////////////////////////////////////////////////////////////////////////////////
/// 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 Jan Steemann
/// @author Daniel H. Larkin
/// @author Simon Grätzer
////////////////////////////////////////////////////////////////////////////////

#include "RocksDBVPackIndex.h"
#include "Aql/AstNode.h"
#include "Aql/SortCondition.h"
#include "Basics/AttributeNameParser.h"
#include "Basics/FixedSizeAllocator.h"
#include "Basics/StaticStrings.h"
#include "Basics/VelocyPackHelper.h"
#include "Indexes/IndexLookupContext.h"
#include "RocksDBEngine/RocksDBCollection.h"
#include "RocksDBEngine/RocksDBCommon.h"
#include "RocksDBEngine/RocksDBComparator.h"
#include "RocksDBEngine/RocksDBPrimaryIndex.h"
#include "RocksDBEngine/RocksDBToken.h"
#include "RocksDBEngine/RocksDBTransactionState.h"
#include "Transaction/Helpers.h"
#include "Transaction/Methods.h"
#include "VocBase/LogicalCollection.h"

#include <rocksdb/iterator.h>
#include <rocksdb/utilities/transaction.h>

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

using namespace arangodb;

/// @brief the _key attribute, which, when used in an index, will implictly make
/// it unique
static std::vector<arangodb::basics::AttributeName> const KeyAttribute{
    arangodb::basics::AttributeName("_key", false)};

static size_t sortWeight(arangodb::aql::AstNode const* node) {
  switch (node->type) {
    case arangodb::aql::NODE_TYPE_OPERATOR_BINARY_EQ:
      return 1;
    case arangodb::aql::NODE_TYPE_OPERATOR_BINARY_IN:
      return 2;
    case arangodb::aql::NODE_TYPE_OPERATOR_BINARY_LT:
      return 3;
    case arangodb::aql::NODE_TYPE_OPERATOR_BINARY_GT:
      return 4;
    case arangodb::aql::NODE_TYPE_OPERATOR_BINARY_LE:
      return 5;
    case arangodb::aql::NODE_TYPE_OPERATOR_BINARY_GE:
      return 6;
    default:
      return 42;
  }
}

// .............................................................................
// recall for all of the following comparison functions:
//
// left < right  return -1
// left > right  return  1
// left == right return  0
//
// furthermore:
//
// the following order is currently defined for placing an order on documents
// undef < null < boolean < number < strings < lists < hash arrays
// note: undefined will be treated as NULL pointer not NULL JSON OBJECT
// within each type class we have the following order
// boolean: false < true
// number: natural order
// strings: lexicographical
// lists: lexicographically and within each slot according to these rules.
// ...........................................................................

RocksDBVPackIndexIterator::RocksDBVPackIndexIterator(
    LogicalCollection* collection, transaction::Methods* trx,
    ManagedDocumentResult* mmdr, arangodb::RocksDBVPackIndex const* index,
    arangodb::RocksDBPrimaryIndex* primaryIndex, bool reverse,
    VPackSlice const& left, VPackSlice const& right)
    : IndexIterator(collection, trx, mmdr, index),
      _index(index),
      _primaryIndex(primaryIndex),
      _cmp(index->_cmp),
      _reverse(reverse),
      _bounds(index->_unique ? RocksDBKeyBounds::UniqueIndexRange(
                                   index->objectId(), left, right)
                             : RocksDBKeyBounds::IndexRange(index->objectId(),
                                                            left, right)) {
  RocksDBTransactionState* state = rocksutils::toRocksTransactionState(trx);
  rocksdb::Transaction* rtrx = state->rocksTransaction();
  auto options = state->readOptions();

  _iterator.reset(rtrx->GetIterator(options));
  if (reverse) {
    _iterator->SeekForPrev(_bounds.end());
  } else {
    _iterator->Seek(_bounds.start());
  }
}

/// @brief Reset the cursor
void RocksDBVPackIndexIterator::reset() {
  if (_reverse) {
    _iterator->SeekForPrev(_bounds.end());
  } else {
    _iterator->Seek(_bounds.start());
  }
}

bool RocksDBVPackIndexIterator::outOfRange() const {
  if (_reverse) {
    return (-1 == _cmp->Compare(_iterator->key(), _bounds.start()));
  } else {
    return (1 == _cmp->Compare(_iterator->key(), _bounds.end()));
  }
}

bool RocksDBVPackIndexIterator::next(TokenCallback const& cb, size_t limit) {
  if (limit == 0 || !_iterator->Valid() || outOfRange()) {
    // No limit no data, or we are actually done. The last call should have
    // returned false
    TRI_ASSERT(limit > 0);  // Someone called with limit == 0. Api broken
    return false;
  }

  while (limit > 0) {
    StringRef primaryKey = _index->_unique
                               ? RocksDBValue::primaryKey(_iterator->value())
                               : RocksDBKey::primaryKey(_iterator->key());
    RocksDBToken token(_primaryIndex->lookupKey(_trx, primaryKey));
    cb(token);

    if (_reverse) {
      _iterator->Prev();
    } else {
      _iterator->Next();
    }

    if (!_iterator->Valid() || outOfRange()) {
      return false;
    }
  }

  return true;
}

/// @brief create the index
RocksDBVPackIndex::RocksDBVPackIndex(TRI_idx_iid_t iid,
                                     arangodb::LogicalCollection* collection,
                                     arangodb::velocypack::Slice const& info)
    : RocksDBIndex(iid, collection, info),
      _useExpansion(false),
      _allowPartialIndex(true) {
  {
    TRI_ASSERT(!_fields.empty());

    TRI_ASSERT(iid != 0);

    fillPaths(_paths, _expanding);

    for (auto const& it : _fields) {
      if (TRI_AttributeNamesHaveExpansion(it)) {
        _useExpansion = true;
        break;
      }
    }
  }
}

/// @brief destroy the index
RocksDBVPackIndex::~RocksDBVPackIndex() {}

size_t RocksDBVPackIndex::memory() const {
  return 0;  // TODO
}

/// @brief return a VelocyPack representation of the index
void RocksDBVPackIndex::toVelocyPack(VPackBuilder& builder,
                                     bool withFigures) const {
  Index::toVelocyPack(builder, withFigures);
  builder.add("unique", VPackValue(_unique));
  builder.add("sparse", VPackValue(_sparse));
}

/// @brief return a VelocyPack representation of the index figures
void RocksDBVPackIndex::toVelocyPackFigures(VPackBuilder& builder) const {
  TRI_ASSERT(builder.isOpenObject());
  builder.add("memory", VPackValue(memory()));
}

/// @brief whether or not the index is implicitly unique
/// this can be the case if the index is not declared as unique, but contains
/// a
/// unique attribute such as _key
bool RocksDBVPackIndex::implicitlyUnique() const {
  if (_unique) {
    // a unique index is always unique
    return true;
  }
  if (_useExpansion) {
    // when an expansion such as a[*] is used, the index may not be unique,
    // even
    // if it contains attributes that are guaranteed to be unique
    return false;
  }

  for (auto const& it : _fields) {
    // if _key is contained in the index fields definition, then the index is
    // implicitly unique
    if (it == KeyAttribute) {
      return true;
    }
  }

  // _key not contained
  return false;
}

/// @brief helper function to insert a document into any index type
/// Should result in an elements vector filled with the new index entries
/// uses the _unique field to determine the kind of key structure
int RocksDBVPackIndex::fillElement(
    transaction::Methods* trx, TRI_voc_rid_t revisionId, VPackSlice const& doc,
    std::vector<std::pair<RocksDBKey, RocksDBValue>>& elements) {
  if (doc.isNone()) {
    LOG_TOPIC(ERR, arangodb::Logger::FIXME)
        << "encountered invalid marker with slice of type None";
    return TRI_ERROR_INTERNAL;
  }

  TRI_IF_FAILURE("FillElementIllegalSlice") { return TRI_ERROR_INTERNAL; }

  if (!_useExpansion) {
    // fast path for inserts... no array elements used

    transaction::BuilderLeaser indexVals(trx);
    indexVals->openArray();

    size_t const n = _paths.size();
    for (size_t i = 0; i < n; ++i) {
      TRI_ASSERT(!_paths[i].empty());

      VPackSlice slice = doc.get(_paths[i]);
      if (slice.isNone() || slice.isNull()) {
        // attribute not found
        if (_sparse) {
          // if sparse we do not have to index, this is indicated by result
          // being shorter than n
          return TRI_ERROR_NO_ERROR;
        }
        // null, note that this will be copied later!
        indexVals->add(VPackSlice::nullSlice());
      } else {
        indexVals->add(slice);
      }
    }
    indexVals->close();

    StringRef key(doc.get(StaticStrings::KeyString));
    if (_unique) {
      // Unique VPack index values are stored as follows:
      // - Key: 7 + 8-byte object ID of index + VPack array with index
      // value(s)
      // + separator (NUL) byte
      // - Value: primary key
      elements.emplace_back(
          RocksDBKey::UniqueIndexValue(_objectId, indexVals->slice()),
          RocksDBValue::UniqueIndexValue(key));
    } else {
      // Non-unique VPack index values are stored as follows:
      // - Key: 6 + 8-byte object ID of index + VPack array with index
      // value(s)
      // + separator (NUL) byte + primary key
      // - Value: empty
      elements.emplace_back(
          RocksDBKey::IndexValue(_objectId, key, indexVals->slice()),
          RocksDBValue::IndexValue());
    }
  } else {
    // other path for handling array elements, too
    std::vector<VPackSlice> sliceStack;

    transaction::BuilderLeaser indexVals(trx);

    buildIndexValues(doc, 0, elements, sliceStack);
  }

  return TRI_ERROR_NO_ERROR;
}

void RocksDBVPackIndex::addIndexValue(
    VPackSlice const& document,
    std::vector<std::pair<RocksDBKey, RocksDBValue>>& elements,
    std::vector<VPackSlice>& sliceStack) {
  // TODO maybe use leaded Builder from transaction.
  VPackBuilder b;
  b.openArray();
  for (VPackSlice const& s : sliceStack) {
    b.add(s);
  }
  b.close();

  StringRef key(document.get(StaticStrings::KeyString));
  if (_unique) {
    // Unique VPack index values are stored as follows:
    // - Key: 7 + 8-byte object ID of index + VPack array with index value(s)
    // - Value: primary key
    elements.emplace_back(RocksDBKey::UniqueIndexValue(_objectId, b.slice()),
                          RocksDBValue::UniqueIndexValue(key));
  } else {
    // Non-unique VPack index values are stored as follows:
    // - Key: 6 + 8-byte object ID of index + VPack array with index value(s)
    // + primary key
    // - Value: empty
    elements.emplace_back(
        RocksDBKey::IndexValue(_objectId, StringRef(key), b.slice()),
        RocksDBValue::IndexValue());
  }
}

/// @brief helper function to create a set of index combinations to insert
void RocksDBVPackIndex::buildIndexValues(
    VPackSlice const document, size_t level,
    std::vector<std::pair<RocksDBKey, RocksDBValue>>& elements,
    std::vector<VPackSlice>& sliceStack) {
  // Invariant: level == sliceStack.size()

  // Stop the recursion:
  if (level == _paths.size()) {
    addIndexValue(document, elements, sliceStack);
    return;
  }

  if (_expanding[level] == -1) {  // the trivial, non-expanding case
    VPackSlice slice = document.get(_paths[level]);
    if (slice.isNone() || slice.isNull()) {
      if (_sparse) {
        return;
      }
      sliceStack.emplace_back(arangodb::basics::VelocyPackHelper::NullValue());
    } else {
      sliceStack.emplace_back(slice);
    }
    buildIndexValues(document, level + 1, elements, sliceStack);
    sliceStack.pop_back();
    return;
  }

  // Finally, the complex case, where we have to expand one entry.
  // Note again that at most one step in the attribute path can be
  // an array step. Furthermore, if _allowPartialIndex is true and
  // anything goes wrong with this attribute path, we have to bottom out
  // with None values to be able to use the index for a prefix match.

  // Trivial case to bottom out with Illegal types.
  VPackSlice illegalSlice = arangodb::basics::VelocyPackHelper::IllegalValue();

  auto finishWithNones = [&]() -> void {
    if (!_allowPartialIndex || level == 0) {
      return;
    }
    for (size_t i = level; i < _paths.size(); i++) {
      sliceStack.emplace_back(illegalSlice);
    }
    addIndexValue(document.get(StaticStrings::KeyString), elements, sliceStack);
    for (size_t i = level; i < _paths.size(); i++) {
      sliceStack.pop_back();
    }
  };
  size_t const n = _paths[level].size();
  // We have 0 <= _expanding[level] < n.
  VPackSlice current(document);
  for (size_t i = 0; i <= static_cast<size_t>(_expanding[level]); i++) {
    if (!current.isObject()) {
      finishWithNones();
      return;
    }
    current = current.get(_paths[level][i]);
    if (current.isNone()) {
      finishWithNones();
      return;
    }
  }
  // Now the expansion:
  if (!current.isArray() || current.length() == 0) {
    finishWithNones();
    return;
  }

  std::unordered_set<VPackSlice, arangodb::basics::VelocyPackHelper::VPackHash,
                     arangodb::basics::VelocyPackHelper::VPackEqual>
      seen(2, arangodb::basics::VelocyPackHelper::VPackHash(),
           arangodb::basics::VelocyPackHelper::VPackEqual());

  auto moveOn = [&](VPackSlice something) -> void {
    auto it = seen.find(something);
    if (it == seen.end()) {
      seen.insert(something);
      sliceStack.emplace_back(something);
      buildIndexValues(document, level + 1, elements, sliceStack);
      sliceStack.pop_back();
    }
  };
  for (auto const& member : VPackArrayIterator(current)) {
    VPackSlice current2(member);
    bool doneNull = false;
    for (size_t i = _expanding[level] + 1; i < n; i++) {
      if (!current2.isObject()) {
        if (!_sparse) {
          moveOn(arangodb::basics::VelocyPackHelper::NullValue());
        }
        doneNull = true;
        break;
      }
      current2 = current2.get(_paths[level][i]);
      if (current2.isNone()) {
        if (!_sparse) {
          moveOn(arangodb::basics::VelocyPackHelper::NullValue());
        }
        doneNull = true;
        break;
      }
    }
    if (!doneNull) {
      moveOn(current2);
    }
    // Finally, if, because of sparsity, we have not inserted anything by now,
    // we need to play the above trick with None because of the above
    // mentioned
    // reasons:
    if (seen.empty()) {
      finishWithNones();
    }
  }
}

/// @brief helper function to transform AttributeNames into strings.
void RocksDBVPackIndex::fillPaths(std::vector<std::vector<std::string>>& paths,
                                  std::vector<int>& expanding) {
  paths.clear();
  expanding.clear();
  for (std::vector<arangodb::basics::AttributeName> const& list : _fields) {
    paths.emplace_back();
    std::vector<std::string>& interior(paths.back());
    int expands = -1;
    int count = 0;
    for (auto const& att : list) {
      interior.emplace_back(att.name);
      if (att.shouldExpand) {
        expands = count;
      }
      ++count;
    }
    expanding.emplace_back(expands);
  }
}

/// @brief inserts a document into the index
int RocksDBVPackIndex::insert(transaction::Methods* trx,
                              TRI_voc_rid_t revisionId, VPackSlice const& doc,
                              bool isRollback) {
  std::vector<std::pair<RocksDBKey, RocksDBValue>> elements;

  int res;
  try {
    res = fillElement(trx, revisionId, doc, elements);
  } catch (...) {
    res = TRI_ERROR_OUT_OF_MEMORY;
  }

  if (res != TRI_ERROR_NO_ERROR) {
    return res;
  }

  RocksDBTransactionState* state = rocksutils::toRocksTransactionState(trx);
  rocksdb::Transaction* rtrx = state->rocksTransaction();
  auto options = state->readOptions();

  size_t const count = elements.size();
  for (size_t i = 0; i < count; ++i) {
    RocksDBKey& key = elements[i].first;
    RocksDBValue& value = elements[i].second;
    if (_unique) {
      RocksDBValue existing =
          RocksDBValue::Empty(RocksDBEntryType::UniqueIndexValue);
      auto status = rtrx->Get(options, key.string(), existing.buffer());
      if (!status.IsNotFound()) {
        res = TRI_ERROR_ARANGO_UNIQUE_CONSTRAINT_VIOLATED;
      }
    }

    if (res == TRI_ERROR_NO_ERROR) {
      auto s = rtrx->Put(key.string(), value.string());

      auto status = rocksutils::convertStatus(s, rocksutils::StatusHint::index);
      if (!status.ok()) {
        res = status.errorNumber();
      }
    }

    if (res != TRI_ERROR_NO_ERROR) {
      for (size_t j = 0; j < i; ++j) {
        rtrx->Delete(elements[j].first.string());
      }

      if (res == TRI_ERROR_ARANGO_UNIQUE_CONSTRAINT_VIOLATED && !_unique) {
        // We ignore unique_constraint violated if we are not unique
        res = TRI_ERROR_NO_ERROR;
      }
      break;
    }
  }

  return res;
}

/// @brief removes a document from the index
int RocksDBVPackIndex::remove(transaction::Methods* trx,
                              TRI_voc_rid_t revisionId, VPackSlice const& doc,
                              bool isRollback) {
  std::vector<std::pair<RocksDBKey, RocksDBValue>> elements;

  int res;
  try {
    res = fillElement(trx, revisionId, doc, elements);
  } catch (...) {
    res = TRI_ERROR_OUT_OF_MEMORY;
  }

  if (res != TRI_ERROR_NO_ERROR) {
    return res;
  }

  RocksDBTransactionState* state = rocksutils::toRocksTransactionState(trx);
  rocksdb::Transaction* rtrx = state->rocksTransaction();

  size_t const count = elements.size();
  for (size_t i = 0; i < count; ++i) {
    auto s = rtrx->Delete(elements[i].first.string());

    auto status = rocksutils::convertStatus(s, rocksutils::StatusHint::index);
    if (!status.ok()) {
      res = status.errorNumber();
    }
  }

  return res;
}

int RocksDBVPackIndex::unload() {
  // nothing to do
  return TRI_ERROR_NO_ERROR;
}

/// @brief called when the index is dropped
int RocksDBVPackIndex::drop() {
  if (_unique) {
    return rocksutils::removeLargeRange(
        rocksutils::globalRocksDB(), RocksDBKeyBounds::UniqueIndex(_objectId));
  } else {
    return rocksutils::removeLargeRange(rocksutils::globalRocksDB(),
                                        RocksDBKeyBounds::Index(_objectId));
  }
}

/// @brief attempts to locate an entry in the index
/// Warning: who ever calls this function is responsible for destroying
/// the RocksDBVPackIndexIterator* results
RocksDBVPackIndexIterator* RocksDBVPackIndex::lookup(
    transaction::Methods* trx, ManagedDocumentResult* mmdr,
    VPackSlice const searchValues, bool reverse) const {
  TRI_ASSERT(searchValues.isArray());
  TRI_ASSERT(searchValues.length() <= _fields.size());

  VPackBuilder leftSearch;
  VPackBuilder rightSearch;

  VPackSlice lastNonEq;
  leftSearch.openArray();
  for (auto const& it : VPackArrayIterator(searchValues)) {
    TRI_ASSERT(it.isObject());
    VPackSlice eq = it.get(StaticStrings::IndexEq);
    if (eq.isNone()) {
      lastNonEq = it;
      break;
    }
    leftSearch.add(eq);
  }

  VPackSlice leftBorder;
  VPackSlice rightBorder;

  if (lastNonEq.isNone()) {
    // We only have equality!
    rightSearch = leftSearch;

    leftSearch.add(VPackSlice::minKeySlice());
    leftSearch.close();

    rightSearch.add(VPackSlice::maxKeySlice());
    rightSearch.close();

    leftBorder = leftSearch.slice();
    rightBorder = rightSearch.slice();
  } else {
    // Copy rightSearch = leftSearch for right border
    rightSearch = leftSearch;

    // Define Lower-Bound
    VPackSlice lastLeft = lastNonEq.get(StaticStrings::IndexGe);
    if (!lastLeft.isNone()) {
      TRI_ASSERT(!lastNonEq.hasKey(StaticStrings::IndexGt));
      leftSearch.add(lastLeft);
      leftSearch.add(VPackSlice::minKeySlice());
      leftSearch.close();
      VPackSlice search = leftSearch.slice();
      leftBorder = search;
    } else {
      lastLeft = lastNonEq.get(StaticStrings::IndexGt);
      if (!lastLeft.isNone()) {
        leftSearch.add(lastLeft);
        leftSearch.add(VPackSlice::maxKeySlice());
        leftSearch.close();
        VPackSlice search = leftSearch.slice();
        leftBorder = search;
      } else {
        // No lower bound set default to (null <= x)
        leftSearch.add(VPackSlice::minKeySlice());
        leftSearch.close();
        VPackSlice search = leftSearch.slice();
        leftBorder = search;
      }
    }

    // Define upper-bound
    VPackSlice lastRight = lastNonEq.get(StaticStrings::IndexLe);
    if (!lastRight.isNone()) {
      TRI_ASSERT(!lastNonEq.hasKey(StaticStrings::IndexLt));
      rightSearch.add(lastRight);
      rightSearch.add(VPackSlice::maxKeySlice());
      rightSearch.close();
      VPackSlice search = rightSearch.slice();
      rightBorder = search;
    } else {
      lastRight = lastNonEq.get(StaticStrings::IndexLt);
      if (!lastRight.isNone()) {
        rightSearch.add(lastRight);
        rightSearch.add(VPackSlice::minKeySlice());
        rightSearch.close();
        VPackSlice search = rightSearch.slice();
        rightBorder = search;
      } else {
        // No upper bound set default to (x <= INFINITY)
        rightSearch.add(VPackSlice::maxKeySlice());
        rightSearch.close();
        VPackSlice search = rightSearch.slice();
        rightBorder = search;
      }
    }
  }

  // Secured by trx. The shared_ptr index stays valid in
  // _collection at least as long as trx is running.
  // Same for the iterator
  auto physical = static_cast<RocksDBCollection*>(_collection->getPhysical());
  auto idx = physical->primaryIndex();
  return new RocksDBVPackIndexIterator(_collection, trx, mmdr, this, idx,
                                       reverse, leftBorder, rightBorder);
}

bool RocksDBVPackIndex::accessFitsIndex(
    arangodb::aql::AstNode const* access, arangodb::aql::AstNode const* other,
    arangodb::aql::AstNode const* op, arangodb::aql::Variable const* reference,
    std::unordered_map<size_t, std::vector<arangodb::aql::AstNode const*>>&
        found,
    std::unordered_set<std::string>& nonNullAttributes,
    bool isExecution) const {
  if (!canUseConditionPart(access, other, op, reference, nonNullAttributes,
                           isExecution)) {
    return false;
  }

  arangodb::aql::AstNode const* what = access;
  std::pair<arangodb::aql::Variable const*,
            std::vector<arangodb::basics::AttributeName>>
      attributeData;

  if (op->type != arangodb::aql::NODE_TYPE_OPERATOR_BINARY_IN) {
    if (!what->isAttributeAccessForVariable(attributeData) ||
        attributeData.first != reference) {
      // this access is not referencing this collection
      return false;
    }
    if (arangodb::basics::TRI_AttributeNamesHaveExpansion(
            attributeData.second)) {
      // doc.value[*] == 'value'
      return false;
    }
    if (isAttributeExpanded(attributeData.second)) {
      // doc.value == 'value' (with an array index)
      return false;
    }
  } else {
    // ok, we do have an IN here... check if it's something like 'value' IN
    // doc.value[*]
    TRI_ASSERT(op->type == arangodb::aql::NODE_TYPE_OPERATOR_BINARY_IN);
    bool canUse = false;

    if (what->isAttributeAccessForVariable(attributeData) &&
        attributeData.first == reference &&
        !arangodb::basics::TRI_AttributeNamesHaveExpansion(
            attributeData.second) &&
        attributeMatches(attributeData.second)) {
      // doc.value IN 'value'
      // can use this index
      canUse = true;
    } else {
      // check for  'value' IN doc.value  AND  'value' IN doc.value[*]
      what = other;
      if (what->isAttributeAccessForVariable(attributeData) &&
          attributeData.first == reference &&
          isAttributeExpanded(attributeData.second) &&
          attributeMatches(attributeData.second)) {
        canUse = true;
      }
    }

    if (!canUse) {
      return false;
    }
  }

  std::vector<arangodb::basics::AttributeName> const& fieldNames =
      attributeData.second;

  for (size_t i = 0; i < _fields.size(); ++i) {
    if (_fields[i].size() != fieldNames.size()) {
      // attribute path length differs
      continue;
    }

    if (this->isAttributeExpanded(i) &&
        op->type != arangodb::aql::NODE_TYPE_OPERATOR_BINARY_IN) {
      // If this attribute is correct or not, it could only serve for IN
      continue;
    }

    bool match = arangodb::basics::AttributeName::isIdentical(_fields[i],
                                                              fieldNames, true);

    if (match) {
      // mark ith attribute as being covered
      auto it = found.find(i);

      if (it == found.end()) {
        found.emplace(i, std::vector<arangodb::aql::AstNode const*>{op});
      } else {
        (*it).second.emplace_back(op);
      }
      TRI_IF_FAILURE("PersistentIndex::accessFitsIndex") {
        THROW_ARANGO_EXCEPTION(TRI_ERROR_DEBUG);
      }

      return true;
    }
  }

  return false;
}

void RocksDBVPackIndex::matchAttributes(
    arangodb::aql::AstNode const* node,
    arangodb::aql::Variable const* reference,
    std::unordered_map<size_t, std::vector<arangodb::aql::AstNode const*>>&
        found,
    size_t& values, std::unordered_set<std::string>& nonNullAttributes,
    bool isExecution) const {
  for (size_t i = 0; i < node->numMembers(); ++i) {
    auto op = node->getMember(i);

    switch (op->type) {
      case arangodb::aql::NODE_TYPE_OPERATOR_BINARY_EQ:
      case arangodb::aql::NODE_TYPE_OPERATOR_BINARY_LT:
      case arangodb::aql::NODE_TYPE_OPERATOR_BINARY_LE:
      case arangodb::aql::NODE_TYPE_OPERATOR_BINARY_GT:
      case arangodb::aql::NODE_TYPE_OPERATOR_BINARY_GE:
        TRI_ASSERT(op->numMembers() == 2);
        accessFitsIndex(op->getMember(0), op->getMember(1), op, reference,
                        found, nonNullAttributes, isExecution);
        accessFitsIndex(op->getMember(1), op->getMember(0), op, reference,
                        found, nonNullAttributes, isExecution);
        break;

      case arangodb::aql::NODE_TYPE_OPERATOR_BINARY_IN:
        if (accessFitsIndex(op->getMember(0), op->getMember(1), op, reference,
                            found, nonNullAttributes, isExecution)) {
          auto m = op->getMember(1);
          if (m->isArray() && m->numMembers() > 1) {
            // attr IN [ a, b, c ]  =>  this will produce multiple items, so
            // count them!
            values += m->numMembers() - 1;
          }
        }
        break;

      default:
        break;
    }
  }
}

bool RocksDBVPackIndex::supportsFilterCondition(
    arangodb::aql::AstNode const* node,
    arangodb::aql::Variable const* reference, size_t itemsInIndex,
    size_t& estimatedItems, double& estimatedCost) const {
  std::unordered_map<size_t, std::vector<arangodb::aql::AstNode const*>> found;
  std::unordered_set<std::string> nonNullAttributes;
  size_t values = 0;
  matchAttributes(node, reference, found, values, nonNullAttributes, false);

  bool lastContainsEquality = true;
  size_t attributesCovered = 0;
  size_t attributesCoveredByEquality = 0;
  double equalityReductionFactor = 20.0;
  estimatedCost = static_cast<double>(itemsInIndex);

  for (size_t i = 0; i < _fields.size(); ++i) {
    auto it = found.find(i);

    if (it == found.end()) {
      // index attribute not covered by condition
      break;
    }

    // check if the current condition contains an equality condition
    auto const& nodes = (*it).second;
    bool containsEquality = false;
    for (size_t j = 0; j < nodes.size(); ++j) {
      if (nodes[j]->type == arangodb::aql::NODE_TYPE_OPERATOR_BINARY_EQ ||
          nodes[j]->type == arangodb::aql::NODE_TYPE_OPERATOR_BINARY_IN) {
        containsEquality = true;
        break;
      }
    }

    if (!lastContainsEquality) {
      // unsupported condition. must abort
      break;
    }

    ++attributesCovered;
    if (containsEquality) {
      ++attributesCoveredByEquality;
      estimatedCost /= equalityReductionFactor;

      // decrease the effect of the equality reduction factor
      equalityReductionFactor *= 0.25;
      if (equalityReductionFactor < 2.0) {
        // equalityReductionFactor shouldn't get too low
        equalityReductionFactor = 2.0;
      }
    } else {
      // quick estimate for the potential reductions caused by the conditions
      if (nodes.size() >= 2) {
        // at least two (non-equality) conditions. probably a range with lower
        // and upper bound defined
        estimatedCost /= 7.5;
      } else {
        // one (non-equality). this is either a lower or a higher bound
        estimatedCost /= 2.0;
      }
    }

    lastContainsEquality = containsEquality;
  }

  if (values == 0) {
    values = 1;
  }

  if (attributesCoveredByEquality == _fields.size() && unique()) {
    // index is unique and condition covers all attributes by equality
    if (estimatedItems >= values) {
      // reduce costs due to uniqueness
      estimatedItems = values;
      estimatedCost = static_cast<double>(estimatedItems);
    } else {
      // cost is already low... now slightly prioritize the unique index
      estimatedCost *= 0.995;
    }
    return true;
  }

  if (attributesCovered > 0 &&
      (!_sparse || attributesCovered == _fields.size())) {
    // if the condition contains at least one index attribute and is not
    // sparse,
    // or the index is sparse and all attributes are covered by the condition,
    // then it can be used (note: additional checks for condition parts in
    // sparse indexes are contained in Index::canUseConditionPart)
    estimatedItems = static_cast<size_t>((std::max)(
        static_cast<size_t>(estimatedCost * values), static_cast<size_t>(1)));
    estimatedCost *= static_cast<double>(values);
    return true;
  }

  // no condition
  estimatedItems = itemsInIndex;
  estimatedCost = static_cast<double>(estimatedItems);
  return false;
}

bool RocksDBVPackIndex::supportsSortCondition(
    arangodb::aql::SortCondition const* sortCondition,
    arangodb::aql::Variable const* reference, size_t itemsInIndex,
    double& estimatedCost, size_t& coveredAttributes) const {
  TRI_ASSERT(sortCondition != nullptr);

  if (!_sparse) {
    // only non-sparse indexes can be used for sorting
    if (!_useExpansion && sortCondition->isUnidirectional() &&
        sortCondition->isOnlyAttributeAccess()) {
      coveredAttributes = sortCondition->coveredAttributes(reference, _fields);

      if (coveredAttributes >= sortCondition->numAttributes()) {
        // sort is fully covered by index. no additional sort costs!
        // forward iteration does not have high costs
        estimatedCost = itemsInIndex * 0.001;
        if (sortCondition->isDescending()) {
          // reverse iteration has higher costs than forward iteration
          estimatedCost *= 4;
        }
        return true;
      } else if (coveredAttributes > 0) {
        estimatedCost = (itemsInIndex / coveredAttributes) *
                        std::log2(static_cast<double>(itemsInIndex));
        if (sortCondition->isAscending()) {
          // reverse iteration is more expensive
          estimatedCost *= 4;
        }
        return true;
      }
    }
  }

  coveredAttributes = 0;
  // by default no sort conditions are supported
  if (itemsInIndex > 0) {
    estimatedCost = itemsInIndex * std::log2(static_cast<double>(itemsInIndex));
    // slightly penalize this type of index against other indexes which
    // are in memory
    estimatedCost *= 1.05;
  } else {
    estimatedCost = 0.0;
  }
  return false;
}

IndexIterator* RocksDBVPackIndex::iteratorForCondition(
    transaction::Methods* trx, ManagedDocumentResult* mmdr,
    arangodb::aql::AstNode const* node,
    arangodb::aql::Variable const* reference, bool reverse) {
  VPackBuilder searchValues;
  searchValues.openArray();
  bool needNormalize = false;
  if (node == nullptr) {
    // We only use this index for sort. Empty searchValue
    VPackArrayBuilder guard(&searchValues);

    TRI_IF_FAILURE("PersistentIndex::noSortIterator") {
      THROW_ARANGO_EXCEPTION(TRI_ERROR_DEBUG);
    }
  } else {
    // Create the search Values for the lookup
    VPackArrayBuilder guard(&searchValues);

    std::unordered_map<size_t, std::vector<arangodb::aql::AstNode const*>>
        found;
    std::unordered_set<std::string> nonNullAttributes;
    size_t unused = 0;
    matchAttributes(node, reference, found, unused, nonNullAttributes, true);

    // found contains all attributes that are relevant for this node.
    // It might be less than fields().
    //
    // Handle the first attributes. They can only be == or IN and only
    // one node per attribute

    auto getValueAccess = [&](arangodb::aql::AstNode const* comp,
                              arangodb::aql::AstNode const*& access,
                              arangodb::aql::AstNode const*& value) -> bool {
      access = comp->getMember(0);
      value = comp->getMember(1);
      std::pair<arangodb::aql::Variable const*,
                std::vector<arangodb::basics::AttributeName>>
          paramPair;
      if (!(access->isAttributeAccessForVariable(paramPair) &&
            paramPair.first == reference)) {
        access = comp->getMember(1);
        value = comp->getMember(0);
        if (!(access->isAttributeAccessForVariable(paramPair) &&
              paramPair.first == reference)) {
          // Both side do not have a correct AttributeAccess, this should not
          // happen and indicates
          // an error in the optimizer
          TRI_ASSERT(false);
        }
        return true;
      }
      return false;
    };

    size_t usedFields = 0;
    for (; usedFields < _fields.size(); ++usedFields) {
      auto it = found.find(usedFields);
      if (it == found.end()) {
        // We are either done
        // or this is a range.
        // Continue with more complicated loop
        break;
      }

      auto comp = it->second[0];
      TRI_ASSERT(comp->numMembers() == 2);
      arangodb::aql::AstNode const* access = nullptr;
      arangodb::aql::AstNode const* value = nullptr;
      getValueAccess(comp, access, value);
      // We found an access for this field

      if (comp->type == arangodb::aql::NODE_TYPE_OPERATOR_BINARY_EQ) {
        searchValues.openObject();
        searchValues.add(VPackValue(StaticStrings::IndexEq));
        TRI_IF_FAILURE("PersistentIndex::permutationEQ") {
          THROW_ARANGO_EXCEPTION(TRI_ERROR_DEBUG);
        }
      } else if (comp->type == arangodb::aql::NODE_TYPE_OPERATOR_BINARY_IN) {
        if (isAttributeExpanded(usedFields)) {
          searchValues.openObject();
          searchValues.add(VPackValue(StaticStrings::IndexEq));
          TRI_IF_FAILURE("PersistentIndex::permutationArrayIN") {
            THROW_ARANGO_EXCEPTION(TRI_ERROR_DEBUG);
          }
        } else {
          needNormalize = true;
          searchValues.openObject();
          searchValues.add(VPackValue(StaticStrings::IndexIn));
        }
      } else {
        // This is a one-sided range
        break;
      }
      // We have to add the value always, the key was added before
      value->toVelocyPackValue(searchValues);
      searchValues.close();
    }

    // Now handle the next element, which might be a range
    if (usedFields < _fields.size()) {
      auto it = found.find(usedFields);
      if (it != found.end()) {
        auto rangeConditions = it->second;
        TRI_ASSERT(rangeConditions.size() <= 2);
        VPackObjectBuilder searchElement(&searchValues);
        for (auto& comp : rangeConditions) {
          TRI_ASSERT(comp->numMembers() == 2);
          arangodb::aql::AstNode const* access = nullptr;
          arangodb::aql::AstNode const* value = nullptr;
          bool isReverseOrder = getValueAccess(comp, access, value);
          // Add the key
          switch (comp->type) {
            case arangodb::aql::NODE_TYPE_OPERATOR_BINARY_LT:
              if (isReverseOrder) {
                searchValues.add(VPackValue(StaticStrings::IndexGt));
              } else {
                searchValues.add(VPackValue(StaticStrings::IndexLt));
              }
              break;
            case arangodb::aql::NODE_TYPE_OPERATOR_BINARY_LE:
              if (isReverseOrder) {
                searchValues.add(VPackValue(StaticStrings::IndexGe));
              } else {
                searchValues.add(VPackValue(StaticStrings::IndexLe));
              }
              break;
            case arangodb::aql::NODE_TYPE_OPERATOR_BINARY_GT:
              if (isReverseOrder) {
                searchValues.add(VPackValue(StaticStrings::IndexLt));
              } else {
                searchValues.add(VPackValue(StaticStrings::IndexGt));
              }
              break;
            case arangodb::aql::NODE_TYPE_OPERATOR_BINARY_GE:
              if (isReverseOrder) {
                searchValues.add(VPackValue(StaticStrings::IndexLe));
              } else {
                searchValues.add(VPackValue(StaticStrings::IndexGe));
              }
              break;
            default:
              // unsupported right now. Should have been rejected by
              // supportsFilterCondition
              TRI_ASSERT(false);
              return nullptr;
          }
          value->toVelocyPackValue(searchValues);
        }
      }
    }
  }
  searchValues.close();

  TRI_IF_FAILURE("PersistentIndex::noIterator") {
    THROW_ARANGO_EXCEPTION(TRI_ERROR_DEBUG);
  }

  if (needNormalize) {
    VPackBuilder expandedSearchValues;
    expandInSearchValues(searchValues.slice(), expandedSearchValues);
    VPackSlice expandedSlice = expandedSearchValues.slice();
    std::vector<IndexIterator*> iterators;
    try {
      for (auto const& val : VPackArrayIterator(expandedSlice)) {
        auto iterator = lookup(trx, mmdr, val, reverse);
        try {
          iterators.push_back(iterator);
        } catch (...) {
          // avoid leak
          delete iterator;
          throw;
        }
      }
      if (reverse) {
        std::reverse(iterators.begin(), iterators.end());
      }
    } catch (...) {
      for (auto& it : iterators) {
        delete it;
      }
      throw;
    }
    return new MultiIndexIterator(_collection, trx, mmdr, this, iterators);
  }

  VPackSlice searchSlice = searchValues.slice();
  TRI_ASSERT(searchSlice.length() == 1);
  searchSlice = searchSlice.at(0);
  return lookup(trx, mmdr, searchSlice, reverse);
}

/// @brief specializes the condition for use with the index
arangodb::aql::AstNode* RocksDBVPackIndex::specializeCondition(
    arangodb::aql::AstNode* node,
    arangodb::aql::Variable const* reference) const {
  std::unordered_map<size_t, std::vector<arangodb::aql::AstNode const*>> found;
  std::unordered_set<std::string> nonNullAttributes;
  size_t values = 0;
  matchAttributes(node, reference, found, values, nonNullAttributes, false);

  std::vector<arangodb::aql::AstNode const*> children;
  bool lastContainsEquality = true;

  for (size_t i = 0; i < _fields.size(); ++i) {
    auto it = found.find(i);

    if (it == found.end()) {
      // index attribute not covered by condition
      break;
    }

    // check if the current condition contains an equality condition
    auto& nodes = (*it).second;
    bool containsEquality = false;
    for (size_t j = 0; j < nodes.size(); ++j) {
      if (nodes[j]->type == arangodb::aql::NODE_TYPE_OPERATOR_BINARY_EQ ||
          nodes[j]->type == arangodb::aql::NODE_TYPE_OPERATOR_BINARY_IN) {
        containsEquality = true;
        break;
      }
    }

    if (!lastContainsEquality) {
      // unsupported condition. must abort
      break;
    }

    std::sort(nodes.begin(), nodes.end(),
              [](arangodb::aql::AstNode const* lhs,
                 arangodb::aql::AstNode const* rhs) -> bool {
                return sortWeight(lhs) < sortWeight(rhs);
              });

    lastContainsEquality = containsEquality;
    std::unordered_set<int> operatorsFound;
    for (auto& it : nodes) {
      // do not let duplicate or related operators pass
      if (isDuplicateOperator(it, operatorsFound)) {
        continue;
      }
      operatorsFound.emplace(static_cast<int>(it->type));
      children.emplace_back(it);
    }
  }

  while (node->numMembers() > 0) {
    node->removeMemberUnchecked(0);
  }

  for (auto& it : children) {
    node->addMember(it);
  }
  return node;
}

bool RocksDBVPackIndex::isDuplicateOperator(
    arangodb::aql::AstNode const* node,
    std::unordered_set<int> const& operatorsFound) const {
  auto type = node->type;
  if (operatorsFound.find(static_cast<int>(type)) != operatorsFound.end()) {
    // duplicate operator
    return true;
  }

  if (operatorsFound.find(
          static_cast<int>(arangodb::aql::NODE_TYPE_OPERATOR_BINARY_EQ)) !=
          operatorsFound.end() ||
      operatorsFound.find(
          static_cast<int>(arangodb::aql::NODE_TYPE_OPERATOR_BINARY_IN)) !=
          operatorsFound.end()) {
    return true;
  }

  bool duplicate = false;
  switch (type) {
    case arangodb::aql::NODE_TYPE_OPERATOR_BINARY_LT:
      duplicate = operatorsFound.find(static_cast<int>(
                      arangodb::aql::NODE_TYPE_OPERATOR_BINARY_LE)) !=
                  operatorsFound.end();
      break;
    case arangodb::aql::NODE_TYPE_OPERATOR_BINARY_LE:
      duplicate = operatorsFound.find(static_cast<int>(
                      arangodb::aql::NODE_TYPE_OPERATOR_BINARY_LT)) !=
                  operatorsFound.end();
      break;
    case arangodb::aql::NODE_TYPE_OPERATOR_BINARY_GT:
      duplicate = operatorsFound.find(static_cast<int>(
                      arangodb::aql::NODE_TYPE_OPERATOR_BINARY_GE)) !=
                  operatorsFound.end();
      break;
    case arangodb::aql::NODE_TYPE_OPERATOR_BINARY_GE:
      duplicate = operatorsFound.find(static_cast<int>(
                      arangodb::aql::NODE_TYPE_OPERATOR_BINARY_GT)) !=
                  operatorsFound.end();
      break;
    case arangodb::aql::NODE_TYPE_OPERATOR_BINARY_EQ:
      duplicate = operatorsFound.find(static_cast<int>(
                      arangodb::aql::NODE_TYPE_OPERATOR_BINARY_IN)) !=
                  operatorsFound.end();
      break;
    case arangodb::aql::NODE_TYPE_OPERATOR_BINARY_IN:
      duplicate = operatorsFound.find(static_cast<int>(
                      arangodb::aql::NODE_TYPE_OPERATOR_BINARY_EQ)) !=
                  operatorsFound.end();
      break;
    default: {
      // ignore
    }
  }

  return duplicate;
}