build a tree of paths for UI represention

core/src/main/groovy/com/muwire/core/collections/FileCollection.groovy

@@ -21,6 +21,8 @@ class FileCollection {
     
     private final Set<FileCollectionItem> files = new LinkedHashSet<>()
     
+    final PathTree tree
+    
     FileCollection(long timestamp, Persona author, String comment, Set<FileCollectionItem> files,
         SigningPrivateKey spk) {
         this.timestamp = timestamp;
@@ -28,6 +30,11 @@ class FileCollection {
         this.comment = comment
         this.files = files
         
+        tree = new PathTree(files.first().pathElements.first())
+        for(FileCollectionItem item : files) {
+            tree.add(item.pathElements)
+        }
+        
         byte [] signablePayload = signablePayload()
         Signature signature = DSAEngine.getInstance().sign(signablePayload, spk)
         this.sig = signature.getData()
@@ -112,6 +119,67 @@ class FileCollection {
         os.write(payload)
     }
     
+    static class PathTree {
+        
+        private final Map<PathNode.Key, PathNode> keyToNode = new HashMap<>()
+        
+        final PathNode root
+        PathTree(String root) {
+            this.root = new PathNode(root, null)
+            keyToNode.put(this.root.key(), this.root)
+        }
+        
+        void add(List<String> paths) {
+            PathNode current = null
+            for (String path : paths) {
+                if (current == null) {
+                    if (path == root.path) {
+                        current = root
+                        continue
+                    }
+                }
+                
+                
+                PathNode child = new PathNode(path, current)
+                PathNode.Key childKey = child.key()
+                if (!keyToNode.containsKey(childKey)) {
+                    keyToNode.put(childKey, child)
+                    current.children.add(child)
+                }
+                current = keyToNode.get(childKey)
+            }
+        }
+    }
     
+    static class PathNode {
+        final String path
+        final PathNode parent
+        final Set<PathNode> children = new LinkedHashSet<>()
+        private final int hashCode
+        
+        PathNode(String path, PathNode parent) {
+            this.parent = parent
+            this.path = path
+            this.hashCode = Objects.hash(path, parent)
+        }
+        
 
+        Key key() {
+            return new Key()
+        }
+        
+        private class Key {
+            
+            private final PathNode node = PathNode.this
+            
+            public int hashCode() {
+                hashCode
+            }
+            public boolean equals(Object o) {
+                Key other = (Key)o
+                Objects.equals(path, other.node.path) &&
+                    Objects.equals(parent, other.node.parent)
+            }
+        }         
+    }
 }