Cache topological sort for preformance (no incremental update to the sort)

Author: SegFaultx64

src/directedAcyclicGraph.ts

@@ -10,6 +10,8 @@ export class CycleError<T> extends Error {
     }
 }
 export default class DirectedAcyclicGraph<T> extends DirectedGraph<T> {
+    private _topologicallySortedNodes?: Array<T>;
+
     static fromDirectedGraph<T>(graph: DirectedGraph<T>): DirectedAcyclicGraph<T> {
         if (!graph.isAcyclic()) {
             throw new CycleError("Can't convert that graph to a DAG because it contains a cycle")
@@ -27,11 +29,26 @@ export default class DirectedAcyclicGraph<T> extends DirectedGraph<T> {
         if (this.wouldAddingEdgeCreateCyle(node1Identity, node2Identity)) {
             throw new CycleError(`Can't add edge from ${node1Identity} to ${node2Identity} it would create a cycle`)
         }
+
+        // Invalidate cache of toposorted nodes
+        this._topologicallySortedNodes = undefined;
         super.addEdge(node1Identity, node2Identity, true)
     }
 
+    // Maintain topo sort when inserting node
+    insert(node: T): string {
+        if (this._topologicallySortedNodes)
+            this._topologicallySortedNodes = [node, ...this._topologicallySortedNodes];
+        
+        return super.insert(node)
+    }
+
     // This is an implementation of Kahn's algorithim
     topologicallySortedNodes(): Array<T> {
+        if (this._topologicallySortedNodes != undefined) {
+            return this._topologicallySortedNodes;
+        }
+
         const nodeIndices = Array.from(this.nodes.keys());
         const nodeInDegrees = new Map(Array.from(this.nodes.keys()).map(n => [n, this.inDegreeOfNode(n)]))
 
@@ -40,7 +57,9 @@ export default class DirectedAcyclicGraph<T> extends DirectedGraph<T> {
         let toSearch = Array.from(nodeInDegrees).filter(pair => pair[1] === 0)
 
         if (toSearch.length === this.nodes.size) {
-            return Array.from(this.nodes.values())
+            const arrayOfNodes = Array.from(this.nodes.values());
+            this._topologicallySortedNodes = arrayOfNodes
+            return arrayOfNodes
         }
 
         let toReturn: Array<T> = []
@@ -63,6 +82,9 @@ export default class DirectedAcyclicGraph<T> extends DirectedGraph<T> {
             })
         }
 
+        // Update cache
+        this._topologicallySortedNodes = toReturn;
+
         // we shouldn't need to account for the error case of there being a cycle because it shouldn't
         // be possible to instantiate this class in a state (or put it in a state) where there is a cycle.
 

test/directedAcyclicGraph.test.ts

@@ -81,6 +81,14 @@ describe("Directed Acyclic Graph", () => {
 
     expect([{ name: 'C' }, { name: 'D' }]).toContainEqual(topoList3[1])
     expect([{ name: 'C' }, { name: 'D' }]).toContainEqual(topoList3[2])
+
+    graph.insert({ name: 'F' })
+
+    const topoList4 = graph.topologicallySortedNodes();
+
+    expect(topoList4).toContainEqual({ name: 'F' })
+    expect([{ name: 'A' }, { name: 'F' }]).toContainEqual(topoList4[0])
+    expect([{ name: 'A' }, { name: 'F' }]).toContainEqual(topoList4[1])
   })
 
 })