make some more pedestrian paths possible by forcing turns even when line trimming fails

docs/TODO_phase3.md

@@ -28,8 +28,6 @@
 
 ## pedestrians
 
-- why so many impossible paths? r350 to r41
-
 - UI: draw cars and peds in intersections, even at lower zoom levels, just like on roads
 - make them obey intersections (deterministically!)
 - make them start and end at buildings

geom/src/polyline.rs

@@ -180,6 +180,20 @@ impl PolyLine {
             .map(|pts| pts.iter().map(|pt| pt.to_vec()).collect())
             .collect()
     }
+
+    pub fn intersects(&self, other: &PolyLine) -> bool {
+        // Quadratic
+        for pair1 in self.pts.windows(2) {
+            let l1 = Line::new(pair1[0], pair1[1]);
+            for pair2 in other.pts.windows(2) {
+                let l2 = Line::new(pair2[0], pair2[1]);
+                if l1.intersects(&l2) {
+                    return true;
+                }
+            }
+        }
+        false
+    }
 }
 
 impl fmt::Display for PolyLine {

map_model/src/make/turns.rs

@@ -99,7 +99,9 @@ pub(crate) fn make_crosswalks(i: &Intersection, m: &Map, mut turn_id_start: usiz
             }
             let dst_pt = dst.endpoint(i.id);
 
-            if src_pt != dst_pt {
+            // TODO Just the first check ideally, but until line trimming handles polylines, we
+            // should also do the second check.
+            if src_pt != dst_pt && !src.lane_center_pts.intersects(&dst.lane_center_pts) {
                 continue;
             }