add common ancestor method to FileTree

core/src/main/groovy/com/muwire/core/files/FileTree.groovy

@@ -96,6 +96,13 @@ class FileTree<T> {
         }
     }
     
+    synchronized File commonAncestor() {
+        TreeNode current = root
+        while(current.children.size() == 1)
+            current = current.children.first()
+        current.file
+    }
+    
     public static class TreeNode<T> {
         TreeNode parent
         File file

core/src/test/groovy/com/muwire/core/files/FileTreeTest.groovy

@@ -134,4 +134,32 @@ class FileTreeTest {
         assert values.size() == 1
         assert values.contains("c")
     }
+    
+    @Test
+    public void testCommonAncestor() {
+        File a = new File("a")
+        File b = new File(a,"b")
+        File c = new File(b,"c")
+        File d = new File(b,"d")
+        
+        a.delete()
+        b.delete()
+        c.delete()
+        d.delete()
+        
+        a.mkdir()
+        b.mkdir()
+        c.createNewFile()
+        d.createNewFile()
+        
+        FileTree<Void> tree = new FileTree<>()
+        tree.add(a, null)
+        tree.add(b, null)
+        tree.add(c, null)
+        tree.add(d, null)
+        
+        File common = tree.commonAncestor()
+        
+        assert common == b
+    }
 }