Sample Directed Graph and Topological Sort Code [closed]

Answer 1

Just to augment the great solution by @templatetypedef a bit, I added some unit tests to give some added confidence for myself and others to use. Hope this helps...

import static org.junit.Assert.assertEquals;
import static org.junit.Assert.assertTrue;
import java.util.List;
import org.junit.Test;

public class TestTopologicalSort {

    @Test (expected=java.lang.NullPointerException.class)
    public void testNullGraph() {
        final List orderedLayers = TopologicalSort.sort(null);
    }

    @Test
    public void testEmptyGraph() {
        final DirectedGraph dag = new DirectedGraph();        
        final List orderedLayers = TopologicalSort.sort(dag);
        assertEquals(0, orderedLayers.size());
    }

    @Test
    public void testSingleVertex() {
        final DirectedGraph dag = new DirectedGraph();
        dag.addNode("a");
        final List orderedLayers = TopologicalSort.sort(dag);
        assertEquals(1, orderedLayers.size());
        assertTrue(orderedLayers.contains("a"));
    }

    @Test
    public void testMultipleVertices() {
        final DirectedGraph dag = new DirectedGraph();
        dag.addNode("a");
        dag.addNode("b");
        final List orderedLayers = TopologicalSort.sort(dag);
        assertEquals(2, orderedLayers.size());
        assertTrue(orderedLayers.contains("a"));
        assertTrue(orderedLayers.contains("b"));
    }

    @Test (expected=java.util.NoSuchElementException.class)
    public void testBogusEdge() {
        final DirectedGraph dag = new DirectedGraph();
        dag.addNode("a");
        dag.addEdge("a", "b");
    }

    @Test
    public void testSimpleDag() {
        final DirectedGraph dag = new DirectedGraph();
        dag.addNode("b");        
        dag.addNode("a");
        dag.addEdge("a", "b");
        final List orderedLayers = TopologicalSort.sort(dag);
        assertEquals(2, orderedLayers.size());
        assertTrue(orderedLayers.get(0).equals("a"));
        assertTrue(orderedLayers.get(1).equals("b"));
    }

    @Test
    public void testComplexGraph() {
        // node b has two incoming edges
        final DirectedGraph dag = new DirectedGraph();
        dag.addNode("a");        
        dag.addNode("b");
        dag.addNode("c");        
        dag.addNode("d");
        dag.addNode("e");        
        dag.addNode("f");
        dag.addNode("g");        
        dag.addNode("h");
        dag.addEdge("a", "b");
        dag.addEdge("a", "c");
        dag.addEdge("c", "d");
        dag.addEdge("d", "b");
        dag.addEdge("c", "e");
        dag.addEdge("f", "g");

        final List orderedLayers = TopologicalSort.sort(dag);
        assertEquals(8, orderedLayers.size());
        assertTrue(orderedLayers.indexOf("a") < orderedLayers.indexOf("b")); 
        assertTrue(orderedLayers.indexOf("a") < orderedLayers.indexOf("c"));
        assertTrue(orderedLayers.indexOf("c") < orderedLayers.indexOf("d"));
        assertTrue(orderedLayers.indexOf("c") < orderedLayers.indexOf("e"));
        assertTrue(orderedLayers.indexOf("d") < orderedLayers.indexOf("b"));
        assertTrue(orderedLayers.indexOf("f") < orderedLayers.indexOf("g"));
    }

    @Test (expected=java.lang.IllegalArgumentException.class)
    public void testCycle() {
        // cycle between a, c, and d
        final DirectedGraph dag = new DirectedGraph();
        dag.addNode("a");        
        dag.addNode("b");
        dag.addNode("c");        
        dag.addNode("d");
        dag.addNode("e");        
        dag.addNode("f");
        dag.addNode("g");        
        dag.addNode("h");
        dag.addEdge("a", "b");
        dag.addEdge("a", "c");
        dag.addEdge("c", "d");
        dag.addEdge("d", "a");
        dag.addEdge("c", "e");
        dag.addEdge("f", "g");

        final List orderedLayers = TopologicalSort.sort(dag);
    }
}