do not change fork directions when combining turns

Author: Moritz Kobitzsch

features/guidance/perception.feature

@@ -97,5 +97,7 @@ Feature: Simple Turns
             | ei    | left  | yes    |
 
         When I route I should get
-            | waypoints | route        | turns                    |
-            | g,a       | in,road,road | depart,fork right,arrive |
+            | waypoints | route          | turns                           |
+            | g,a       | in,road,road   | depart,fork slight right,arrive |
+            | g,h       | in,right,right | depart,fork straight,arrive     |
+            | g,i       | in,left,left   | depart,fork slight left,arrive  |

src/engine/guidance/collapse_turns.cpp

@@ -158,6 +158,18 @@ void TransferTurnTypeStrategy::operator()(RouteStep &step_at_turn_location,
 void AdjustToCombinedTurnAngleStrategy::operator()(RouteStep &step_at_turn_location,
                                                    const RouteStep &transfer_from_step) const
 {
+    // Forks point to left/right. By doing a combined angle, we would risk ending up with
+    // unreasonable fork instrucitons. The direction of a fork only depends on the forking location,
+    // not further angles coming up
+    //
+    //          d
+    //       .  c
+    // a - b
+    //
+    // could end up as `fork left` for `a-b-c`, instead of fork-right
+    if (hasTurnType(step_at_turn_location, TurnType::Fork))
+        return;
+
     // TODO assert transfer_from_step == step_at_turn_location + 1
     const auto angle = findTotalTurnAngle(step_at_turn_location, transfer_from_step);
     step_at_turn_location.maneuver.instruction.direction_modifier = getTurnDirection(angle);
@@ -173,7 +185,19 @@ void AdjustToCombinedTurnStrategy::operator()(RouteStep &step_at_turn_location,
                                               const RouteStep &transfer_from_step) const
 {
     const auto angle = findTotalTurnAngle(step_at_turn_location, transfer_from_step);
-    const auto new_modifier = getTurnDirection(angle);
+
+    // Forks point to left/right. By doing a combined angle, we would risk ending up with
+    // unreasonable fork instrucitons. The direction of a fork only depends on the forking location,
+    // not further angles coming up
+    //
+    //          d
+    //       .  c
+    // a - b
+    //
+    // could end up as `fork left` for `a-b-c`, instead of fork-right
+    const auto new_modifier = hasTurnType(step_at_turn_location, TurnType::Fork)
+                                  ? step_at_turn_location.maneuver.instruction.direction_modifier
+                                  : getTurnDirection(angle);
 
     // a turn that is a new name or straight (turn/continue)
     const auto is_non_turn = [](const RouteStep &step) {