Resolves #1042: Optimisation - removed the n^2 visits of the tree.

Author: andrzejj0

versions-maven-plugin/src/main/java/org/codehaus/mojo/versions/SetMojo.java

@@ -413,22 +413,31 @@ public void execute() throws MojoExecutionException, MojoFailureException {
     }
 
     static Map<Pair<String, String>, Set<Model>> computeChildren(Map<File, Model> reactor) {
-        Map<Pair<String, String>, Set<Model>> result = new HashMap<>();
-        for (Map.Entry<File, Model> entry : reactor.entrySet()) {
-            Optional.ofNullable(entry.getValue().getParent())
-                    .map(p -> new ImmutablePair<>(p.getGroupId(), p.getArtifactId()))
-                    .ifPresent(parent -> result.compute(parent, (f1, s) -> Optional.ofNullable(s)
-                            .map(children -> {
-                                children.add(entry.getValue());
-                                return children;
-                            })
-                            .orElse(new HashSet<Model>() {
-                                {
-                                    add(entry.getValue());
-                                }
-                            })));
-        }
-        return result;
+        return reactor.values().stream()
+                .filter(v -> v.getParent() != null)
+                .reduce(
+                        new HashMap<>(),
+                        (map, child) -> {
+                            map.compute(
+                                    new ImmutablePair<>(
+                                            child.getParent().getGroupId(),
+                                            child.getParent().getArtifactId()),
+                                    (pair, set) -> Optional.ofNullable(set)
+                                            .map(children -> {
+                                                children.add(child);
+                                                return children;
+                                            })
+                                            .orElse(new HashSet<Model>() {
+                                                {
+                                                    add(child);
+                                                }
+                                            }));
+                            return map;
+                        },
+                        (m1, m2) -> {
+                            m1.putAll(m2);
+                            return m1;
+                        });
     }
 
     /**

versions-maven-plugin/src/test/java/org/codehaus/mojo/versions/SetMojoTest.java

@@ -252,9 +252,9 @@ public void testComputeChildren() {
                         });
                     }
                 });
-                put(new File("parent2"), new Model() {
+                put(new File("child2"), new Model() {
                     {
-                        setArtifactId("parent2");
+                        setArtifactId("child2");
                         setParent(new Parent() {
                             {
                                 setGroupId("default");