Add IntersectionTools::collinear

Author: loganharbour

include/geom/intersection_tools.h

@@ -69,6 +69,19 @@ WithinSegmentResult within_segment(const Point & s1,
                                    const Point & p,
                                    const Real tol = TOLERANCE);
 
+/**
+ * Checks whether or not the given points are collinear
+ * @param p1 The first point
+ * @param p2 The second point
+ * @param p3 The third point
+ * @param tol The tolerance to use
+ * @return Whether or not the given points are collinear
+ */
+bool collinear(const Point & p1,
+               const Point & p2,
+               const Point & p3,
+               const Real tol = TOLERANCE);
+
 } // namespace IntersectionTools
 } // namespace libMesh
 

src/geom/intersection_tools.C

@@ -20,6 +20,7 @@
 #include "libmesh/intersection_tools.h"
 
 #include "libmesh/point.h"
+#include "libmesh/int_range.h"
 
 namespace libMesh::IntersectionTools
 {
@@ -62,6 +63,19 @@ WithinSegmentResult within_segment(const Point & s1,
   return within_segment(s1, s2, (s1 - s2).norm(), p, tol);
 }
 
+bool collinear(const Point & p1,
+               const Point & p2,
+               const Point & p3,
+               const Real tol)
+{
+  // (p1 -> p2) X (p1 - > p3) should be == the zero vector
+  const auto p1p2_cross_p1p3 = (p2 - p1).cross(p3 - p1);
+  for (const auto i : make_range(LIBMESH_DIM))
+    if (std::abs(p1p2_cross_p1p3(i)) > tol)
+      return false;
+  return true;
+}
+
 } // namespace libMesh::IntersectionTools
 
 

tests/geom/intersection_tools_test.C

@@ -12,6 +12,7 @@ class IntersectionToolsTest : public CppUnit::TestCase
 public:
   LIBMESH_CPPUNIT_TEST_SUITE( IntersectionToolsTest );
   CPPUNIT_TEST( within_segment );
+  CPPUNIT_TEST( collinear );
   CPPUNIT_TEST_SUITE_END();
 
 public:
@@ -47,6 +48,35 @@ public:
                          IntersectionTools::WithinSegmentResult::NOT_WITHIN);
   }
 
+  void collinear()
+  {
+    LOG_UNIT_TEST;
+
+    const auto do_assert = [](const Point & p1,
+                              const Point & p2,
+                              const Point & p3,
+                              const bool collinear)
+    {
+      CPPUNIT_ASSERT_EQUAL(IntersectionTools::collinear(p1, p2, p3), collinear);
+      CPPUNIT_ASSERT_EQUAL(IntersectionTools::collinear(p1, p3, p2), collinear);
+      CPPUNIT_ASSERT_EQUAL(IntersectionTools::collinear(p2, p1, p3), collinear);
+      CPPUNIT_ASSERT_EQUAL(IntersectionTools::collinear(p2, p3, p1), collinear);
+      CPPUNIT_ASSERT_EQUAL(IntersectionTools::collinear(p3, p1, p2), collinear);
+      CPPUNIT_ASSERT_EQUAL(IntersectionTools::collinear(p3, p2, p1), collinear);
+    };
+
+    // two of the same points
+    do_assert(Point(1, 2, 3), Point(1, 2, 3), Point(4, 5, 6), true);
+
+    // three of the same points
+    do_assert(Point(1, 2, 3), Point(1, 2, 3), Point(1, 2, 3), true);
+
+    // all in a line
+    do_assert(Point(1, 1, 2), Point(1, 1, 3), Point(1, 1, 0), true);
+
+    // not collinear
+    do_assert(Point(0, 1, 2), Point(0, 1, 3), Point(1, 5, 10), false);
+  }
 };
 
 CPPUNIT_TEST_SUITE_REGISTRATION( IntersectionToolsTest );