🐛 fix snapping of route segments on long lines without points

app/Services/GeodesicService.php

@@ -72,6 +72,30 @@ public function haversineDistance(Coordinate $from, Coordinate $to): int
         return (int) round($this->exactHaversineDistance($from, $to));
     }
 
+    /**
+     * Distance in meters from a coordinate to the closest place on the straight line between two
+     * others. Projects onto a local flat plane, which is exact enough for lines of a few dozen
+     * kilometres.
+     */
+    public function distanceToSegment(Coordinate $needle, Coordinate $from, Coordinate $to): int
+    {
+        $metersPerDegree = self::EARTH_RADIUS_METERS * M_PI / 180;
+        $metersPerDegreeLongitude = $metersPerDegree * cos(deg2rad($needle->latitude));
+
+        $fromX = ($from->longitude - $needle->longitude) * $metersPerDegreeLongitude;
+        $fromY = ($from->latitude - $needle->latitude) * $metersPerDegree;
+        $deltaX = ($to->longitude - $from->longitude) * $metersPerDegreeLongitude;
+        $deltaY = ($to->latitude - $from->latitude) * $metersPerDegree;
+
+        $lengthSquared = $deltaX * $deltaX + $deltaY * $deltaY;
+        // Share of the way from $from to $to where the perpendicular from $needle hits the line
+        $share = $lengthSquared > 0.0
+            ? max(0.0, min(1.0, -($fromX * $deltaX + $fromY * $deltaY) / $lengthSquared))
+            : 0.0;
+
+        return (int) round(hypot($fromX + $share * $deltaX, $fromY + $share * $deltaY));
+    }
+
     /**
      * Find the index of the path point closest to the given coordinate.
      *

app/Services/Trip/ProviderPolylineService.php

@@ -339,7 +339,7 @@ private function snapStopoversToGeometry(Collection $stopovers, array $coordinat
                 return null;
             }
 
-            $snapDistance = $this->geodesicService->haversineDistance($location, $coordinates[$bestIndex]);
+            $snapDistance = $this->distanceToGeometryAround($location, $coordinates, $bestIndex);
             if ($snapDistance > self::MAX_SNAP_DISTANCE_METERS) {
                 Log::debug('ProviderPolyline: Stopover too far away from geometry', [
                     'station' => $stopover->station?->name,
@@ -388,6 +388,28 @@ private function snapStopoversToGeometry(Collection $stopovers, array $coordinat
         return $indices;
     }
 
+    /**
+     * Distance from a location to the line leading into and out of the given geometry point. On
+     * straight stretches the provider leaves kilometres between two points, so a stop right on the
+     * line can still be far away from the closest point itself.
+     *
+     * (Example: Long AMTRAK tracks in the US)
+     *
+     * @param  Coordinate[]  $coordinates
+     */
+    private function distanceToGeometryAround(Coordinate $location, array $coordinates, int $index): int
+    {
+        $distance = $this->geodesicService->haversineDistance($location, $coordinates[$index]);
+
+        foreach ([$index - 1, $index + 1] as $neighbour) {
+            if (isset($coordinates[$neighbour])) {
+                $distance = min($distance, $this->geodesicService->distanceToSegment($location, $coordinates[$index], $coordinates[$neighbour]));
+            }
+        }
+
+        return $distance;
+    }
+
     /**
      * Straight-line distance between two stopovers, used to judge how much of a detour the
      * sliced shape describes. Returns 0 when either side has no usable coordinates.