[feature] Add 2 DAG Algorithms: ancestors and descendants

src/algorithms/__tests__/dag-test.js

@@ -1,6 +1,7 @@
 /*globals assert, utils*/
 'use strict';
 
+import { Set } from '../../_internals/';
 import {Graph, DiGraph} from '../../classes';
 import JSNetworkXError from '../../exceptions/JSNetworkXError';
 import JSNetworkXUnfeasible from '../../exceptions/JSNetworkXUnfeasible';
@@ -162,5 +163,34 @@ export var TestDAG = {
     G.addCycle([0,1,2]);
     G.addEdge(3,3);
     assert(!dag.isAperiodic(G));
+  },
+
+  // dag.ancestors
+  // https://github.com/networkx/networkx/blob/master/networkx/algorithms/tests/test_dag.py#L281
+  testAncestors: function () {
+    var DG = new DiGraph();
+    DG.addEdgesFrom([[1, 2], [1, 3], [4, 2], [4, 3], [4, 5], [2, 6], [5, 6]]);
+    assert.deepEqual(dag.ancestors(DG, 6), new Set([1, 2, 4, 5]));
+    assert.deepEqual(dag.ancestors(DG, 3), new Set([1, 4]));
+    assert.deepEqual(dag.ancestors(DG, 1), new Set());
+  },
+  testAncestorsInvalidSource: function () {
+    var DG = new DiGraph();
+    DG.addEdgesFrom([[1, 2], [1, 3]]); // tested node is not in graph
+    assert.throws(() => dag.ancestors(DG, 0), JSNetworkXError);
+  },
+
+  // dag.descendants
+  testDescendants: function () {
+    var DG = new DiGraph();
+    DG.addEdgesFrom([[1, 2], [1, 3], [4, 2], [4, 3], [4, 5], [2, 6], [5, 6]]);
+    assert.deepEqual(dag.descendants(DG, 1), new Set([2, 3, 6]));
+    assert.deepEqual(dag.descendants(DG, 4), new Set([2, 3, 5, 6]));
+    assert.deepEqual(dag.descendants(DG, 3), new Set([]));
+  },
+  testDescendantsInvalidSource: function () {
+    var DG = new DiGraph();
+    DG.addEdgesFrom([[1, 2], [1, 3]]);
+    assert.throws(() => dag.descendants(DG, 0), JSNetworkXError);
   }
 };

src/algorithms/dag.js

@@ -4,6 +4,8 @@
 import JSNetworkXError from '../exceptions/JSNetworkXError';
 import JSNetworkXUnfeasible from '../exceptions/JSNetworkXUnfeasible';
 
+import { shortestPathLength } from './shortestPaths';
+
 import {
 /*jshint ignore:start*/
   Map,
@@ -13,8 +15,41 @@ import {
   gcd
 } from '../_internals';
 
-// TODO: descendants
-// TODO: ancestors
+/**
+ * Returns all nodes reachable from `source` in `G`
+ *
+ * @param {Graph} G A NetworkX directed acyclic graph
+ * @param {Node} Source: node in `G`
+ * @return {Set} The descendants of `source` in `G`
+ */
+export async function descendants(G, source) {
+  if (!G.hasNode(source)) {
+    throw new JSNetworkXError(
+      `The node ${source} is not in the graph`
+    )
+  }
+  const descendantNodes = new Set(shortestPathLength(G, { source }).keys());
+  descendantNodes.delete(source); // cannot be own parent
+  return descendantNodes;
+}
+
+/**
+ * Returns all nodes having a path to `source` in `G`
+ *
+ * @param {Graph} G A NetworkX directed acyclic graph
+ * @param {Node} Source: node in `G`
+ * @return {Set} Set of all nodes that source is descended from
+ */
+export async function ancestors(G, source) {
+  if (!G.hasNode(source)) {
+    throw new JSNetworkXError(
+      `The node ${source} is not in the graph`
+    )
+  }
+  const ancestorNodes = new Set(shortestPathLength(G, { target: source }).keys());
+  ancestorNodes.delete(source); // cannot be own parent
+  return ancestorNodes;
+}
 
 /**
  * Return `true` if the graph G is a directed acyclic graph (DAG) or