Function TryToGetExitDirection

libs/routing/car_directions.cpp:212–259

\brief Return |CarDirection::ExitHighwayToRight| or |CarDirection::ExitHighwayToLeft| or return |CarDirection::None| if no exit is detected. \note The function makes a decision about turn based on geometry of the route and turn candidates, so it works correctly for both left and right hand traffic.

/// \note The function makes a decision about turn based on geometry of the route and turn
/// candidates, so it works correctly for both left and right hand traffic.
CarDirection TryToGetExitDirection(TurnCandidates const & possibleTurns, TurnInfo const & turnInfo,
                                   Segment const & firstOutgoingSeg, CarDirection const intermediateDirection)
{
  if (!possibleTurns.isCandidatesAngleValid)
    return CarDirection::None;

  if (!IsHighway(turnInfo.m_ingoing->m_highwayClass, turnInfo.m_ingoing->m_isLink))
    return CarDirection::None;

  if (!turnInfo.m_outgoing->m_isLink &&
      !(IsSmallRoad(turnInfo.m_outgoing->m_highwayClass) && IsGoStraightOrSlightTurn(intermediateDirection)))
    return CarDirection::None;

  // At this point it is known that the route goes form a highway to a link road or to a small road
  // which has a slight angle with the highway.

  // Considering cases when the route goes from a highway to a link or a small road.
  // Checking all turn candidates (sorted by their angles) and looking for the road which is a
  // continuation of the ingoing segment. If the continuation is on the right hand of the route
  // it's an exit to the left. If the continuation is on the left hand of the route
  // it's an exit to the right.
  // Note. The angle which is used for sorting turn candidates in |possibleTurns.candidates|
  // is a counterclockwise angle between the ingoing route edge and corresponding candidate.
  // For left turns the angle is less than zero and for right ones it is more than zero.
  bool isCandidateBeforeOutgoing = true;
  bool isHighwayCandidateBeforeOutgoing = true;
  size_t highwayCandidateNumber = 0;

  for (auto const & c : possibleTurns.candidates)
  {
    if (c.m_segment == firstOutgoingSeg)
    {
      isCandidateBeforeOutgoing = false;
      continue;
    }

    if (IsHighway(c.m_highwayClass, c.m_isLink))
    {
      ++highwayCandidateNumber;
      if (highwayCandidateNumber >= 2)
        return CarDirection::None;  // There are two or more highway candidates from the junction.
      isHighwayCandidateBeforeOutgoing = isCandidateBeforeOutgoing;
    }
  }
  if (highwayCandidateNumber == 1)
    return isHighwayCandidateBeforeOutgoing ? CarDirection::ExitHighwayToRight : CarDirection::ExitHighwayToLeft;
  return CarDirection::None;
}

// Min difference of route and alternative turn abs angles in degrees
// to ignore alternative turn (when route direction is GoStraight).