Fix line trim when line begins on polygon edge

CHANGELOG.md

@@ -3,6 +3,7 @@
 ### Fixed
 - CodeGen was failing intermittently
 - Elements schemas with Dictionary types were failing to serialize
+- [#355](https://github.com/hypar-io/Elements/issues/355)
 ### Added
 - Elements supports the [Hub beta](https://hypar-io.github.io/Elements/Hub.html)
 - CodeGen supports `input_schema`

src/Elements/Geometry/Line.cs

@@ -1,4 +1,3 @@
-using Elements.Geometry.Interfaces;
 using System;
 using System.Collections.Generic;
 using System.Linq;
@@ -470,7 +469,8 @@ public List<Line> Trim(Polygon polygon, out List<Line> outsideSegments)
 
             // Add the segment's starting point.
             intersections.Add(this.Start);
-            var StartsOutsidePolygon = !polygon.Contains(this.Start);
+            polygon.Contains(this.Start, out var containment);
+            var StartsOutsidePolygon = containment == Containment.Outside;
 
             var hasVertexIntersections = false;
 
@@ -514,7 +514,7 @@ public List<Line> Trim(Polygon polygon, out List<Line> outsideSegments)
                     continue;
                 }
                 var segment = new Line(A, B);
-                if (hasVertexIntersections) // if it passed through a vertex, we can't rely on alternating, so check each midpoint
+                if (hasVertexIntersections || containment == Containment.CoincidesAtEdge) // if it passed through a vertex, or started at an edge, we can't rely on alternating, so check each midpoint
                 {
                     currentlyIn = polygon.Contains((A + B) / 2);
                 }

test/Elements.Tests/LineTests.cs

@@ -1,5 +1,8 @@
 using System;
+using System.Collections.Generic;
+using System.IO;
 using Elements.Tests;
+using Newtonsoft.Json;
 using Xunit;
 
 namespace Elements.Geometry.Tests
@@ -174,6 +177,16 @@ public void LineTrimToInYZ()
             Assert.NotNull(l3);
         }
 
+        [Fact]
+        public void TrimLineThatStartsAtPolygonEdge()
+        {
+            var polygon = JsonConvert.DeserializeObject<Polygon>(File.ReadAllText("../../../models/Geometry/ConcavePolygon.json"));
+            var line = JsonConvert.DeserializeObject<Line>(File.ReadAllText("../../../models/Geometry/LineThatFailsTrim.json"));
+            var lines = line.Trim(polygon, out var _);
+
+            Assert.Equal(4.186147, lines[0].Length(), 2);
+        }
+
 
         [Fact]
         public void LineTrimWithPolygon()

test/Elements.Tests/models/Geometry/ConcavePolygon.json

@@ -0,0 +1,105 @@
+{
+    "discriminator": "Elements.Geometry.Polygon",
+    "Vertices": [
+        {
+            "X": -10.276991193869598,
+            "Y": -1.1511904595984275,
+            "Z": 0.0
+        },
+        {
+            "X": -3.1186796087302824,
+            "Y": -1.1511904595984301,
+            "Z": 0.0
+        },
+        {
+            "X": -3.1186796087302824,
+            "Y": 6.007121125540885,
+            "Z": 0.0
+        },
+        {
+            "X": 6.0908440680571374,
+            "Y": 6.007121125540885,
+            "Z": 0.0
+        },
+        {
+            "X": 6.0908440680571374,
+            "Y": 0.98374457456592879,
+            "Z": 0.0
+        },
+        {
+            "X": 10.6118829639346,
+            "Y": 0.98374457456592879,
+            "Z": 0.0
+        },
+        {
+            "X": 10.6118829639346,
+            "Y": -4.5001081602483968,
+            "Z": 0.0
+        },
+        {
+            "X": 14.002662135842693,
+            "Y": -4.5001081602483968,
+            "Z": 0.0
+        },
+        {
+            "X": 14.002662135842693,
+            "Y": 5.5047834704433907,
+            "Z": 0.0
+        },
+        {
+            "X": 18.188809261655159,
+            "Y": 5.5047834704433907,
+            "Z": 0.0
+        },
+        {
+            "X": 18.188809261655159,
+            "Y": -8.5606708722864866,
+            "Z": 0.0
+        },
+        {
+            "X": 5.2954761141527626,
+            "Y": -8.5606708722864866,
+            "Z": 0.0
+        },
+        {
+            "X": 5.2954761141527626,
+            "Y": -3.1605410799884073,
+            "Z": 0.0
+        },
+        {
+            "X": 0.14651514940343802,
+            "Y": -3.1605410799884073,
+            "Z": 0.0
+        },
+        {
+            "X": 0.14651514940343691,
+            "Y": -9.1885929411583547,
+            "Z": 0.0
+        },
+        {
+            "X": -5.0861687578621426,
+            "Y": -9.1885929411583547,
+            "Z": 0.0
+        },
+        {
+            "X": -5.0861687578621426,
+            "Y": -4.9605843440877688,
+            "Z": 0.0
+        },
+        {
+            "X": -7.1792423207683749,
+            "Y": -4.9605843440877688,
+            "Z": 0.0
+        },
+        {
+            "X": -7.1792423207683749,
+            "Y": -7.3885496770589976,
+            "Z": 0.0
+        },
+        {
+            "X": -10.276991193869598,
+            "Y": -6.0908440680571321,
+            "Z": 0.0
+        }
+    ]
+}

test/Elements.Tests/models/Geometry/LineThatFailsTrim.json

@@ -0,0 +1,13 @@
+{
+    "discriminator": "Elements.Geometry.Line",
+    "Start": {
+        "X": 18.188809261655159,
+        "Y": 1.5664562544790255,
+        "Z": 0.0
+    },
+    "End": {
+        "X": -10.276991193869598,
+        "Y": 1.5664562544790255,
+        "Z": 0.0
+    }
+}