refactor: extract shared DirectedAdj and safeNeighbors into GraphUtils

Add GraphUtils.DirectedAdj class and buildDirectedAdjacencyMap() method,
generalized from TopologicalSortAnalyzer's private DirectedAdj.

Add GraphUtils.safeNeighbors() to eliminate null-check boilerplate.

Refactor TopologicalSortAnalyzer to use shared DirectedAdj (removes ~25
lines of duplicated adjacency construction).

Refactor CycleAnalyzer's getNeighbors helper to use GraphUtils.safeNeighbors.

This establishes the shared utilities for consolidating adjacency/BFS
construction across the remaining 11 analyzers listed in #43.

Partial fix for #43

Author: sauravbhattacharya001

Gvisual/src/gvisual/CycleAnalyzer.java

@@ -661,8 +661,7 @@ private Iterable<String> getSuccessors(String v) {
     }
 
     private Iterable<String> getNeighbors(String v) {
-        Collection<String> nbrs = graph.getNeighbors(v);
-        return nbrs != null ? nbrs : Collections.<String>emptyList();
+        return GraphUtils.safeNeighbors(graph, v);
     }
 
     private String edgeKey(String v1, String v2) {

Gvisual/src/gvisual/GraphUtils.java

@@ -85,6 +85,66 @@ public static Map<String, Set<String>> buildAdjacencyMap(
         return adj;
     }
 
+    /**
+     * Directed adjacency data: vertices, successor map, and predecessor map.
+     */
+    public static class DirectedAdj {
+        public final Set<String> vertices;
+        public final Map<String, Set<String>> successors;
+        public final Map<String, Set<String>> predecessors;
+
+        public DirectedAdj(Set<String> vertices,
+                    Map<String, Set<String>> successors,
+                    Map<String, Set<String>> predecessors) {
+            this.vertices = vertices;
+            this.successors = successors;
+            this.predecessors = predecessors;
+        }
+    }
+
+    /**
+     * Builds directed adjacency maps from the graph. Each edge is
+     * interpreted as vertex1 → vertex2.
+     *
+     * @param graph the JUNG graph
+     * @return DirectedAdj containing vertices, successors, and predecessors
+     */
+    public static DirectedAdj buildDirectedAdjacencyMap(Graph<String, edge> graph) {
+        Map<String, Set<String>> successors = new HashMap<String, Set<String>>();
+        Map<String, Set<String>> predecessors = new HashMap<String, Set<String>>();
+        Set<String> allVertices = new HashSet<String>();
+
+        for (String v : graph.getVertices()) {
+            allVertices.add(v);
+            successors.put(v, new HashSet<String>());
+            predecessors.put(v, new HashSet<String>());
+        }
+
+        for (edge e : graph.getEdges()) {
+            String from = e.getVertex1();
+            String to = e.getVertex2();
+            if (from != null && to != null
+                    && allVertices.contains(from) && allVertices.contains(to)) {
+                successors.get(from).add(to);
+                predecessors.get(to).add(from);
+            }
+        }
+
+        return new DirectedAdj(allVertices, successors, predecessors);
+    }
+
+    /**
+     * Safe neighbor lookup that never returns null.
+     *
+     * @param graph the JUNG graph
+     * @param v     the vertex
+     * @return neighbors collection, empty if null
+     */
+    public static Collection<String> safeNeighbors(Graph<String, edge> graph, String v) {
+        Collection<String> nbrs = graph.getNeighbors(v);
+        return nbrs != null ? nbrs : Collections.<String>emptyList();
+    }
+
     /**
      * BFS from a source vertex, returning distances (hop counts) to all
      * reachable vertices.

Gvisual/src/gvisual/TopologicalSortAnalyzer.java

@@ -183,46 +183,11 @@ public VertexDependencyInfo(String vertex, Set<String> allDependencies,
      * {@link #analyze()}, {@link #analyzeDependencies(String)}, and
      * {@link #countChoicePoints()}.
      */
-    private static class DirectedAdj {
-        final Set<String> vertices;
-        final Map<String, Set<String>> successors;
-        final Map<String, Set<String>> predecessors;
-
-        DirectedAdj(Set<String> vertices,
-                    Map<String, Set<String>> successors,
-                    Map<String, Set<String>> predecessors) {
-            this.vertices = vertices;
-            this.successors = successors;
-            this.predecessors = predecessors;
-        }
-    }
-
     /**
-     * Builds the directed adjacency maps from the graph.  Each edge is
-     * interpreted as vertex1 → vertex2 (vertex1 must come before vertex2).
+     * Builds the directed adjacency maps from the graph using shared GraphUtils.
      */
-    private DirectedAdj buildDirectedAdj() {
-        Map<String, Set<String>> successors = new HashMap<String, Set<String>>();
-        Map<String, Set<String>> predecessors = new HashMap<String, Set<String>>();
-        Set<String> allVertices = new HashSet<String>();
-
-        for (String v : graph.getVertices()) {
-            allVertices.add(v);
-            successors.put(v, new HashSet<String>());
-            predecessors.put(v, new HashSet<String>());
-        }
-
-        for (edge e : graph.getEdges()) {
-            String from = e.getVertex1();
-            String to = e.getVertex2();
-            if (from != null && to != null
-                    && allVertices.contains(from) && allVertices.contains(to)) {
-                successors.get(from).add(to);
-                predecessors.get(to).add(from);
-            }
-        }
-
-        return new DirectedAdj(allVertices, successors, predecessors);
+    private GraphUtils.DirectedAdj buildDirectedAdj() {
+        return GraphUtils.buildDirectedAdjacencyMap(graph);
     }
 
     // ── Core algorithms ─────────────────────────────────────────
@@ -240,7 +205,7 @@ private DirectedAdj buildDirectedAdj() {
      * @return complete topological sort analysis
      */
     public TopologicalSortResult analyze() {
-        DirectedAdj adj = buildDirectedAdj();
+        GraphUtils.DirectedAdj adj = buildDirectedAdj();
         Map<String, Set<String>> successors = adj.successors;
         Map<String, Set<String>> predecessors = adj.predecessors;
         Set<String> allVertices = adj.vertices;
@@ -351,7 +316,7 @@ public VertexDependencyInfo analyzeDependencies(String vertex) {
             return null;
         }
 
-        DirectedAdj adj = buildDirectedAdj();
+        GraphUtils.DirectedAdj adj = buildDirectedAdj();
         Map<String, Set<String>> successors = adj.successors;
         Map<String, Set<String>> predecessors = adj.predecessors;
 
@@ -416,7 +381,7 @@ public int countChoicePoints() {
 
         // Re-use shared adjacency builder and count points where
         // multiple vertices are ready simultaneously
-        DirectedAdj adj = buildDirectedAdj();
+        GraphUtils.DirectedAdj adj = buildDirectedAdj();
         Map<String, Set<String>> successors = adj.successors;
         Map<String, Integer> inDegree = new HashMap<String, Integer>();