From 7daf042876ebe4b21a4b1ad98067317cb0216b80 Mon Sep 17 00:00:00 2001
From: Adithya Anilkumar <adithyaanilkumar1@gmail.com>
Date: Sat, 30 Oct 2021 19:23:28 +0530
Subject: [PATCH] Create dijkstra.js

---
 Javascript/Stack/dijkstra.js | 80 ++++++++++++++++++++++++++++++++++++
 1 file changed, 80 insertions(+)
 create mode 100644 Javascript/Stack/dijkstra.js

diff --git a/Javascript/Stack/dijkstra.js b/Javascript/Stack/dijkstra.js
new file mode 100644
index 0000000..c5b47b0
--- /dev/null
+++ b/Javascript/Stack/dijkstra.js
@@ -0,0 +1,80 @@
+import PriorityQueue from '../../../data-structures/priority-queue/PriorityQueue';
+
+/**
+ * @typedef {Object} ShortestPaths
+ * @property {Object} distances - shortest distances to all vertices
+ * @property {Object} previousVertices - shortest paths to all vertices.
+ */
+
+/**
+ * Implementation of Dijkstra algorithm of finding the shortest paths to graph nodes.
+ * @param {Graph} graph - graph we're going to traverse.
+ * @param {GraphVertex} startVertex - traversal start vertex.
+ * @return {ShortestPaths}
+ */
+export default function dijkstra(graph, startVertex) {
+  // Init helper variables that we will need for Dijkstra algorithm.
+  const distances = {};
+  const visitedVertices = {};
+  const previousVertices = {};
+  const queue = new PriorityQueue();
+
+  // Init all distances with infinity assuming that currently we can't reach
+  // any of the vertices except the start one.
+  graph.getAllVertices().forEach((vertex) => {
+    distances[vertex.getKey()] = Infinity;
+    previousVertices[vertex.getKey()] = null;
+  });
+
+  // We are already at the startVertex so the distance to it is zero.
+  distances[startVertex.getKey()] = 0;
+
+  // Init vertices queue.
+  queue.add(startVertex, distances[startVertex.getKey()]);
+
+  // Iterate over the priority queue of vertices until it is empty.
+  while (!queue.isEmpty()) {
+    // Fetch next closest vertex.
+    const currentVertex = queue.poll();
+
+    // Iterate over every unvisited neighbor of the current vertex.
+    currentVertex.getNeighbors().forEach((neighbor) => {
+      // Don't visit already visited vertices.
+      if (!visitedVertices[neighbor.getKey()]) {
+        // Update distances to every neighbor from current vertex.
+        const edge = graph.findEdge(currentVertex, neighbor);
+
+        const existingDistanceToNeighbor = distances[neighbor.getKey()];
+        const distanceToNeighborFromCurrent = distances[currentVertex.getKey()] + edge.weight;
+
+        // If we've found shorter path to the neighbor - update it.
+        if (distanceToNeighborFromCurrent < existingDistanceToNeighbor) {
+          distances[neighbor.getKey()] = distanceToNeighborFromCurrent;
+
+          // Change priority of the neighbor in a queue since it might have became closer.
+          if (queue.hasValue(neighbor)) {
+            queue.changePriority(neighbor, distances[neighbor.getKey()]);
+          }
+
+          // Remember previous closest vertex.
+          previousVertices[neighbor.getKey()] = currentVertex;
+        }
+
+        // Add neighbor to the queue for further visiting.
+        if (!queue.hasValue(neighbor)) {
+          queue.add(neighbor, distances[neighbor.getKey()]);
+        }
+      }
+    });
+
+    // Add current vertex to visited ones to avoid visiting it again later.
+    visitedVertices[currentVertex.getKey()] = currentVertex;
+  }
+
+  // Return the set of shortest distances to all vertices and the set of
+  // shortest paths to all vertices in a graph.
+  return {
+    distances,
+    previousVertices,
+  };
+}