blk
+	 *                The block to put at the back.
+	 */
+	public void addBlock(final Block blk) {
+		waiting.add(blk);
+	}
+
+	/**
+	 * Insert a block at the front of the queue of pending blocks.
+	 *
+	 * @param blk
+	 *                The block to put at the front.
+	 */
+	public void pushBlock(final Block blk) {
+		waiting.push(blk);
+	}
+
+	@Override
+	public String toString() {
+		return String.format("PushbackBlockReader [waiting=%s, curBlock=%s, blockNo=%s]", waiting, curBlock,
+				blockNo);
+	}
+}