s2: Add initial pieces of Loop validation.

rsned · dsymonds · commit 770b483fa00a · 2017-09-22T07:57:21.000+10:00
This is a precursor to Polygon validation.

Signed-off-by: David Symonds &lt;dsymonds@golang.org&gt;
diff --git a/s2/loop.go b/s2/loop.go
@@ -204,6 +204,59 @@ func (l *Loop) initBound() {
 	l.subregionBound = ExpandForSubregions(l.bound)
 }
 
+// IsValid reports whether this is a valid loop or not.
+func (l *Loop) IsValid() bool {
+	return l.findValidationError() == nil
+}
+
+// findValidationError reports whether this is not a valid loop and if so
+// returns an error describing why. This function requires the Loops ShapeIndex
+// to have been intialized.
+func (l *Loop) findValidationError() error {
+	if err := l.findValidationErrorNoIndex(); err != nil {
+		return err
+	}
+	// Check for intersections between non-adjacent edges (including at vertices)
+	// TODO(roberts): Once shapeutil gets findAnyCrossing uncomment this.
+	// return findAnyCrossing(l.index)
+	return nil
+}
+
+// findValidationErrorNoIndex reports whether this is not a valid loop, but
+// skips checks that would require a ShapeIndex to be built for the loop. This
+// is primarily used by Polygon to do validation so it doesn't trigger the
+// creation of unneeded ShapeIndices.
+func (l *Loop) findValidationErrorNoIndex() error {
+	// All vertices must be unit length.
+	for i, v := range l.vertices {
+		if !v.IsUnit() {
+			return fmt.Errorf("vertex %d is not unit length", i)
+		}
+	}
+
+	// Loops must have at least 3 vertices (except for empty and full).
+	if len(l.vertices) < 3 {
+		if l.isEmptyOrFull() {
+			return nil // Skip remaining tests.
+		}
+		return fmt.Errorf("non-empty, non-full loops must have at least 3 vertices")
+	}
+
+	// Loops are not allowed to have any duplicate vertices or edge crossings.
+	// We split this check into two parts. First we check that no edge is
+	// degenerate (identical endpoints). Then we check that there are no
+	// intersections between non-adjacent edges (including at vertices). The
+	// second check needs the ShapeIndex, so it does not fall within the scope
+	// of this method.
+	for i, v := range l.vertices {
+		if v == l.Vertex(i+1) {
+			return fmt.Errorf("edge %d is degenerate (duplicate vertex)", i)
+		}
+	}
+
+	return nil
+}
+
 // ContainsOrigin reports true if this loop contains s2.OriginPoint().
 func (l *Loop) ContainsOrigin() bool {
 	return l.originInside
diff --git a/s2/loop_test.go b/s2/loop_test.go
@@ -1582,6 +1582,63 @@ func TestLoopNormalizedCompatibleWithContains(t *testing.T) {
 	}
 }
 
+func TestLoopIsValidDetectsInvalidLoops(t *testing.T) {
+	tests := []struct {
+		msg    string
+		points []Point
+	}{
+		// Not enough vertices. Note that all single-vertex loops are valid; they
+		// are interpreted as being either "empty" or "full".
+		{
+			msg:    "loop has no vertices",
+			points: parsePoints(""),
+		},
+		{
+			msg:    "loop has too few vertices",
+			points: parsePoints("20:20, 21:21"),
+		},
+		// degenerate edge checks happen in validation before duplicate vertices.
+		{
+			msg:    "loop has degenerate first edge",
+			points: parsePoints("20:20, 20:20, 20:21"),
+		},
+		{
+			msg:    "loop has degenerate third edge",
+			points: parsePoints("20:20, 20:21, 20:20"),
+		},
+		// TODO(roberts): Uncomment these cases when FindAnyCrossings is in.
+		/*
+			{
+				msg:    "loop has duplicate points",
+				points: parsePoints("20:20, 21:21, 21:20, 20:20, 20:21"),
+			},
+			{
+				msg:    "loop has crossing edges",
+				points: parsePoints("20:20, 21:21, 21:20.5, 21:20, 20:21"),
+			},
+		*/
+		{
+			// Ensure points are not normalized.
+			msg: "loop with non-normalized vertices",
+			points: []Point{
+				Point{r3.Vector{2, 0, 0}},
+				Point{r3.Vector{0, 1, 0}},
+				Point{r3.Vector{0, 0, 1}},
+			},
+		},
+	}
+
+	for _, test := range tests {
+		loop := LoopFromPoints(test.points)
+		err := loop.findValidationError()
+		if err == nil {
+			t.Errorf("%s. %v.findValidationError() = nil, want err to be non-nil", test.msg, loop)
+		}
+		// The C++ tests also tests that the returned error message string contains
+		// a specific set of text. That part of the test is skipped here.
+	}
+}
+
 const (
 	// TODO(roberts): Convert these into changeable flags or parameters.
 	// A loop with a 10km radius and 4096 vertices has an edge length of 15 meters.
