simplify intersection polygon code a bit, then handle the degenerate cases

docs/design/code.md

@@ -22,6 +22,8 @@ ControlMap, DrawMap, etc.
 Need to figure out how to handle reaping old IDs for transient objects like
 cars, but also things like modified roads. Slot maps?
 
+Sort of related -- http://smallcultfollowing.com/babysteps/blog/2018/11/01/after-nll-interprocedural-conflicts/
+
 ## Everything as FSMs
 
 Driving and walking layer are both kind of broken, since they know about

docs/design/walking.md

@@ -55,3 +55,5 @@ Small complication with two directional sidewalks
 	  original direction always? How to start/end walking?
 	- do they belong to children {forwards, backwards}? They'd no longer be
 	  in order.
+
+And what about modeling shared left-turn lanes?

map_model/src/make/intersections.rs

@@ -1,53 +1,102 @@
-use geom::{PolyLine, Pt2D};
+use dimensioned::si;
+use geom::{PolyLine, Angle, Pt2D};
 use std::collections::BTreeSet;
+use std::marker;
 use {Intersection, Road, RoadID, LANE_THICKNESS};
 
+const DEGENERATE_INTERSECTION_HALF_LENGTH: si::Meter<f64> = si::Meter {
+    value_unsafe: 5.0,
+    _marker: marker::PhantomData,
+};
+
+// The polygon should exist entirely within the thick bands around all original roads -- it just
+// carves up part of that space, doesn't reach past it.
 pub fn intersection_polygon(
     i: &Intersection,
     road_ids: BTreeSet<RoadID>,
     roads: &Vec<Road>,
 ) -> Vec<Pt2D> {
-    // Turn all of the incident roads into the center PolyLine, always pointing at the intersection
+    // Turn all of the incident roads into two PolyLines (the "forwards" and "backwards" borders of
-    // (endpoint is pt). The f64's are the width to shift without transforming the points, and then
+    // the road), both with an endpoint at i.point, and the angle of the last segment of the center
-    // the width to shift when reversing the points.
+    // line.
-    // TODO Simplify code by just stashing the two transformed PolyLines here. ;)
+    let mut lines: Vec<(RoadID, Angle, PolyLine, PolyLine)> = road_ids
-    let mut center_lines: Vec<(PolyLine, RoadID, f64, f64)> = road_ids
         .into_iter()
         .map(|id| {
             let r = &roads[id.0];
-            let line = &r.center_pts;
+            let center_line = &r.center_pts;
             let fwd_width = LANE_THICKNESS * (r.children_forwards.len() as f64);
             let back_width = LANE_THICKNESS * (r.children_backwards.len() as f64);
 
-            if line.first_pt() == i.point {
+            let (line, width_normal, width_reverse) = if center_line.first_pt() == i.point {
-                (line.reversed(), id, back_width, fwd_width)
+                (center_line.reversed(), back_width, fwd_width)
-            } else if line.last_pt() == i.point {
+            } else if center_line.last_pt() == i.point {
-                (line.clone(), id, fwd_width, back_width)
+                (center_line.clone(), fwd_width, back_width)
             } else {
                 panic!("Incident road {} doesn't have an endpoint at {}", id, i.id);
-            }
+            };
+
+            let pl_normal = line.shift(width_normal).unwrap();
+            let pl_reverse = line.reversed().shift(width_reverse).unwrap().reversed();
+            (id, line.last_line().angle(), pl_normal, pl_reverse)
         }).collect();
 
     // Sort the polylines by the angle of their last segment.
     // TODO This might break weirdly for polylines with very short last lines!
-    center_lines.sort_by_key(|(pl, _, _, _)| pl.last_line().angle().normalized_degrees() as i64);
+    lines.sort_by_key(|(_, angle, _, _)| angle.normalized_degrees() as i64);
 
-    // Now look at adjacent pairs of these polylines...
+    // Special cases for degenerate intersections.
     let mut endpoints: Vec<Pt2D> = Vec::new();
-    for idx1 in 0..center_lines.len() as isize {
+    if lines.len() == 1 {
+        // Dead-ends!
+        let (id, _, pl_a, pl_b) = &lines[0];
+        let pt1 = pl_a
+            .reversed()
+            .safe_dist_along(DEGENERATE_INTERSECTION_HALF_LENGTH * 2.0)
+            .map(|(pt, _)| pt);
+        let pt2 = pl_b
+            .reversed()
+            .safe_dist_along(DEGENERATE_INTERSECTION_HALF_LENGTH * 2.0)
+            .map(|(pt, _)| pt);
+        if pt1.is_some() && pt2.is_some() {
+            endpoints.extend(vec![
+                pt1.unwrap(),
+                pt2.unwrap(),
+                pl_b.last_pt(),
+                pl_a.last_pt(),
+            ]);
+        } else {
+            error!("{} is a dead-end for {}, which is too short to make degenerate intersection geometry", i.id, id);
+            endpoints.extend(vec![pl_a.last_pt(), pl_b.last_pt()]);
+        }
+    } else if lines.len() == 2 {
+        let (id1, _, pl1_a, pl1_b) = &lines[0];
+        let (id2, _, pl2_a, pl2_b) = &lines[1];
+        endpoints.extend(
+            vec![pl1_a, pl1_b, pl2_a, pl2_b]
+                .into_iter()
+                .filter_map(|l| {
+                    l.reversed()
+                        .safe_dist_along(DEGENERATE_INTERSECTION_HALF_LENGTH)
+                        .map(|(pt, _)| pt)
+                }).collect::<Vec<Pt2D>>(),
+        );
+        if endpoints.len() != 4 {
+            error!("{} has only {} and {}, some of which are too short to make degenerate intersection geometry", i.id, id1, id2);
+            endpoints.clear();
+            endpoints.extend(vec![
+                pl1_a.last_pt(),
+                pl1_b.last_pt(),
+                pl2_a.last_pt(),
+                pl2_b.last_pt(),
+            ]);
+        }
+    } else {
+        // Look at adjacent pairs of these polylines...
+        for idx1 in 0..lines.len() as isize {
             let idx2 = idx1 + 1;
 
-        let (center1, id1, _, width1_reverse) = wraparound_get(&center_lines, idx1);
+            let (id1, _, _, pl1) = wraparound_get(&lines, idx1);
-        let (center2, id2, width2_normal, _) = wraparound_get(&center_lines, idx2);
+            let (id2, _, pl2, _) = wraparound_get(&lines, idx2);
-
-        // Turn the center polylines into one of the road's border polylines. Every road should
-        // have a chance to be shifted in both directions.
-        let pl1 = center1
-            .reversed()
-            .shift(*width1_reverse)
-            .unwrap()
-            .reversed();
-        let pl2 = center2.shift(*width2_normal).unwrap();
 
             // If the two lines are too close in angle, they'll either not hit or even if they do, it
             // won't be right.
@@ -66,15 +115,7 @@ pub fn intersection_polygon(
             let mut ok = true;
 
             // Use the next adjacent road, doing line to line segment intersection instead.
-        let inf_line1 = {
+            let inf_line1 = wraparound_get(&lines, idx1 - 1).3.last_line();
-            let (center, _, _, width_reverse) = wraparound_get(&center_lines, idx1 - 1);
-            center
-                .reversed()
-                .shift(*width_reverse)
-                .unwrap()
-                .reversed()
-                .last_line()
-        };
             if let Some(hit) = pl1.intersection_infinite_line(inf_line1) {
                 endpoints.push(hit);
             } else {
@@ -82,10 +123,7 @@ pub fn intersection_polygon(
                 ok = false;
             }
 
-        let inf_line2 = {
+            let inf_line2 = wraparound_get(&lines, idx2 + 1).2.last_line();
-            let (center, _, width_normal, _) = wraparound_get(&center_lines, idx2 + 1);
-            center.shift(*width_normal).unwrap().last_line()
-        };
             if let Some(hit) = pl2.intersection_infinite_line(inf_line2) {
                 endpoints.push(hit);
             } else {
@@ -99,10 +137,11 @@ pub fn intersection_polygon(
                     id1,
                     id2,
                     i.id,
-                center_lines.len()
+                    lines.len()
                 );
             }
         }
+    }
 
     // Close off the polygon
     let first_pt = endpoints[0].clone();