fix minMaxDist floating point rounding errors

This manifests itsef in

#29

rtree.go

@@ -32,17 +32,21 @@ type Rtree struct {
 	// deleted is a temporary buffer to avoid memory allocations in Delete.
 	// It is just an optimization and not part of the data structure.
 	deleted []*node
+
+	// FloatingPointTolerance is the tolerance to guard against floating point rounding errors during minMaxDist calculations.
+	FloatingPointTolerance float64
 }
 
 // NewTree returns an Rtree. If the number of objects given on initialization
 // is larger than max, the Rtree will be initialized using the Overlap
 // Minimizing Top-down bulk-loading algorithm.
 func NewTree(dim, min, max int, objs ...Spatial) *Rtree {
 	rt := &Rtree{
-		Dim:         dim,
-		MinChildren: min,
-		MaxChildren: max,
-		height:      1,
+		Dim:                    dim,
+		MinChildren:            min,
+		MaxChildren:            max,
+		height:                 1,
+		FloatingPointTolerance: 1e-6,
 		root: &node{
 			entries: []entry{},
 			leaf:    true,
@@ -765,7 +769,8 @@ func (tree *Rtree) nearestNeighbor(p Point, n *node, d float64, nearest Spatial)
 
 		for _, e := range n.entries {
 			minDist := p.minDist(e.bb)
-			if minDist > minMinMaxDist {
+			// Add a bit of tolerance to guard against floating point rounding errors.
+			if minDist > minMinMaxDist+tree.FloatingPointTolerance {
 				continue
 			}
 

rtree_test.go

@@ -1354,6 +1354,24 @@ func TestNearestNeighborsHalf(t *testing.T) {
 	}
 }
 
+func TestMinMaxDistFloatingPointRoundingError(t *testing.T) {
+	rects := []Rect{
+		Point{1134900, 15600}.ToRect(0),
+		Point{1134900, 25600}.ToRect(0),
+		Point{1134900, 22805}.ToRect(0),
+		Point{1134900, 29116}.ToRect(0),
+	}
+	things := make([]Spatial, 0, len(rects))
+	for i := range rects {
+		things = append(things, &rects[i])
+	}
+	rt := NewTree(2, 1, 2, things...)
+	n := rt.NearestNeighbor(Point{1134851.8, 25570.8})
+	if n != things[1] {
+		t.Fatalf("wrong neighbor, expected %v, got %v", things[1], n)
+	}
+}
+
 func ensureOrderedSubset(t *testing.T, actual []Spatial, expected []Spatial) {
 	for i := range actual {
 		if len(expected)-1 < i || actual[i] != expected[i] {