Fix missing double rotation height update

pom.xml

@@ -43,6 +43,16 @@
 			<version>3.2.0</version>
 			<scope>test</scope>
 		</dependency>
+		<dependency>
+			<groupId>org.hamcrest</groupId>
+			<artifactId>hamcrest</artifactId>
+			<version>2.1</version>
+		</dependency>
+		<dependency>
+			<groupId>org.mockito</groupId>
+			<artifactId>mockito-core</artifactId>
+			<version>2.27.0</version>
+		</dependency>
 	</dependencies>
 
 	<properties>

src/main/java/com/lodborg/intervaltree/TreeNode.java

@@ -196,6 +196,7 @@ private TreeNode<T> balanceOut(){
 			// The tree is right-heavy.
 			if (height(right.left) > height(right.right)){
 				this.right = this.right.rightRotate();
+				this.right.height = Math.max(height(this.right.left), height(this.right.right)) + 1;
 				return leftRotate();
 			} else{
 				return leftRotate();
@@ -204,6 +205,7 @@ private TreeNode<T> balanceOut(){
 			// The tree is left-heavy.
 			if (height(left.right) > height(left.left)){
 				this.left = this.left.leftRotate();
+				this.left.height = Math.max(height(this.left.left), height(this.left.right)) + 1;
 				return rightRotate();
 			} else
 				return rightRotate();

src/test/java/com/lodborg/intervaltree/TreeNodeTest.java

@@ -1,12 +1,16 @@
 package com.lodborg.intervaltree;
 
+import com.lodborg.intervaltree.Interval.Bounded;
+import org.hamcrest.CoreMatchers;
 import org.junit.Test;
+import org.mockito.Mockito;
 
+import java.util.*;
+
+import static org.hamcrest.Matchers.greaterThan;
+import static org.hamcrest.Matchers.lessThanOrEqualTo;
 import static org.hamcrest.core.Is.is;
 import static org.junit.Assert.*;
-import com.lodborg.intervaltree.Interval.*;
-
-import java.util.*;
 
 public class TreeNodeTest {
 	@Test
@@ -234,4 +238,87 @@ public void test_iteratorRemoveDeletesInnerNodeAndPromotesTheSubtreeRoot(){
 				assertTrue(list.contains(next));
 		}
 	}
+
+	@Test
+	public void leftRightRotation() {
+
+		IntervalTree<Integer> tree = Mockito.mock(IntervalTree.class);
+		TreeNode<Integer> node = TreeNode.addInterval(tree, null, new IntegerInterval(100, 100, Bounded.CLOSED));
+		node = TreeNode.addInterval(tree, node, new IntegerInterval(50, 50, Bounded.CLOSED));
+		node = TreeNode.addInterval(tree, node, new IntegerInterval(75, 75, Bounded.CLOSED));
+
+		assertAVLHeightProperty(node);
+		assertHeight(node);
+	}
+
+	@Test
+	public void rightLeftRotation() {
+
+		IntervalTree<Integer> tree = Mockito.mock(IntervalTree.class);
+		TreeNode<Integer> node = TreeNode.addInterval(tree, null, new IntegerInterval(50, 50, Bounded.CLOSED));
+		node = TreeNode.addInterval(tree, node, new IntegerInterval(100, 100, Bounded.CLOSED));
+		node = TreeNode.addInterval(tree, node, new IntegerInterval(75, 75, Bounded.CLOSED));
+
+		assertAVLHeightProperty(node);
+		assertHeight(node);
+	}
+
+
+	@Test
+	public void randomIntervals() {
+
+		IntervalTree<Integer> tree = Mockito.mock(IntervalTree.class);
+		Random random = new Random();
+
+		TreeNode<Integer> node = TreeNode.addInterval(tree, null, new IntegerInterval(100, 100, Bounded.CLOSED));
+
+		for (int i = 0; i < 100_000; i++) {
+
+			int num = random.nextInt();
+			node = TreeNode.addInterval(tree, node, new IntegerInterval(num - 1, num + 1, Bounded.CLOSED));
+		}
+
+		assertAVLHeightProperty(node);
+		assertHeight(node);
+	}
+
+	private <T extends Comparable<T>> int assertHeight(TreeNode<T> node) {
+
+		if (node == null) {
+			return 0;
+		}
+
+		int leftHeight = assertHeight(node.left);
+		int rightHeight = assertHeight(node.right);
+
+		int height = Math.max(leftHeight, rightHeight) + 1;
+
+		assertThat(node.height, is(height));
+
+		return height;
+	}
+
+	private <T extends Comparable<T>> void assertAVLHeightProperty(TreeNode<T> root) {
+
+		if (root == null) {
+			return;
+		}
+
+		assertThat(root.height, greaterThan(0));
+
+		if (root.left == null && root.right == null) {
+			assertThat(root.height, CoreMatchers.is(1));
+		} else {
+			assertThat("Height of two subtrees should not differ by more than 1",
+					Math.abs(height(root.left) - height(root.right)), lessThanOrEqualTo(1));
+		}
+
+		assertAVLHeightProperty(root.left);
+		assertAVLHeightProperty(root.right);
+	}
+
+	private <T extends Comparable<T>> int height(TreeNode<T> root) {
+
+		return root == null ? 0 : root.height;
+	}
 }