From 47a6cd5462822184ad03feff6198b6c1b597168f Mon Sep 17 00:00:00 2001
From: Moritz Kobitzsch <moritz@mapbox.com>
Date: Mon, 5 Sep 2016 12:41:33 +0200
Subject: [PATCH] make sure collapse only happens on valid indices

---
 src/engine/guidance/post_processing.cpp | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/engine/guidance/post_processing.cpp b/src/engine/guidance/post_processing.cpp
index 0e4a40d4a..5c678711b 100644
--- a/src/engine/guidance/post_processing.cpp
+++ b/src/engine/guidance/post_processing.cpp
@@ -832,8 +832,8 @@ std::vector<RouteStep> collapseTurns(std::vector<RouteStep> steps)
                 invalidateStep(steps[step_index]);
                 invalidateStep(steps[next_step_index]);
             }
-            else if (choiceless(current_step, one_back_step) ||
-                     one_back_step.distance <= MAX_COLLAPSE_DISTANCE)
+            else if (one_back_index > 0 && (choiceless(current_step, one_back_step) ||
+                     one_back_step.distance <= MAX_COLLAPSE_DISTANCE))
             {
                 // check for one of the multiple collapse scenarios and, if possible, collapse the
                 // turn