refactor: Enhance docs, add more tests in JohnsonsAlgorithm

Author: Hardvan

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

@@ -21,17 +21,18 @@
  */
 public final class JohnsonsAlgorithm {
 
-    // Constant representing infinity
     private static final double INF = Double.POSITIVE_INFINITY;
 
-    /**
-     * A private constructor to hide the implicit public one.
-     */
     private JohnsonsAlgorithm() {
     }
 
     /**
      * Executes Johnson's algorithm on the given graph.
+     * Steps:
+     * 1. Add a new vertex to the graph and run Bellman-Ford to compute modified weights
+     * 2. t the graph using the modified weights
+     * 3. Run Dijkstra's algorithm for each vertex to compute the shortest paths
+     * The final result is a 2D array of shortest distances between all pairs of vertices.
      *
      * @param graph The input graph represented as an adjacency matrix.
      * @return A 2D array representing the shortest distances between all pairs of vertices.
@@ -40,13 +41,10 @@ public static double[][] johnsonAlgorithm(double[][] graph) {
         int numVertices = graph.length;
         double[][] edges = convertToEdgeList(graph);
 
-        // Step 1: Add a new vertex and run Bellman-Ford
         double[] modifiedWeights = bellmanFord(edges, numVertices);
 
-        // Step 2: Reweight the graph
         double[][] reweightedGraph = reweightGraph(graph, modifiedWeights);
 
-        // Step 3: Run Dijkstra's algorithm for each vertex
         double[][] shortestDistances = new double[numVertices][numVertices];
         for (int source = 0; source < numVertices; source++) {
             shortestDistances[source] = dijkstra(reweightedGraph, source, modifiedWeights);
@@ -74,7 +72,6 @@ public static double[][] convertToEdgeList(double[][] graph) {
             }
         }
 
-        // Convert the List to a 2D array
         return edgeList.toArray(new double[0][]);
     }
 
@@ -89,7 +86,7 @@ public static double[][] convertToEdgeList(double[][] graph) {
     private static double[] bellmanFord(double[][] edges, int numVertices) {
         double[] dist = new double[numVertices + 1];
         Arrays.fill(dist, INF);
-        dist[numVertices] = 0; // Distance to the new source vertex is 0
+        dist[numVertices] = 0;
 
         // Add edges from the new vertex to all original vertices
         double[][] allEdges = Arrays.copyOf(edges, edges.length + numVertices);

src/test/java/com/thealgorithms/datastructures/graphs/JohnsonsAlgorithmTest.java

@@ -23,114 +23,120 @@ class JohnsonsAlgorithmTest {
      */
     @Test
     void testSimpleGraph() {
-        // Test case for a simple graph without negative edges
         double[][] graph = {{0, 4, INF, INF}, {INF, 0, 1, INF}, {INF, INF, 0, 2}, {INF, INF, INF, 0}};
-
         double[][] result = JohnsonsAlgorithm.johnsonAlgorithm(graph);
-
         double[][] expected = {{0, 4, 5, 7}, {INF, 0, 1, 3}, {INF, INF, 0, 2}, {INF, INF, INF, 0}};
-
         assertArrayEquals(expected, result);
     }
 
     /**
-     * Tests Johnson's Algorithm on a graph with negative edges but no
-     * negative weight cycles. Verifies the algorithm handles negative
-     * edge weights correctly.
+     * Tests Johnson's Algorithm on a graph with negative edges but no negative weight cycles.
      */
     @Test
     void testGraphWithNegativeEdges() {
-        // Graph with negative edges but no negative weight cycles
         double[][] graph = {{0, -1, 4}, {INF, 0, 3}, {INF, INF, 0}};
-
         double[][] result = JohnsonsAlgorithm.johnsonAlgorithm(graph);
-
         double[][] expected = {{0, INF, 4}, {INF, 0, 3}, {INF, INF, 0}};
-
         assertArrayEquals(expected, result);
     }
 
     /**
-     * Tests the behavior of Johnson's Algorithm on a graph with a negative
-     * weight cycle. Expects an IllegalArgumentException to be thrown
-     * due to the presence of the cycle.
+     * Tests Johnson's Algorithm on a graph with a negative weight cycle.
      */
     @Test
     void testNegativeWeightCycle() {
-        // Graph with a negative weight cycle
         double[][] graph = {{0, 1, INF}, {INF, 0, -1}, {-1, INF, 0}};
-
-        // Johnson's algorithm should throw an exception when a negative cycle is detected
-        assertThrows(IllegalArgumentException.class, () -> { JohnsonsAlgorithm.johnsonAlgorithm(graph); });
+        assertThrows(IllegalArgumentException.class, () -> JohnsonsAlgorithm.johnsonAlgorithm(graph));
     }
 
     /**
-     * Tests Dijkstra's algorithm as a part of Johnson's algorithm implementation
-     * on a small graph. Verifies that the shortest path is correctly calculated.
+     * Tests Dijkstra's algorithm on a small graph as part of Johnson's Algorithm.
      */
     @Test
     void testDijkstra() {
-        // Testing Dijkstra's algorithm with a small graph
         double[][] graph = {{0, 1, 2}, {INF, 0, 3}, {INF, INF, 0}};
-
-        double[] modifiedWeights = {0, 0, 0}; // No reweighting in this simple case
-
+        double[] modifiedWeights = {0, 0, 0};
         double[] result = JohnsonsAlgorithm.dijkstra(graph, 0, modifiedWeights);
         double[] expected = {0, 1, 2};
-
         assertArrayEquals(expected, result);
     }
 
     /**
      * Tests the conversion of an adjacency matrix to an edge list.
-     * Verifies that the conversion process generates the correct edge list.
      */
     @Test
     void testEdgeListConversion() {
-        // Test the conversion of adjacency matrix to edge list
         double[][] graph = {{0, 5, INF}, {INF, 0, 2}, {INF, INF, 0}};
-
-        // Running convertToEdgeList
         double[][] edges = JohnsonsAlgorithm.convertToEdgeList(graph);
-
-        // Expected edge list: (0 -> 1, weight 5), (1 -> 2, weight 2)
         double[][] expected = {{0, 1, 5}, {1, 2, 2}};
-
-        // Verify the edge list matches the expected values
         assertArrayEquals(expected, edges);
     }
 
     /**
-     * Tests the reweighting of a graph as a part of Johnson's Algorithm.
-     * Verifies that the reweighted graph produces correct results.
+     * Tests the reweighting of a graph.
      */
     @Test
     void testReweightGraph() {
-        // Test reweighting of the graph
         double[][] graph = {{0, 2, 9}, {INF, 0, 1}, {INF, INF, 0}};
-        double[] modifiedWeights = {1, 2, 3}; // Arbitrary weight function
-
+        double[] modifiedWeights = {1, 2, 3};
         double[][] reweightedGraph = JohnsonsAlgorithm.reweightGraph(graph, modifiedWeights);
-
-        // Expected reweighted graph:
         double[][] expected = {{0, 1, 7}, {INF, 0, 0}, {INF, INF, 0}};
-
         assertArrayEquals(expected, reweightedGraph);
     }
 
     /**
-     * Tests the minDistance method used in Dijkstra's algorithm to find
-     * the vertex with the minimum distance that has not yet been visited.
+     * Tests the minDistance method used in Dijkstra's algorithm.
      */
     @Test
     void testMinDistance() {
-        // Test minDistance method
         double[] dist = {INF, 3, 1, INF};
         boolean[] visited = {false, false, false, false};
-
         int minIndex = JohnsonsAlgorithm.minDistance(dist, visited);
-
-        // The vertex with minimum distance is vertex 2 with a distance of 1
         assertEquals(2, minIndex);
     }
+
+    /**
+     * Tests Johnson's Algorithm on a graph where all vertices are disconnected.
+     */
+    @Test
+    void testDisconnectedGraph() {
+        double[][] graph = {{0, INF, INF}, {INF, 0, INF}, {INF, INF, 0}};
+        double[][] result = JohnsonsAlgorithm.johnsonAlgorithm(graph);
+        double[][] expected = {{0, INF, INF}, {INF, 0, INF}, {INF, INF, 0}};
+        assertArrayEquals(expected, result);
+    }
+
+    /**
+     * Tests Johnson's Algorithm on a fully connected graph.
+     */
+    @Test
+    void testFullyConnectedGraph() {
+        double[][] graph = {{0, 1, 2}, {1, 0, 1}, {2, 1, 0}};
+        double[][] result = JohnsonsAlgorithm.johnsonAlgorithm(graph);
+        double[][] expected = {{0, 1, 2}, {1, 0, 1}, {2, 1, 0}};
+        assertArrayEquals(expected, result);
+    }
+
+    /**
+     * Tests Dijkstra's algorithm on a graph with multiple shortest paths.
+     */
+    @Test
+    void testDijkstraMultipleShortestPaths() {
+        double[][] graph = {{0, 1, 2, INF}, {INF, 0, INF, 1}, {INF, INF, 0, 1}, {INF, INF, INF, 0}};
+        double[] modifiedWeights = {0, 0, 0, 0};
+        double[] result = JohnsonsAlgorithm.dijkstra(graph, 0, modifiedWeights);
+        double[] expected = {0, 1, 2, 2};
+        assertArrayEquals(expected, result);
+    }
+
+    /**
+     * Tests Johnson's Algorithm with a graph where all edge weights are zero.
+     */
+    @Test
+    void testGraphWithZeroWeights() {
+        double[][] graph = {{0, 0, 0}, {0, 0, 0}, {0, 0, 0}};
+        double[][] result = JohnsonsAlgorithm.johnsonAlgorithm(graph);
+        double[][] expected = {{0, INF, INF}, {INF, 0, INF}, {INF, INF, 0}};
+        assertArrayEquals(expected, result);
+    }
 }