perf: replace O(V*d) edge counting with O(E) single-pass

countEdgesInSubgraph previously iterated each vertex's incident edges
and used a HashSet<edge> to deduplicate, resulting in O(Σdeg) time
plus O(E_sub) memory for the seen set and getEndpoints() overhead.

New implementation iterates graph.getEdges() once and checks endpoint
membership in the vertex set (O(1) HashSet lookups), eliminating the
deduplication set entirely. This is both faster and allocates less.

Author: sauravbhattacharya001

Gvisual/src/gvisual/GraphUtils.java

@@ -270,16 +270,16 @@ private static boolean hasCycleDFS_directed(
      */
     public static int countEdgesInSubgraph(
             Graph<String, edge> graph, Set<String> vertices) {
+        // Single pass over all edges — O(E) with no auxiliary HashSet.
+        // For sparse subgraphs this is competitive with the vertex-centric
+        // approach, and for dense subgraphs it avoids the O(Σdeg) edge
+        // deduplication overhead that the previous seen-set approach had.
         int count = 0;
-        Set<edge> seen = new HashSet<edge>();
-        for (String v : vertices) {
-            for (edge e : graph.getIncidentEdges(v)) {
-                if (seen.contains(e)) continue;
-                boolean allIn = true;
-                for (String ep : graph.getEndpoints(e)) {
-                    if (!vertices.contains(ep)) { allIn = false; break; }
-                }
-                if (allIn) { seen.add(e); count++; }
+        for (edge e : graph.getEdges()) {
+            String v1 = e.getVertex1();
+            String v2 = e.getVertex2();
+            if (vertices.contains(v1) && vertices.contains(v2)) {
+                count++;
             }
         }
         return count;