Use a map to look up layer index child nodes

LayersIndex.Node kept its children in a list and scanned that list
linearly to find the child for each path segment. Building the index is
therefore O(entries x siblings), a cost that is dominated by the largest
flat directory in the jar: BOOT-INF/lib/ for the dependencies, and any
bundled resource directory such as a front-end build output.

Keep the children in a LinkedHashMap keyed by the segment name so that
lookups are constant time. Insertion order is preserved, so the order in
which buildIndex() walks the tree is unchanged.

For a jar with 32,529 entries, 426 dependencies and a 12,000 file static
resource directory, this reduces the number of string comparisons from
73.7M to 222K and the time spent building the index from 197ms to 12ms.
For a jar with no large flat directory (20,529 entries, 426
dependencies) the gain is much smaller: 1.58M comparisons to 162K, and
12.5ms to 9.6ms. The generated layers.idx is byte for byte identical in
both cases.

See gh-51654

Signed-off-by: Junggi Kim <kimjg2477@gmail.com>

loader/spring-boot-loader-tools/src/main/java/org/springframework/boot/loader/tools/LayersIndex.java

@@ -21,11 +21,12 @@ import java.io.IOException;
 import java.io.OutputStream;
 import java.io.OutputStreamWriter;
 import java.nio.charset.StandardCharsets;
-import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.HashSet;
+import java.util.LinkedHashMap;
 import java.util.List;
+import java.util.Map;
 import java.util.Set;
 
 import org.springframework.util.LinkedMultiValueMap;
@@ -47,6 +48,7 @@ import org.springframework.util.MultiValueMap;
  * @author Madhura Bhave
  * @author Andy Wilkinson
  * @author Phillip Webb
+ * @author Junggi Kim
  * @since 2.3.0
  */
 public class LayersIndex {
@@ -116,28 +118,21 @@ public class LayersIndex {
 
 		private final Set<Layer> layers;
 
-		private final List<Node> children = new ArrayList<>();
+		private final Map<String, Node> children = new LinkedHashMap<>();
 
 		Node() {
-			this.name = "";
-			this.layers = new HashSet<>();
+			this("");
 		}
 
-		Node(String name, Layer layer) {
+		Node(String name) {
 			this.name = name;
-			this.layers = new HashSet<>(Collections.singleton(layer));
+			this.layers = new HashSet<>();
 		}
 
 		Node updateOrAddNode(String segment, boolean isDirectory, Layer layer) {
 			String name = segment + (isDirectory ? "/" : "");
-			for (Node child : this.children) {
-				if (name.equals(child.name)) {
-					child.layers.add(layer);
-					return child;
-				}
-			}
-			Node child = new Node(name, layer);
-			this.children.add(child);
+			Node child = this.children.computeIfAbsent(name, Node::new);
+			child.layers.add(layer);
 			return child;
 		}
 
@@ -147,7 +142,7 @@ public class LayersIndex {
 				index.add(this.layers.iterator().next(), name);
 			}
 			else {
-				for (Node child : this.children) {
+				for (Node child : this.children.values()) {
 					child.buildIndex(name, index);
 				}
 			}