move TimeStampDeltaDecoder to BSFile

Now the encoding and decoding code is in the same class.

block-storage/src/main/java/org/lucares/pdb/blockstorage/BSFile.java

@@ -6,6 +6,7 @@ import java.util.NoSuchElementException;
 import java.util.Optional;
 import java.util.Spliterator;
 import java.util.Spliterators;
+import java.util.function.Function;
 import java.util.stream.Stream;
 import java.util.stream.StreamSupport;
 
@@ -37,11 +38,30 @@ import org.slf4j.LoggerFactory;
  */
 public class BSFile implements AutoCloseable {
 
+	private static class TimeStampDeltaDecoder implements Function<LongList, LongList> {
+
+		/**
+		 * Computes the inverse of the delta encoding in {@link BSFile#appendTimeValue}
+		 */
+		@Override
+		public LongList apply(final LongList t) {
+			long lastTimeValue = 0;
+			for (int i = 0; i < t.size(); i += 2) {
+				lastTimeValue += t.get(i);
+				t.set(i, lastTimeValue);
+			}
+
+			return t;
+		}
+	}
+
 	private static final Logger LOGGER = LoggerFactory.getLogger(BSFile.class);
 
 	private static final ThreadLocal<LongSequenceEncoderDecoder> INT_ENCODER = ThreadLocal
 			.withInitial(LongSequenceEncoderDecoder::new);
 
+	private static final TimeStampDeltaDecoder TIME_DELTA_DECODER = new TimeStampDeltaDecoder();
+
 	/*
 	 * The last disk block of this sequence. This is the block new values will be
 	 * appended to.
@@ -78,9 +98,32 @@ public class BSFile implements AutoCloseable {
 			buffer = diskStorage.getDiskBlock(lastBlockNumber);
 		}
 		offsetInBuffer = determineWriteOffsetInExistingBuffer(buffer);
+		lastEpochMilli = determineLastEpochMilli(buffer);
 		LOGGER.trace("create bsFile={} lastBlockNumber={}", rootBlockNumber, lastBlockNumber);
 	}
 
+	private long determineLastEpochMilli(final DiskBlock diskBlock) {
+
+		LongList longList = new LongList();
+
+		// get the time/value delta encoded longs
+		final byte[] buf = diskBlock.getBuffer();
+		INT_ENCODER.get().decodeInto(buf, longList);
+		final long result;
+		if (longList.isEmpty()) {
+			// only new files have empty disk blocks
+			// and empty disk blocks have time offset 0
+			result = 0;
+		} else {
+			// decode the deltas to get the correct timestamps
+			longList = TIME_DELTA_DECODER.apply(longList);
+
+			// return the last timestamp
+			result = longList.get(longList.size() - 2);
+		}
+		return result;
+	}
+
 	private int determineWriteOffsetInExistingBuffer(final DiskBlock buffer) {
 
 		final byte[] buf = buffer.getBuffer();
@@ -106,9 +149,9 @@ public class BSFile implements AutoCloseable {
 	public void appendTimeValue(final long epochMilli, final long value) throws IOException {
 		final LongSequenceEncoderDecoder intEncoder = INT_ENCODER.get();
 
-		final long epochMilliDiff = epochMilli - lastEpochMilli;
+		final long epochMilliDelta = epochMilli - lastEpochMilli;
 
-		final int bytesWritten = intEncoder.encodeInto(epochMilliDiff, value, buffer.getBuffer(), offsetInBuffer);
+		final int bytesWritten = intEncoder.encodeInto(epochMilliDelta, value, buffer.getBuffer(), offsetInBuffer);
 
 		if (bytesWritten == 0) {
 			flushFullBufferAndCreateNew();
@@ -194,6 +237,11 @@ public class BSFile implements AutoCloseable {
 		return StreamSupport.stream(Spliterators.spliteratorUnknownSize(iterator, Spliterator.ORDERED), false);
 	}
 
+	public Stream<LongList> streamOfTimeValueLongLists() {
+		final Stream<LongList> stream = streamOfLongLists();
+		return stream.map(TIME_DELTA_DECODER);
+	}
+
 	private static class LongListIterator implements Iterator<LongList> {
 
 		private LongList next = null;
@@ -239,6 +287,13 @@ public class BSFile implements AutoCloseable {
 		return result;
 	}
 
+	public LongList asTimeValueLongList() {
+
+		final LongList result = new LongList();
+		streamOfTimeValueLongLists().forEachOrdered(result::addAll);
+		return result;
+	}
+
 	public long getRootBlockNumber() {
 
 		return rootBlockNumber;