@Override
  public void setChainHead(StoredBlock chainHead) throws BlockStoreException {
    final MappedByteBuffer buffer = this.buffer;
    if (buffer == null) throw new BlockStoreException("Store closed");

    lock.lock();
    try {
      lastChainHead = chainHead;
      byte[] headHash = chainHead.getHeader().getHash().getBytes();
      buffer.position(8);
      buffer.put(headHash);
    } finally {
      lock.unlock();
    }
  }
 private void initNewStore(NetworkParameters params) throws Exception {
   byte[] header;
   header = HEADER_MAGIC.getBytes("US-ASCII");
   buffer.put(header);
   // Insert the genesis block.
   lock.lock();
   try {
     setRingCursor(buffer, FILE_PROLOGUE_BYTES);
   } finally {
     lock.unlock();
   }
   Block genesis = params.getGenesisBlock().cloneAsHeader();
   StoredBlock storedGenesis = new StoredBlock(genesis, genesis.getWork(), 0);
   put(storedGenesis);
   setChainHead(storedGenesis);
 }
  @Override
  @Nullable
  public StoredBlock get(Sha256Hash hash) throws BlockStoreException {
    final MappedByteBuffer buffer = this.buffer;
    if (buffer == null) throw new BlockStoreException("Store closed");

    lock.lock();
    try {
      StoredBlock cacheHit = blockCache.get(hash);
      if (cacheHit != null) return cacheHit;
      if (notFoundCache.get(hash) != null) return null;

      // Starting from the current tip of the ring work backwards until we have either found the
      // block or
      // wrapped around.
      int cursor = getRingCursor(buffer);
      final int startingPoint = cursor;
      final int fileSize = getFileSize();
      final byte[] targetHashBytes = hash.getBytes();
      byte[] scratch = new byte[32];
      do {
        cursor -= RECORD_SIZE;
        if (cursor < FILE_PROLOGUE_BYTES) {
          // We hit the start, so wrap around.
          cursor = fileSize - RECORD_SIZE;
        }
        // Cursor is now at the start of the next record to check, so read the hash and compare it.
        buffer.position(cursor);
        buffer.get(scratch);
        if (Arrays.equals(scratch, targetHashBytes)) {
          // Found the target.
          StoredBlock storedBlock = StoredBlock.deserializeCompact(params, buffer);
          blockCache.put(hash, storedBlock);
          return storedBlock;
        }
      } while (cursor != startingPoint);
      // Not found.
      notFoundCache.put(hash, notFoundMarker);
      return null;
    } catch (ProtocolException e) {
      throw new RuntimeException(e); // Cannot happen.
    } finally {
      lock.unlock();
    }
  }
  @Override
  public StoredBlock getChainHead() throws BlockStoreException {
    final MappedByteBuffer buffer = this.buffer;
    if (buffer == null) throw new BlockStoreException("Store closed");

    lock.lock();
    try {
      if (lastChainHead == null) {
        byte[] headHash = new byte[32];
        buffer.position(8);
        buffer.get(headHash);
        Sha256Hash hash = new Sha256Hash(headHash);
        StoredBlock block = get(hash);
        if (block == null)
          throw new BlockStoreException(
              "Corrupted block store: could not find chain head: " + hash);
        lastChainHead = block;
      }
      return lastChainHead;
    } finally {
      lock.unlock();
    }
  }
  @Override
  public void put(StoredBlock block) throws BlockStoreException {
    final MappedByteBuffer buffer = this.buffer;
    if (buffer == null) throw new BlockStoreException("Store closed");

    lock.lock();
    try {
      int cursor = getRingCursor(buffer);
      if (cursor == getFileSize()) {
        // Wrapped around.
        cursor = FILE_PROLOGUE_BYTES;
      }
      buffer.position(cursor);
      Sha256Hash hash = block.getHeader().getHash();
      notFoundCache.remove(hash);
      buffer.put(hash.getBytes());
      block.serializeCompact(buffer);
      setRingCursor(buffer, buffer.position());
      blockCache.put(hash, block);
    } finally {
      lock.unlock();
    }
  }