////////////////////////////////////////////////////////////////////////////////
/// 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
////////////////////////////////////////////////////////////////////////////////

#include "BreadthFirstEnumerator.h"

#include "VocBase/Traverser.h"
#include "VocBase/TraverserCache.h"
#include "VocBase/TraverserOptions.h"

#include <velocypack/Slice.h>
#include <velocypack/velocypack-aliases.h>

using namespace arangodb;
using namespace arangodb::traverser;

using BreadthFirstEnumerator = arangodb::graph::BreadthFirstEnumerator;

BreadthFirstEnumerator::PathStep::PathStep(StringRef const vertex) 
  : sourceIdx(0),
    vertex(vertex) {
}


BreadthFirstEnumerator::PathStep::PathStep(size_t sourceIdx,
                                           StringRef const edge,
                                           StringRef const vertex) :
  sourceIdx(sourceIdx), edge(edge), vertex(vertex) {
}

BreadthFirstEnumerator::BreadthFirstEnumerator(Traverser* traverser,
                                               VPackSlice startVertex,
                                               TraverserOptions* opts)
    : PathEnumerator(traverser, startVertex.copyString(), opts),
      _schreierIndex(1),
      _lastReturned(0),
      _currentDepth(0),
      _toSearchPos(0) {
  _schreier.reserve(32);
  StringRef startVId = _opts->cache()->persistString(StringRef(startVertex));

  _schreier.emplace_back(startVId);
  _toSearch.emplace_back(NextStep(0));
}

bool BreadthFirstEnumerator::next() {
  if (_isFirst) {
    _isFirst = false;
    if (_opts->minDepth == 0) {
      return true;
    }
  }
  _lastReturned++;

  if (_lastReturned < _schreierIndex) {
    // We still have something on our stack.
    // Paths have been read but not returned.
    return true;
  }

  if (_opts->maxDepth == 0) {
    // Short circuit.
    // We cannot find any path of length 0 or less
    return false;
  }
  // Avoid large call stacks.
  // Loop will be left if we are either finished
  // with searching.
  // Or we found vertices in the next depth for
  // a vertex.
  while (true) {
    if (_toSearchPos >= _toSearch.size()) {
      // This depth is done. GoTo next
      if (_nextDepth.empty()) {
        // That's it. we are done.
        return false;
      }
      // Save copies:
      // We clear current
      // we swap current and next.
      // So now current is filled
      // and next is empty.
      _toSearch.clear();
      _toSearchPos = 0;
      _toSearch.swap(_nextDepth);
      _currentDepth++;
      TRI_ASSERT(_toSearchPos < _toSearch.size());
      TRI_ASSERT(_nextDepth.empty());
      TRI_ASSERT(_currentDepth < _opts->maxDepth);
    }
    // This access is always safe.
    // If not it should have bailed out before.
    TRI_ASSERT(_toSearchPos < _toSearch.size());

    _tmpEdges.clear();
    auto const nextIdx = _toSearch[_toSearchPos++].sourceIdx;
    auto const nextVertex = _schreier[nextIdx].vertex;
    StringRef vId;

    std::unique_ptr<arangodb::traverser::EdgeCursor> cursor(_opts->nextCursor(_traverser->mmdr(), nextVertex, _currentDepth));
    if (cursor != nullptr) {
      bool shouldReturnPath = _currentDepth + 1 >= _opts->minDepth;
      bool didInsert = false;

      auto callback = [&] (arangodb::StringRef const& eid, VPackSlice e, size_t cursorIdx) -> void {
        if (_opts->uniqueEdges ==
            TraverserOptions::UniquenessLevel::GLOBAL) {
          if (_returnedEdges.find(eid) == _returnedEdges.end()) {
            // Edge not yet visited. Mark and continue.
            // TODO FIXME the edge will run out of scope
            _returnedEdges.emplace(eid);
          } else {
            // Edge filtered due to unique_constraint
            _traverser->_filteredPaths++;
            return;
          }
        }

        if (!_traverser->edgeMatchesConditions(e, nextVertex,
                                               _currentDepth,
                                               cursorIdx)) {
          return;
        }

        if (_traverser->getSingleVertex(e, nextVertex, _currentDepth, vId)) {
          _schreier.emplace_back(nextIdx, eid, vId);
          if (_currentDepth < _opts->maxDepth - 1) {
            _nextDepth.emplace_back(NextStep(_schreierIndex));
          }
          _schreierIndex++;
          didInsert = true;
        }
      };

      cursor->readAll(callback);

      if (!shouldReturnPath) {
        _lastReturned = _schreierIndex;
        didInsert = false;
      }
      if (didInsert) {
        // We exit the loop here.
        // _schreierIndex is moved forward
        break;
      }
    }
    // Nothing found for this vertex.
    // _toSearchPos is increased so
    // we are not stuck in an endless loop
  }

  // _lastReturned points to the last used
  // entry. We compute the path to it.
  return true;
}

arangodb::aql::AqlValue BreadthFirstEnumerator::lastVertexToAqlValue() {
  TRI_ASSERT(_lastReturned < _schreier.size());
  PathStep const& current = _schreier[_lastReturned];
  return _traverser->fetchVertexData(StringRef(current.vertex));
}

arangodb::aql::AqlValue BreadthFirstEnumerator::lastEdgeToAqlValue() {
  TRI_ASSERT(_lastReturned < _schreier.size());
  if (_lastReturned == 0) {
    // This is the first Vertex. No Edge Pointing to it
    return arangodb::aql::AqlValue(arangodb::basics::VelocyPackHelper::NullValue());
  }
  PathStep const& current = _schreier[_lastReturned];
  return _traverser->fetchEdgeData(StringRef(current.edge));
}

arangodb::aql::AqlValue BreadthFirstEnumerator::pathToAqlValue(
    arangodb::velocypack::Builder& result) {
  // TODO make deque class variable
  std::deque<size_t> fullPath;
  size_t cur = _lastReturned;
  while (cur != 0) {
    // Walk backwards through the path and push everything found on the local stack
    fullPath.emplace_front(cur);
    cur = _schreier[cur].sourceIdx;
  }

  result.clear();
  result.openObject();
  result.add(VPackValue("edges"));
  result.openArray();
  for (auto const& idx : fullPath) {
    _traverser->addEdgeToVelocyPack(StringRef(_schreier[idx].edge), result);
  }
  result.close(); // edges
  result.add(VPackValue("vertices"));
  result.openArray();
  // Always add the start vertex
  _traverser->addVertexToVelocyPack(_schreier[0].vertex, result);
  for (auto const& idx : fullPath) {
    _traverser->addVertexToVelocyPack(_schreier[idx].vertex, result);
  }
  result.close(); // vertices
  result.close();
  return arangodb::aql::AqlValue(result.slice());
}