Improve adding intersecting segment using interpolation

shared/src/main/java/org/odk/collect/shared/geometry/Geometry.kt

@@ -101,6 +101,12 @@ fun LineSegment.intersects(other: LineSegment, allowConnection: Boolean = false)
     }
 }
 
+fun LineSegment.interpolate(position: Double): Point {
+    val x = start.x + position * (end.x - start.x)
+    val y = start.y + position * (end.y - start.y)
+    return Point(x, y)
+}
+
 /**
  * Calculate the "orientation" (or "direction") of three points using the cross product of the
  * vectors of the pairs of points (see

shared/src/test/java/org/odk/collect/shared/geometry/GeometryTest.kt

@@ -214,27 +214,34 @@ class GeometryTest {
 
             // Check intersects is consistent when trace is reversed
             val reversedTrace = Trace(trace.points.reversed())
-            assertThat(reversedTrace.intersects(), equalTo(intersects))
+            assertThat(
+                "Expected intersects=$intersects:\n$reversedTrace",
+                reversedTrace.intersects(),
+                equalTo(intersects)
+            )
 
             // Check intersects is consistent when trace is scaled
             val scaleFactor = Random.nextDouble(0.1, 10.0)
             val scaledTrace = Trace(trace.points.map {
                 Point(it.x * scaleFactor, it.y * scaleFactor)
             })
-            assertThat(scaledTrace.intersects(), equalTo(intersects))
+            assertThat(
+                "Expected intersects=$intersects:\n$scaledTrace",
+                scaledTrace.intersects(),
+                equalTo(intersects)
+            )
 
             // Check adding an intersection makes intersects true
             if (!intersects) {
-                val randomSegment = if (trace.segments().size > 1) {
-                    trace.segments().drop(1).random()
-                } else {
-                    trace.segments().first()
-                }
-
-                val intersectionPoint = randomSegment.start
+                val intersectPosition = Random.nextDouble(0.0, 1.0)
+                val intersectionPoint = trace.segments().first().interpolate(intersectPosition)
                 val intersectingTrace =
                     Trace(trace.points + listOf(trace.points.last(), intersectionPoint))
-                assertThat(intersectingTrace.intersects(), equalTo(true))
+                assertThat(
+                    "Expected intersect=true:\n$intersectingTrace",
+                    intersectingTrace.intersects(),
+                    equalTo(true)
+                )
             }
         }
     }