Adding reverse post order

graphs-core/src/main/java/com/github/moaxcp/graphs/PropertyGraph.java

@@ -971,6 +971,49 @@ default Stream<Vertex<ID>> breadthFirstStream(ID... start) {
     return StreamSupport.stream(Spliterators.spliteratorUnknownSize(iterator, Spliterator.ORDERED), false);
   }
 
+  /**
+   * Returns a reverse-post-order {@link Iterator} of every {@link Vertex} in this graph. This is a reverse of a
+   * postOrderIterator starting at the provided vertices.
+   * @throws NullPointerException if start is null or any id in start
+   * @throws IllegalArgumentException if start contains ids that are not in the graph
+   * @param start ids of traversal
+   * @return
+   */
+  default Iterator<Vertex<ID>> reversePostOrderIterator(ID... start) {
+    var iterator = postOrderIterator(start);
+    var list = new LinkedList<Vertex<ID>>();
+    while(iterator.hasNext()) {
+      list.addFirst(iterator.next());
+    }
+    return new Iterator<Vertex<ID>>() {
+      @Override
+      public boolean hasNext() {
+        return list.size() != 0;
+      }
+
+      @Override
+      public Vertex<ID> next() {
+        if(list.size() == 0) {
+          throw new NoSuchElementException("Could not find next element.");
+        }
+        return list.removeFirst();
+      }
+    };
+  }
+
+  /**
+   * Returns a reverse-post-order {@link Stream} of every {@link Vertex} in this graph. This is a reverse of a
+   * postOrderIterator starting at the provided vertices.
+   * @throws NullPointerException if start is null or any id in start
+   * @throws IllegalArgumentException if start contains ids that are not in the graph
+   * @param start ids of traversal
+   * @return
+   */
+  default Stream<Vertex<ID>> reversePostOrderStream(ID... start) {
+    var iterator = reversePostOrderIterator(start);
+    return StreamSupport.stream(Spliterators.spliteratorUnknownSize(iterator, Spliterator.ORDERED), false);
+  }
+
   /**
    * Returns true if this graph is empty.
    * @return

graphs-manual/src/docs/asciidoc/index.adoc

@@ -345,6 +345,24 @@ include::{sourcedir}/com/github/moaxcp/graphs/manual/PostorderTraversal.java[tag
 
 image::images/postOrderTraversal.gif[post order]
 
+== reverse-post-order iterator and post-order stream
+
+An iterator and stream are provided to perform reverse-post-order depth first traversal.
+
+.reversePostOrderIterator()
+[source,java,indent=0]
+----
+include::{sourcedir}/com/github/moaxcp/graphs/manual/ReversePostorderTraversal.java[tags=reversePostOrderIterator]
+----
+
+.reversePostOrderStream()
+[source,java,indent=0]
+----
+include::{sourcedir}/com/github/moaxcp/graphs/manual/ReversePostorderTraversal.java[tags=reversePostOrderStream]
+----
+
+image::images/reversePostOrderTraversal.gif[reverse post order]
+
 == breadth first iterator and breadth first stream
 
 An iterator and stream are provided to perform breadth first traversals.
@@ -365,6 +383,10 @@ image::images/breadthFirstTraversal.gif[post order]
 
 == Releases
 
+== 0.16.0-SNAPSHOT
+
+Adding reversePostOrder iterator and stream.
+
 == 0.15.0
 
 Adding new project `graphs-guava` and `graphs-graphviz-guava-gif`.

graphs-manual/src/test/java/com/github/moaxcp/graphs/manual/ReversePostorderTraversal.java

@@ -0,0 +1,59 @@
+package com.github.moaxcp.graphs.manual;
+
+import com.github.moaxcp.graphs.DirectedPropertyGraph;
+import com.github.moaxcp.graphs.PropertyGraph.Vertex;
+import com.github.moaxcp.graphs.graphviz.greenrobotgif.GreenrobotGifSubscriber;
+import com.github.moaxcp.graphs.greenrobot.DirectedEventPropertyGraph;
+import java.nio.file.Path;
+import java.util.Iterator;
+import org.greenrobot.eventbus.EventBus;
+import org.junit.jupiter.api.Test;
+
+public class ReversePostorderTraversal {
+
+  @Test
+  void reversePostOrderTraversal() {
+    var bus = new EventBus();
+    var graph = new DirectedEventPropertyGraph<String>(bus);
+
+    graph.edge("A", "B")
+        .edge("B", "C")
+        .edge("B", "D")
+        .edge("D", "E")
+        .edge("D", "C")
+        .edge("A", "D")
+        .edge("D", "A")
+        .edge("A", "E")
+        .edge("F", "G")
+        .edge("G", "D");
+    var gif = new GreenrobotGifSubscriber<>(graph, Path.of("src/docs/asciidoc/images/reversePostOrderTraversal.gif"));
+
+    // tag::reversePostOrderIterator[]
+    Iterator<Vertex<String>> iterator = graph.reversePostOrderIterator("A");
+    while(iterator.hasNext()) {
+      Vertex<String> vertex = iterator.next();
+      vertex.property("color", "green");
+    }
+    // end::reversePostOrderIterator[]
+    gif.writeFile();
+  }
+
+  @Test
+  void postOrderStream() {
+    var graph = new DirectedPropertyGraph<String>();
+
+    graph.edge("A", "B")
+        .edge("B", "C")
+        .edge("B", "D")
+        .edge("D", "E")
+        .edge("D", "C")
+        .edge("A", "D")
+        .edge("D", "A")
+        .edge("A", "E")
+        .edge("F", "G")
+        .edge("G", "D");
+    // tag::reversePostOrderStream[]
+    graph.reversePostOrderStream("A").forEach(v -> v.property("color", "green"));
+    // end::reversePostOrderStream[]
+  }
+}

graphs-test/src/test/java/publicapi/PostOrderDepthFirstIteratorTest.java

@@ -1,19 +1,17 @@
 package publicapi;
 
-import com.github.moaxcp.graphs.*;
-import com.github.moaxcp.graphs.PropertyGraph.*;
-import com.github.moaxcp.graphs.testframework.*;
-import org.junit.jupiter.api.*;
-import org.junit.jupiter.params.*;
-import org.junit.jupiter.params.provider.*;
-
-import java.util.*;
-
-import static com.github.moaxcp.graphs.testframework.MethodSources.*;
-import static com.github.moaxcp.graphs.testframework.PathOrder.*;
-import static com.google.common.truth.Truth.*;
-import static java.util.stream.Collectors.*;
-import static org.junit.jupiter.api.Assertions.*;
+import com.github.moaxcp.graphs.PropertyGraph;
+import com.github.moaxcp.graphs.PropertyGraph.Vertex;
+import com.github.moaxcp.graphs.testframework.TestGraphs;
+import java.util.List;
+import java.util.NoSuchElementException;
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.MethodSource;
+
+import static com.google.common.truth.Truth.assertThat;
+import static java.util.stream.Collectors.toList;
+import static org.junit.jupiter.api.Assertions.assertThrows;
 
 public class PostOrderDepthFirstIteratorTest {
 
@@ -98,18 +96,6 @@ void postOrderIterator(String name, PropertyGraph<String> graph, String[] start,
     }
   }
 
-  @TestDirectedGraphs
-  void postOrderIteratorStart(PropertyGraph<String> graph) {
-    complexTwoComponents(graph, POST_ORDER);
-    var result = new ArrayList<String>();
-    var iterator = graph.postOrderIterator("D", "G", "W");
-    while(iterator.hasNext()) {
-      result.add(iterator.next().getId());
-    }
-
-    assertThat(result).isEqualTo(List.of("E", "C", "B", "A", "D", "G", "W", "F", "Y", "Z", "X"));
-  }
-
   @MethodSource("com.github.moaxcp.graphs.testframework.MethodSources#graphsPostOrder")
   @DisplayName("postOrderStream matches expected order")
   @ParameterizedTest(name = "{index} - {0} {2}")
@@ -120,14 +106,4 @@ void postOrderStream(String name, PropertyGraph<String> graph, String[] start, L
 
     assertThat(result).isEqualTo(expectedOrder);
   }
-
-  @TestDirectedGraphs
-  void postOrderStreamStart(PropertyGraph<String> graph) {
-    complexTwoComponents(graph, POST_ORDER);
-    var result = graph.postOrderStream("D", "G", "W")
-      .map(Vertex::getId)
-      .collect(toList());
-
-    assertThat(result).isEqualTo(List.of("E", "C", "B", "A", "D", "G", "W", "F", "Y", "Z", "X"));
-  }
 }

graphs-test/src/test/java/publicapi/ReversePostOrderDepthFirstIteratorTest.java

@@ -0,0 +1,109 @@
+package publicapi;
+
+import com.github.moaxcp.graphs.PropertyGraph;
+import com.github.moaxcp.graphs.PropertyGraph.Vertex;
+import com.github.moaxcp.graphs.testframework.TestGraphs;
+import java.util.List;
+import java.util.NoSuchElementException;
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.MethodSource;
+
+import static com.google.common.truth.Truth.assertThat;
+import static java.util.stream.Collectors.toList;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+public class ReversePostOrderDepthFirstIteratorTest {
+
+  @TestGraphs
+  void nullStart(PropertyGraph<String> graph) {
+    var exception = assertThrows(NullPointerException.class, () -> graph.reversePostOrderIterator((String[]) null));
+    assertThat(exception).hasMessageThat().isEqualTo("start is marked non-null but is null");
+  }
+
+  @TestGraphs
+  void nullInOther(PropertyGraph<String> graph) {
+    graph.vertex("A").vertex("B");
+    var exception = assertThrows(NullPointerException.class, () -> graph.reversePostOrderIterator("A", null, "B"));
+    assertThat(exception).hasMessageThat().isEqualTo("\"id\" in \"start\" must not be null.");
+  }
+
+  @TestGraphs
+  void startNotInGraph(PropertyGraph<String> graph) {
+    var exception = assertThrows(IllegalArgumentException.class, () -> graph.reversePostOrderIterator("A"));
+    assertThat(exception).hasMessageThat().isEqualTo("vertex \"A\" not found in graph.");
+  }
+
+  @TestGraphs
+  void hasNext_EmptyGraph(PropertyGraph<String> graph) {
+    var iterator = graph.reversePostOrderIterator();
+    assertThat(iterator.hasNext()).isFalse();
+  }
+
+  @TestGraphs
+  void next_EmptyGraph(PropertyGraph<String> graph) {
+    var iterator = graph.reversePostOrderIterator();
+    var exception = assertThrows(NoSuchElementException.class, () -> iterator.next());
+    assertThat(exception).hasMessageThat().isEqualTo("Could not find next element.");
+  }
+
+  @TestGraphs
+  void hasNext_beforeIteration(PropertyGraph<String> graph) {
+    graph.vertex("A");
+    var iterator = graph.reversePostOrderIterator();
+    assertThat(iterator.hasNext()).isTrue();
+  }
+
+  @TestGraphs
+  void hasNext_MultipleBeforeIteration(PropertyGraph<String> graph) {
+    graph.vertex("A");
+    var iterator = graph.reversePostOrderIterator();
+    assertThat(iterator.hasNext()).isTrue();
+    assertThat(iterator.hasNext()).isTrue();
+    assertThat(iterator.hasNext()).isTrue();
+    assertThat(iterator.next().getId()).isEqualTo("A");
+  }
+
+  @TestGraphs
+  void hasNext_MultipleBetweenComponents(PropertyGraph<String> graph) {
+    graph.vertex("A");
+    graph.vertex("B");
+    var iterator = graph.reversePostOrderIterator();
+    assertThat(iterator.hasNext()).isTrue();
+    assertThat(iterator.next().getId()).isEqualTo("B");
+    assertThat(iterator.hasNext()).isTrue();
+    assertThat(iterator.hasNext()).isTrue();
+    assertThat(iterator.hasNext()).isTrue();
+    assertThat(iterator.next().getId()).isEqualTo("A");
+  }
+
+  @TestGraphs
+  void next_withoutCallingHasNext(PropertyGraph<String> graph) {
+    graph.vertex("A");
+    var iterator = graph.reversePostOrderIterator();
+    assertThat(iterator.next().getId()).isEqualTo("A");
+    assertThat(iterator.hasNext()).isFalse();
+  }
+
+  @MethodSource("com.github.moaxcp.graphs.testframework.MethodSources#graphsReversePostOrder")
+  @DisplayName("reversePostOrderIterator matches expected order")
+  @ParameterizedTest(name = "{index} - {0} {2}")
+  void reversePostOrderIterator(String name, PropertyGraph<String> graph, String[] start, List<String> expectedOrder) {
+    var iterator = graph.reversePostOrderIterator(start);
+    for(String expected : expectedOrder) {
+      String result = iterator.next().getId();
+      assertThat(result).isEqualTo(expected);
+    }
+  }
+
+  @MethodSource("com.github.moaxcp.graphs.testframework.MethodSources#graphsReversePostOrder")
+  @DisplayName("reversePostOrderStream matches expected order")
+  @ParameterizedTest(name = "{index} - {0} {2}")
+  void reversePostOrderStream(String name, PropertyGraph<String> graph, String[] start, List<String> expectedOrder) {
+    var result = graph.reversePostOrderStream(start)
+    .map(Vertex::getId)
+    .collect(toList());
+
+    assertThat(result).isEqualTo(expectedOrder);
+  }
+}