text: ////////////////////////////////////////////////////////////////////////////////
  /// DISCLAIMER
  ///
  /// Copyright 2014-2016 ArangoDB GmbH, Cologne, Germany
  /// Copyright 2004-2014 triAGENS GmbH, Cologne, Germany
  ///
  /// 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.
  ///
  /// Copyright holder is ArangoDB GmbH, Cologne, Germany
  ///
  /// @author Michael Hackstein
  ////////////////////////////////////////////////////////////////////////////////
  
  #ifndef ARANGOD_AQL_TRAVERSAL_BLOCK_H
  #define ARANGOD_AQL_TRAVERSAL_BLOCK_H 1
  
  #include "Aql/ExecutionBlock.h"
  #include "Aql/TraversalNode.h"
  #include "Basics/VelocyPackHelper.h"
  #include "VocBase/Traverser.h"
  
  #include <velocypack/Slice.h>
  #include <velocypack/velocypack-aliases.h>
  
  namespace arangodb {
  namespace aql {
  
  class TraversalBlock : public ExecutionBlock {
   public:
    TraversalBlock(ExecutionEngine* engine, TraversalNode const* ep);
  
    ~TraversalBlock();
  
    /// @brief initialize, here we fetch all docs from the database
    int initialize() override;
  
    /// @brief initializeCursor
    int initializeCursor(AqlItemBlock* items, size_t pos) override;
  
    /// @brief shutdown send destroy to all engines.
    int shutdown(int errorCode) override final;
  
    /// @brief getSome
    AqlItemBlock* getSome(size_t atLeast, size_t atMost) override final;
  
    // skip between atLeast and atMost, returns the number actually skipped . . .
    // will only return less than atLeast if there aren't atLeast many
    // things to skip overall.
    size_t skipSome(size_t atLeast, size_t atMost) override final;
  
   private:
  
    /// @brief cleanup, here we clean up all internally generated values
    void freeCaches();
  
    /// @brief vertices buffer
    std::vector<arangodb::aql::AqlValue> _vertices;
  
    /// @brief edges buffer
    std::vector<arangodb::aql::AqlValue> _edges;
  
    /// @brief path buffer
    std::vector<arangodb::aql::AqlValue> _paths;
  
    /// @brief current position in _paths, _edges, _vertices
    size_t _posInPaths;
  
    /// @brief Options for the travereser
    arangodb::traverser::TraverserOptions* _opts;
  
    /// @brief Traverser object
    std::unique_ptr<arangodb::traverser::Traverser> _traverser;
  
    /// @brief The information to get the starting point, when a register id is
    /// used
    arangodb::aql::RegisterId _reg;
  
    /// @brief Keep a copy of the start vertex id-string. Can be freed if this
    /// start vertex is not in use any more.
    std::string _vertexId;
  
    /// @brief Indicator if we use a register for input variable.
    ///        Invariant: _useRegister == true <=> _reg != undefined
    bool _useRegister;
  
    /// @brief Indicator if we have used the constant input parameter for
    /// computation
    ///        We use it to check if we are done with enumerating.
    bool _usedConstant;
  
    /// @brief Variable for the vertex output
    Variable const* _vertexVar;
  
    /// @brief Register for the vertex output
    RegisterId _vertexReg;
  
    /// @brief Variable for the edge output
    Variable const* _edgeVar;
  
    /// @brief Register for the edge output
    RegisterId _edgeReg;
  
    /// @brief Variable for the full path output
    Variable const* _pathVar;
  
    /// @brief Register for the full path output
    RegisterId _pathReg;
  
    /// @brief _inVars, a vector containing all variables required
    ///        for the filtering conditions.
    std::vector<Variable const*> _inVars;
  
    /// @brief _inRegs, a vector containing for each expression above
    /// a vector of RegisterId, used to execute the expression
    std::vector<RegisterId> _inRegs;
  
    std::unordered_map<ServerID, traverser::TraverserEngineID> const* _engines;
  
    /// @brief continue fetching of paths
    bool morePaths(size_t hint);
  
    /// @brief skip the next paths
    size_t skipPaths(size_t hint);
  
    /// @brief Initialize the filter expressions
    void initializeExpressions(AqlItemBlock const*, size_t pos);
  
    /// @brief Initialize the path enumerator
    void initializePaths(AqlItemBlock const*, size_t pos);
  
    /// @brief Checks if we output the vertex
    bool usesVertexOutput() { return _vertexVar != nullptr; }
  
    /// @brief Checks if we output the edge
    bool usesEdgeOutput() { return _edgeVar != nullptr; }
  
    /// @brief Checks if we output the path
    bool usesPathOutput() { return _pathVar != nullptr; }
  
  };
  }  // namespace arangodb::aql
  }  // namespace arangodb
  
  #endif
  