Add Tarjan SCC algorithm implementation

Adds an implementation of Tarjan's algorithm to find strongly connected
components.

This will be used on the experimental bundler to simplify cycle handling and
can be used on other parts of the codebase to simplify the symbol propagation
implementation.

On the bundler, we will use this to convert the graph to an acyclic graph
(replace the strongly connected components with a single 'group' node) before
doing other work.

Test Plan: yarn test

Reviewers: JakeLane

Reviewed By: JakeLane

Pull Request: #332

packages/bundlers/bundler-experimental/src/DominatorBundler/cycleBreaker/findStronglyConnectedComponents.js

@@ -0,0 +1,79 @@
+// @flow strict-local
+
+import type {Graph, NodeId} from '@atlaspack/graph';
+
+export type StronglyConnectedComponent = NodeId[];
+
+/**
+ * Robert Tarjan's algorithm to find strongly connected components in a graph.
+ *
+ * Time complexity: O(V + E)
+ * Space complexity (worst case): O(V)
+ *
+ * * https://web.archive.org/web/20170829214726id_/http://www.cs.ucsb.edu/~gilbert/cs240a/old/cs240aSpr2011/slides/TarjanDFS.pdf
+ * * https://en.wikipedia.org/wiki/Tarjan%27s_strongly_connected_components_algorithm
+ */
+export function findStronglyConnectedComponents<T, E: number>(
+  graph: Graph<T, E>,
+): StronglyConnectedComponent[] {
+  type State = {|
+    index: null | NodeId,
+    lowlink: null | NodeId,
+    onStack: boolean,
+  |};
+  const result = [];
+  let index = 0;
+  const stack = [];
+  const state: State[] = new Array(graph.nodes.length).fill(null).map(() => ({
+    index: null,
+    lowlink: null,
+    onStack: false,
+  }));
+  graph.nodes.forEach((node, nodeId) => {
+    if (node != null && state[nodeId].index == null) {
+      strongConnect(nodeId);
+    }
+  });
+  function strongConnect(nodeId) {
+    const nodeState = state[nodeId];
+    nodeState.index = index;
+    nodeState.lowlink = index;
+
+    index += 1;
+
+    stack.push(nodeId);
+    nodeState.onStack = true;
+
+    for (let neighborId of graph.getNodeIdsConnectedFrom(nodeId)) {
+      const neighborState = state[neighborId];
+      if (neighborState.index == null) {
+        strongConnect(neighborId);
+
+        nodeState.lowlink = Math.min(
+          Number(nodeState.lowlink),
+          Number(neighborState.lowlink),
+        );
+      } else if (neighborState.onStack) {
+        nodeState.lowlink = Math.min(
+          Number(nodeState.lowlink),
+          Number(neighborState.index),
+        );
+      }
+    }
+
+    if (nodeState.lowlink === nodeState.index) {
+      const component = [];
+      let member;
+
+      do {
+        member = stack.pop();
+        state[member].onStack = false;
+        component.push(member);
+      } while (member !== nodeId);
+
+      result.push(component);
+    }
+  }
+
+  return result;
+}

packages/bundlers/bundler-experimental/test/DominatorBundler/cycleBreaker/findStronglyConnectedComponents.js

@@ -0,0 +1,81 @@
+// @flow strict-local
+
+import {Graph} from '@atlaspack/graph';
+import assert from 'assert';
+import {findStronglyConnectedComponents} from '../../../src/DominatorBundler/cycleBreaker/findStronglyConnectedComponents';
+
+describe('findStronglyConnectedComponents', () => {
+  it('should find strongly connected components', () => {
+    /*
+      digraph graph {
+        root -> a;
+        a -> b;
+        b -> a;
+        c;
+      }
+     */
+    const graph = new Graph();
+    const root = graph.addNode('root');
+    graph.setRootNodeId(root);
+    const a = graph.addNode('a');
+    const b = graph.addNode('b');
+    const c = graph.addNode('c');
+
+    graph.addEdge(root, a);
+    graph.addEdge(a, b);
+    graph.addEdge(b, a);
+
+    const result = findStronglyConnectedComponents(graph);
+
+    assert.deepStrictEqual(result, [[b, a], [root], [c]]);
+  });
+
+  it('works over an empty graph with a root', () => {
+    /*
+      digraph graph {
+        root;
+      }
+     */
+    const graph = new Graph();
+    const root = graph.addNode('root');
+    graph.setRootNodeId(root);
+
+    const result = findStronglyConnectedComponents(graph);
+    assert.deepStrictEqual(result, [[root]]);
+  });
+
+  it('works over an empty graph', () => {
+    /*
+      digraph graph {
+      }
+     */
+    const graph = new Graph();
+
+    const result = findStronglyConnectedComponents(graph);
+    assert.deepStrictEqual(result, []);
+  });
+
+  it('returns all nodes if there are no cycles', () => {
+    /*
+      digraph graph {
+        root -> a;
+        a -> b;
+        b -> c;
+      }
+     */
+    const graph = new Graph();
+    const root = graph.addNode('root');
+    graph.setRootNodeId(root);
+    const a = graph.addNode('a');
+    const b = graph.addNode('b');
+    const c = graph.addNode('c');
+
+    graph.addEdge(root, a);
+    graph.addEdge(a, b);
+    graph.addEdge(b, c);
+
+    const result = findStronglyConnectedComponents(graph);
+
+    assert.deepStrictEqual(result, [[c], [b], [a], [root]]);
+  });
+});