Rework comments to reduce confusion about "leaf" entries

rtree/rtree.go

@@ -5,22 +5,23 @@ import (
 )
 
 const (
-	minChildren = 2
-	maxChildren = 4
+	minEntries = 2
+	maxEntries = 4
 )
 
-// node is a node in an R-Tree. nodes can either be leaf nodes holding entries
-// for terminal items, or intermediate nodes holding entries for more nodes.
+// node is a node in an R-Tree, holding user record IDs and/or links to deeper
+// nodes in the tree.
 type node struct {
-	entries    [maxChildren]entry
+	entries    [maxEntries]entry
 	numEntries int
 }
 
-// entry is an entry under a node, leading either to terminal items, or more nodes.
+// entry is an entry contained inside a node. An entry can either hold a user
+// record ID, or point to a deeper node in the tree (but not both). Because 0
+// is a valid record ID, the child pointer should be used to distinguish
+// between the two types of entries.
 type entry struct {
-	box Box
-
-	// For leaf nodes, recordID is populated. For non-leaf nodes, child is populated.
+	box      Box
 	child    *node
 	recordID int
 }

rtree/rtree_test.go

@@ -139,9 +139,6 @@ func checkInvariants(t *testing.T, rt *RTree, boxes []Box) {
 			if e.child == nil {
 				minLeafLevel = min(minLeafLevel, level)
 				maxLeafLevel = max(maxLeafLevel, level)
-				if e.child != nil {
-					t.Fatalf("leaf node has child (entry %d)", i)
-				}
 				if _, ok := unfound[e.recordID]; !ok {
 					t.Fatal("record ID found in tree but wasn't in unfound map")
 				}
@@ -162,12 +159,12 @@ func checkInvariants(t *testing.T, rt *RTree, boxes []Box) {
 		}
 		for i := current.numEntries; i < len(current.entries); i++ {
 			e := current.entries[i]
-			if e.box != (Box{}) || e.child != nil || e.recordID != 0 {
+			if e != (entry{}) {
 				t.Fatal("entry past numEntries is not the zero value")
 			}
 		}
-		if current.numEntries > maxChildren ||
-			(current != rt.root && current.numEntries < minChildren) {
+		if current.numEntries > maxEntries ||
+			(current != rt.root && current.numEntries < minEntries) {
 			t.Fatalf("%p: unexpected number of entries", current)
 		}
 	}