Improve speed of finding cycles (swiftlang#332)

Author: mpsnp

Sources/TSCBasic/GraphAlgorithms.swift

@@ -115,10 +115,13 @@ public func findCycle<T: Hashable>(
 ) rethrows -> (path: [T], cycle: [T])? {
     // Ordered set to hold the current traversed path.
     var path = OrderedSet<T>()
+    var validNodes = Set<T>()
 
     // Function to visit nodes recursively.
     // FIXME: Convert to stack.
     func visit(_ node: T, _ successors: (T) throws -> [T]) rethrows -> (path: [T], cycle: [T])? {
+        if validNodes.contains(node) { return nil }
+        
         // If this node is already in the current path then we have found a cycle.
         if !path.append(node) {
             let index = path.firstIndex(of: node)!
@@ -133,6 +136,7 @@ public func findCycle<T: Hashable>(
         // No cycle found for this node, remove it from the path.
         let item = path.removeLast()
         assert(item == node)
+        validNodes.insert(node)
         return nil
     }
 

Tests/TSCBasicTests/GraphAlgorithmsTests.swift

@@ -54,6 +54,23 @@ class GraphAlgorithmsTests: XCTestCase {
         XCTAssertEqual([1, 2], transitiveClosure(1, [1: [2], 2: [1]]))
     }
 
+    func testLayeredAllToAllGraph() throws {
+        let count = 100
+        let items = Array(0...count)
+        let layers = (0...count).flatMap { layerNumber in
+            items.map {
+                (
+                    $0 + 1000 * layerNumber,
+                    layerNumber == count ? [] : items.map {
+                        $0 + 1000 * (layerNumber + 1)
+                    }
+                )
+            }
+        }
+        
+        XCTAssertNotCycle(findCycle(1, Dictionary(uniqueKeysWithValues: layers)))
+    }
+
     func testTopologicalSort() throws {
         // A trival graph.
         XCTAssertEqual([1, 2], try topologicalSort(1, [1: [2]]))