Add Johnson's algorithm (#5712)

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

@@ -0,0 +1,136 @@
+package com.thealgorithms.datastructures.graphs;
+
+import static org.junit.jupiter.api.Assertions.assertArrayEquals;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import org.junit.jupiter.api.Test;
+
+/**
+ * Unit tests for {@link JohnsonsAlgorithm} class. This class
+ * contains test cases to verify the correct implementation of
+ * various methods used in Johnson's Algorithm such as shortest path
+ * calculations, graph reweighting, and more.
+ */
+class JohnsonsAlgorithmTest {
+
+    // Constant representing infinity
+    private static final double INF = Double.POSITIVE_INFINITY;
+
+    /**
+     * Tests the Johnson's Algorithm with a simple graph without negative edges.
+     * Verifies that the algorithm returns the correct shortest path distances.
+     */
+    @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.
+     */
+    @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.
+     */
+    @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); });
+    }
+
+    /**
+     * Tests Dijkstra's algorithm as a part of Johnson's algorithm implementation
+     * on a small graph. Verifies that the shortest path is correctly calculated.
+     */
+    @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[] 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.
+     */
+    @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[][] 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.
+     */
+    @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);
+    }
+}