From 453a44a8af1772413baf37d73423a000796d4ab2 Mon Sep 17 00:00:00 2001
From: Soumya Pandey <43310171+100mya@users.noreply.github.com>
Date: Fri, 13 Oct 2023 16:15:57 +0530
Subject: [PATCH] Create bi_directional_dijkstra.js

---
 Graphs/bi_directional_dijkstra.js | 151 ++++++++++++++++++++++++++++++
 1 file changed, 151 insertions(+)
 create mode 100644 Graphs/bi_directional_dijkstra.js

diff --git a/Graphs/bi_directional_dijkstra.js b/Graphs/bi_directional_dijkstra.js
new file mode 100644
index 0000000000..76d6c56bd7
--- /dev/null
+++ b/Graphs/bi_directional_dijkstra.js
@@ -0,0 +1,151 @@
+/**
+ * Bidirectional Dijkstra's algorithm to find the shortest path between two nodes in a graph.
+ *
+ * @param {string} source - The starting node.
+ * @param {string} destination - The destination node.
+ * @param {object} graphForward - The forward graph representation with node distances.
+ * @param {object} graphBackward - The backward graph representation with node distances.
+ * @returns {number} - The length of the shortest path. Returns -1 if the destination is not reachable.
+ */
+function bidirectionalDijkstra(source, destination, graphForward, graphBackward) {
+    let shortestPathDistance = -1;
+
+    const visitedForward = new Set();
+    const visitedBackward = new Set();
+    const costForward = { [source]: 0 };
+    const costBackward = { [destination]: 0 };
+    const parentForward = { [source]: null };
+    const parentBackward = { [destination]: null };
+    const queueForward = new PriorityQueue();
+    const queueBackward = new PriorityQueue();
+
+    let shortestDistance = Number.POSITIVE_INFINITY;
+
+    queueForward.enqueue(0, source);
+    queueBackward.enqueue(0, destination);
+
+    if (source === destination) {
+        return 0;
+    }
+
+    while (!queueForward.isEmpty() && !queueBackward.isEmpty()) {
+        const [_, vertexForward] = queueForward.dequeue();
+        visitedForward.add(vertexForward);
+
+        const [__, vertexBackward] = queueBackward.dequeue();
+        visitedBackward.add(vertexBackward);
+
+        shortestDistance = passAndRelaxation(
+            graphForward,
+            vertexForward,
+            visitedForward,
+            visitedBackward,
+            costForward,
+            costBackward,
+            queueForward,
+            parentForward,
+            shortestDistance
+        );
+
+        shortestDistance = passAndRelaxation(
+            graphBackward,
+            vertexBackward,
+            visitedBackward,
+            visitedForward,
+            costBackward,
+            costForward,
+            queueBackward,
+            parentBackward,
+            shortestDistance
+        );
+
+        if (costForward[vertexForward] + costBackward[vertexBackward] >= shortestDistance) {
+            break;
+        }
+    }
+
+    if (shortestDistance !== Number.POSITIVE_INFINITY) {
+        shortestPathDistance = shortestDistance;
+    }
+
+    return shortestPathDistance;
+}
+
+/**
+ * Helper function for relaxing edges and updating costs.
+ *
+ * @param {object} graph - The graph representation with node distances.
+ * @param {string} vertex - The current vertex being processed.
+ * @param {Set} visitedForward - The set of visited nodes in the forward search.
+ * @param {Set} visitedBackward - The set of visited nodes in the backward search.
+ * @param {object} costForward - The cost of reaching each node from the source in the forward search.
+ * @param {object} costBackward - The cost of reaching each node from the destination in the backward search.
+ * @param {PriorityQueue} queue - The priority queue for nodes to be processed.
+ * @param {object} parent - The parent node of each node in the path.
+ * @param {number} shortestDistance - The current shortest distance.
+ * @returns {number} - The updated shortest distance.
+ */
+function passAndRelaxation(
+    graph,
+    vertex,
+    visitedForward,
+    visitedBackward,
+    costForward,
+    costBackward,
+    queue,
+    parent,
+    shortestDistance
+) {
+    for (const [next, distance] of graph[vertex]) {
+        if (visitedForward.has(next)) {
+            continue;
+        }
+
+        const oldCostForward = costForward[next] || Number.POSITIVE_INFINITY;
+        const newCostForward = costForward[vertex] + distance;
+
+        if (newCostForward < oldCostForward) {
+            queue.enqueue(newCostForward, next);
+            costForward[next] = newCostForward;
+            parent[next] = vertex;
+        }
+
+        if (visitedBackward.has(next)) {
+            if (costForward[vertex] + distance + costBackward[next] < shortestDistance) {
+                shortestDistance = costForward[vertex] + distance + costBackward[next];
+            }
+        }
+    }
+
+    return shortestDistance;
+}
+
+// Example usage:
+const graphForward = {
+    B: [["C", 1]],
+    C: [["D", 1]],
+    D: [["F", 1]],
+    E: [["B", 1], ["G", 2]],
+    F: [],
+    G: [["F", 1]],
+};
+
+const graphBackward = {
+    B: [["E", 1]],
+    C: [["B", 1]],
+    D: [["C", 1]],
+    F: [["D", 1], ["G", 1]],
+    E: [[null, Number.POSITIVE_INFINITY]],
+    G: [["E", 2]],
+};
+
+const source = "E";
+const destination = "F";
+
+const shortestPathDistance = bidirectionalDijkstra(source, destination, graphForward, graphBackward);
+
+if (shortestPathDistance !== -1) {
+    console.log(`Shortest path from ${source} to ${destination}: ${shortestPathDistance}`);
+} else {
+    console.log(`No path found from ${source} to ${destination}`);
+}