Select the best possible namespace instead of the last one to match any of the elements.

src/PSR4/PSR4Finder.php

@@ -20,16 +20,7 @@ class PSR4Finder implements FinderInterface
      */
     public function findClasses($namespace)
     {
-        $composerNamespaces = $this->factory->getPSR4Namespaces();
-
-        /** @var PSR4Namespace $bestNamespace */
-        $bestNamespace = array_reduce($composerNamespaces, function($carry, PSR4Namespace $potentialNamespace) use ($namespace) {
-            if ($potentialNamespace->matches($namespace)) {
-                return $potentialNamespace;
-            } else {
-                return $carry;
-            }
-        }, null);
+        $bestNamespace = $this->findBestPSR4Namespace($namespace);
 
         if ($bestNamespace instanceof PSR4Namespace) {
             return $bestNamespace->findClasses($namespace);
@@ -40,4 +31,31 @@ class PSR4Finder implements FinderInterface
             ));
         }
     }
+
+    /**
+     * @param $namespace
+     * @return PSR4Namespace
+     */
+    private function findBestPSR4Namespace($namespace)
+    {
+        $composerNamespaces = $this->factory->getPSR4Namespaces();
+
+        $carry = new \stdClass();
+        $carry->highestMatchingSegments = 0;
+        $carry->bestNamespace = null;
+
+        /** @var PSR4Namespace $bestNamespace */
+        $bestNamespace = array_reduce($composerNamespaces, function ($carry, PSR4Namespace $potentialNamespace) use ($namespace) {
+            $matchingSegments = $potentialNamespace->countMatchingNamespaceSegments($namespace);
+
+            if ($matchingSegments > $carry->highestMatchingSegments) {
+                $carry->highestMatchingSegments = $matchingSegments;
+                $carry->bestNamespace = $potentialNamespace;
+            }
+
+            return $carry;
+        }, $carry);
+
+        return $bestNamespace->bestNamespace;
+    }
 }

src/PSR4/PSR4Namespace.php

@@ -14,7 +14,28 @@ class PSR4Namespace
         $this->directories = $directories;
     }
 
-    public function matches($namespace)
+    /**
+     * Determines how many namespace segments match the internal namespace. This is useful because multiple namespaces
+     * may technically match a registered namespace root, but one of the matches may be a better match. Namespaces that
+     * match, but are not _the best_ match are incorrect matches. TestApp1\\ is **not** the best match when searching for
+     * namespace TestApp1\\Multi\\Foo if TestApp1\\Multi was explicitly registered.
+     *
+     * PSR4Namespace $a;
+     * $a->namespace = "TestApp1\\";
+     * $a->countMatchingNamespaceSegments("TestApp1\\Multi") -> 1, TestApp1 matches.
+     *
+     * PSR4Namespace $b;
+     * $b->namespace = "TestApp1\\Multi";
+     * $b->countMatchingNamespaceSegments("TestApp1\\Multi") -> 2, TestApp1\\Multi matches
+     *
+     * PSR4Namespace $c;
+     * $c->namespace = "HaydenPierce\\Foo\\Bar";
+     * $c->countMatchingNamespaceSegments("TestApp1\\Multi") -> 0, No matches.
+     *
+     * @param $namespace
+     * @return int
+     */
+    public function countMatchingNamespaceSegments($namespace)
     {
         $namespaceFragments = explode('\\', $namespace);
         $undefinedNamespaceFragments = [];
@@ -23,13 +44,13 @@ class PSR4Namespace
             $possibleNamespace = implode('\\', $namespaceFragments) . '\\';
 
             if($this->namespace === $possibleNamespace){
-                return true;
+                return count(explode('\\', $possibleNamespace));
             }
 
             array_unshift($undefinedNamespaceFragments, array_pop($namespaceFragments));
         }
 
-        return false;
+        return 0;
     }
 
     public function findClasses($namespace)