Add Adaptive A* pathfinding with semantic risk cost

src/main/java/com/thealgorithms/datastructures/graphs/AdaptiveAStar.java

@@ -0,0 +1,263 @@
+package com.thealgorithms.datastructures.graphs;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Comparator;
+import java.util.List;
+import java.util.PriorityQueue;
+
+/**
+ * Adaptive A* (A-star) pathfinding algorithm with semantic cost weighting.
+ *
+ * This implementation extends the classical A* algorithm by introducing
+ * a semantic risk cost layer, as proposed in:
+ *
+ *
+ *   Hong Yun, "An Adaptive Path Planning Method for Indoor and Outdoor
+ *   Integrated Navigation," 2025 IEEE International Conference on Machine
+ *   Learning and Intelligent Systems Engineering (MLISE 2025).
+ *
+ *
+ * Cost Function
+ *   f(n) = g(n) + h(n) + lambda * R_sem(n)
+ *
+ *
+ *
+ *   g(n) — actual cost from the start node to node n</li>
+ *   h(n) — heuristic estimate from node n to the goal</li>
+ *   lambda — global semantic weight multiplier</li>
+ *   R_sem(n) — per-node semantic risk value
+ *       (e.g., 2.0 for construction zones, 0.5 for sidewalks, 0.0 for normal)
+ *
+ *
+ * The semantic cost enables the algorithm to prefer safer or more convenient
+ * routes in indoor/outdoor navigation scenarios, such as avoiding construction
+ * areas, preferring well-lit paths at night, or prioritizing barrier-free routes.
+ *
+ * When all semantic risk values are zero and lambda is zero, the algorithm
+ * behaves identically to classical A*.
+ *
+ * Time Complexity: O((V + E) log V) where V is the number of vertices
+ * and E is the number of edges. In the worst case, this reduces to O(E) when
+ * the heuristic provides perfect guidance.
+ *
+ * @see <a href="https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/graphs/AStar.java">
+ *      Classical AStar (without semantic cost)</a>
+ */
+public final class AdaptiveAStar {
+
+    private AdaptiveAStar() {
+    }
+
+    /**
+     * Directed or undirected edge in the graph.
+     */
+    public static class Edge {
+        private final int from;
+        private final int to;
+        private final int weight;
+
+        public Edge(int from, int to, int weight) {
+            this.from = from;
+            this.to = to;
+            this.weight = weight;
+        }
+
+        public int getFrom() {
+            return from;
+        }
+
+        public int getTo() {
+            return to;
+        }
+
+        public int getWeight() {
+            return weight;
+        }
+    }
+
+    /**
+     * Graph represented as an adjacency list.
+     */
+    public static class Graph {
+        private final ArrayList<ArrayList<Edge>> adjacencyList;
+
+        public Graph(int nodeCount) {
+            this.adjacencyList = new ArrayList<>(nodeCount);
+            for (int i = 0; i < nodeCount; i++) {
+                this.adjacencyList.add(new ArrayList<>());
+            }
+        }
+
+        /**
+         * Adds a bidirectional (undirected) edge.
+         */
+        public void addBidirectionalEdge(int from, int to, int weight) {
+            adjacencyList.get(from).add(new Edge(from, to, weight));
+            adjacencyList.get(to).add(new Edge(to, from, weight));
+        }
+
+        /**
+         * Adds a directed edge.
+         */
+        public void addDirectedEdge(int from, int to, int weight) {
+            adjacencyList.get(from).add(new Edge(from, to, weight));
+        }
+
+        public int nodeCount() {
+            return adjacencyList.size();
+        }
+
+        public ArrayList<Edge> getNeighbors(int node) {
+            return adjacencyList.get(node);
+        }
+    }
+
+    /**
+     * Holds the result of a pathfinding operation.
+     */
+    public static class PathResult {
+        private final int totalCost;
+        private final List<Integer> path;
+        private final boolean found;
+
+        public PathResult(int totalCost, List<Integer> path, boolean found) {
+            this.totalCost = totalCost;
+            this.path = path;
+            this.found = found;
+        }
+
+        public int getTotalCost() {
+            return totalCost;
+        }
+
+        public List<Integer> getPath() {
+            return path;
+        }
+
+        public boolean isFound() {
+            return found;
+        }
+    }
+
+    /**
+     * Internal node wrapper used in the priority queue.
+     */
+    private static class NodeState {
+        final int node;
+        final int gCost;       // actual cost from start
+        final int fCost;       // f(n) = g(n) + h(n) + lambda * R_sem(n)
+
+        NodeState(int node, int gCost, int fCost) {
+            this.node = node;
+            this.gCost = gCost;
+            this.fCost = fCost;
+        }
+    }
+
+    /**
+     * Runs the Adaptive A* algorithm.
+     *
+     * @param start        the starting node index
+     * @param goal         the target node index
+     * @param graph        the graph (adjacency list)
+     * @param heuristic    heuristic values h[n] for each node (e.g., Euclidean distance to goal)
+     * @param semanticRisk per-node semantic risk values (e.g., 0.0 = normal, 2.0 = construction zone)
+     * @param lambda       global semantic weight multiplier
+     * @return a {@link PathResult} containing the total cost and path if found
+     */
+    public static PathResult findPath(int start, int goal, Graph graph,
+                                       int[] heuristic, double[] semanticRisk,
+                                       double lambda) {
+        int nodeCount = graph.nodeCount();
+        if (start < 0 || start >= nodeCount || goal < 0 || goal >= nodeCount) {
+            return new PathResult(-1, null, false);
+        }
+
+        // gCost[i] = actual cost from start to node i
+        int[] gCost = new int[nodeCount];
+        Arrays.fill(gCost, Integer.MAX_VALUE);
+        gCost[start] = 0;
+
+        // parent[i] = predecessor of node i on the best path
+        int[] parent = new int[nodeCount];
+        Arrays.fill(parent, -1);
+
+        // closed[i] = true if node i has been fully explored
+        boolean[] closed = new boolean[nodeCount];
+
+        // Priority queue orders by fCost = gCost + heuristic + semantic penalty
+        PriorityQueue<NodeState> openSet = new PriorityQueue<>(
+            Comparator.comparingInt(ns -> ns.fCost));
+
+        int initialFCost = computeFCost(0, heuristic[start],
+            semanticRisk[start], lambda);
+        openSet.add(new NodeState(start, 0, initialFCost));
+
+        while (!openSet.isEmpty()) {
+            NodeState current = openSet.poll();
+
+            // If the current node is the goal, reconstruct and return the path
+            if (current.node == goal) {
+                List<Integer> path = reconstructPath(parent, goal);
+                return new PathResult(current.gCost, path, true);
+            }
+
+            if (closed[current.node]) {
+                continue;
+            }
+            closed[current.node] = true;
+
+            // Expand neighbors
+            for (Edge edge : graph.getNeighbors(current.node)) {
+                int neighbor = edge.getTo();
+
+                if (closed[neighbor]) {
+                    continue;
+                }
+
+                int tentativeGCost = current.gCost + edge.getWeight();
+
+                if (tentativeGCost < gCost[neighbor]) {
+                    gCost[neighbor] = tentativeGCost;
+                    parent[neighbor] = current.node;
+
+                    int fCost = computeFCost(tentativeGCost, heuristic[neighbor],
+                        semanticRisk[neighbor], lambda);
+                    openSet.add(new NodeState(neighbor, tentativeGCost, fCost));
+                }
+            }
+        }
+
+        return new PathResult(-1, null, false);
+    }
+
+    /**
+     * Computes the adaptive cost function:
+     * <pre>f(n) = g(n) + h(n) + lambda * R_sem(n)</pre>
+     *
+     * @param gCost       actual cost from start to current node
+     * @param heuristic   heuristic estimate to goal
+     * @param semanticRisk per-node semantic risk
+     * @param lambda      semantic weight multiplier
+     * @return the total f-cost
+     */
+    private static int computeFCost(int gCost, int heuristic,
+                                     double semanticRisk, double lambda) {
+        int semanticPenalty = (int) Math.round(lambda * semanticRisk);
+        return gCost + heuristic + semanticPenalty;
+    }
+
+    /**
+     * Reconstructs the path from start to goal using the parent array.
+     */
+    private static List<Integer> reconstructPath(int[] parent, int goal) {
+        List<Integer> path = new ArrayList<>();
+        int current = goal;
+        while (current != -1) {
+            path.add(0, current);
+            current = parent[current];
+        }
+        return path;
+    }
+}