null safety

.github/workflows/ci.yml

@@ -14,13 +14,13 @@ env:
 
 jobs:
   # Check code formatting and static analysis on a single OS (linux)
-  # against Dart dev.
+  # against Dart beta.
   analyze:
     runs-on: ubuntu-latest
     strategy:
       fail-fast: false
       matrix:
-        sdk: [dev]
+        sdk: [beta]
     steps:
       - uses: actions/checkout@v2
       - uses: dart-lang/[email protected]
@@ -38,7 +38,7 @@ jobs:
 
   # Run tests on a matrix consisting of two dimensions:
   # 1. OS: ubuntu-latest, (macos-latest, windows-latest)
-  # 2. release channel: dev
+  # 2. release channel: beta
   test:
     needs: analyze
     runs-on: ${{ matrix.os }}
@@ -47,7 +47,7 @@ jobs:
       matrix:
         # Add macos-latest and/or windows-latest if relevant for this package.
         os: [ubuntu-latest]
-        sdk: [2.9.0, dev]
+        sdk: [beta]
     steps:
       - uses: actions/checkout@v2
       - uses: dart-lang/[email protected]

CHANGELOG.md

@@ -1,12 +1,9 @@
 # 1.0.0-dev
 
+
+- Migrate to null safety.
 - **Breaking**: Paths from `shortestPath[s]` are now returned as iterables to
   reduce memory consumption of the algorithm to O(n).
-- Require Dart SDK `>=2.9.0 <3.0.0`.
-
-# 0.2.1
-
-- Require Dart SDK `>=2.2.0 <3.0.0`.
 
 # 0.2.0
 

benchmark/connected_components_benchmark.dart

@@ -32,7 +32,8 @@ void main() {
     while (watch.elapsed < duration) {
       count++;
       final length =
-          stronglyConnectedComponents(graph.keys, (e) => graph[e] ?? []).length;
+          stronglyConnectedComponents(graph.keys, (e) => graph[e] ?? <Never>[])
+              .length;
       assert(length == 244, '$length');
     }
 

benchmark/shortest_path_benchmark.dart

@@ -21,11 +21,11 @@ void main() {
     }
   }
 
-  int minTicks;
+  int? minTicks;
   var maxIteration = 0;
 
   final testOutput =
-      shortestPath(0, size - 1, (e) => graph[e] ?? []).toString();
+      shortestPath(0, size - 1, (e) => graph[e] ?? <Never>[]).toString();
   print(testOutput);
   assert(testOutput == '(258, 252, 819, 999)', testOutput);
 
@@ -34,7 +34,7 @@ void main() {
     watch
       ..reset()
       ..start();
-    final result = shortestPath(0, size - 1, (e) => graph[e] ?? []);
+    final result = shortestPath(0, size - 1, (e) => graph[e] ?? <Never>[])!;
     final length = result.length;
     final first = result.first;
     watch.stop();

benchmark/shortest_path_worst_case_benchmark.dart

@@ -22,11 +22,11 @@ void main() {
     }
   }
 
-  int minTicks;
+  int? minTicks;
   var maxIteration = 0;
 
   final testOutput =
-      shortestPath(0, size - 1, (e) => graph[e] ?? []).toString();
+      shortestPath(0, size - 1, (e) => graph[e] ?? <Never>[]).toString();
   print(testOutput);
   assert(testOutput == Iterable.generate(size - 1, (i) => i + 1).toString(),
       '$testOutput');
@@ -36,7 +36,7 @@ void main() {
     watch
       ..reset()
       ..start();
-    final result = shortestPath(0, size - 1, (e) => graph[e] ?? []);
+    final result = shortestPath(0, size - 1, (e) => graph[e] ?? <Never>[])!;
     final length = result.length;
     final first = result.first;
     watch.stop();

example/crawl_async_example.dart

@@ -26,10 +26,11 @@ Future<Null> main() async {
   print(allImports.map((s) => s.uri).toList());
 }
 
-AnalysisContext _analysisContext;
+AnalysisContext? _analysisContext;
 
 Future<AnalysisContext> get analysisContext async {
-  if (_analysisContext == null) {
+  var context = _analysisContext;
+  if (context == null) {
     var libUri = Uri.parse('package:graphs/');
     var libPath = await pathForUri(libUri);
     var packagePath = p.dirname(libPath);
@@ -39,16 +40,17 @@ Future<AnalysisContext> get analysisContext async {
       throw StateError('Expected to find exactly one context root, got $roots');
     }
 
-    _analysisContext = ContextBuilder().createContext(contextRoot: roots[0]);
+    context = _analysisContext =
+        ContextBuilder().createContext(contextRoot: roots[0]);
   }
 
-  return _analysisContext;
+  return context;
 }
 
 Future<Iterable<Uri>> findImports(Uri from, Source source) async {
   return source.unit.directives
       .whereType<UriBasedDirective>()
-      .map((d) => d.uri.stringValue)
+      .map((d) => d.uri.stringValue!)
       .where((uri) => !uri.startsWith('dart:'))
       .map((import) => resolveImport(import, from));
 }

example/example.dart

@@ -41,7 +41,7 @@ void main() {
   });
 
   var components = stronglyConnectedComponents<Node>(
-      graph.nodes.keys, (node) => graph.nodes[node]);
+      graph.nodes.keys, (node) => graph.nodes[node] ?? []);
 
   print(components);
 }

lib/src/crawl_async.dart

@@ -33,7 +33,9 @@ final _empty = Future<Null>.value(null);
 /// Crawling is eager, so calls to [edges] may overlap with other calls that
 /// have not completed. If the [edges] callback needs to be limited or throttled
 /// that must be done by wrapping it before calling [crawlAsync].
-Stream<V> crawlAsync<K, V>(Iterable<K> roots, FutureOr<V> Function(K) readNode,
+Stream<V> crawlAsync<K extends Object, V>(
+    Iterable<K> roots,
+    FutureOr<V> Function(K) readNode,
     FutureOr<Iterable<K>> Function(K, V) edges) {
   final crawl = _CrawlAsync(roots, readNode, edges)..run();
   return crawl.result.stream;
@@ -69,7 +71,7 @@ class _CrawlAsync<K, V> {
     if (value == null) return;
     if (result.isClosed) return;
     result.add(value);
-    var next = await edges(key, value) ?? const [];
+    var next = await edges(key, value);
     await Future.wait(next.map(_visit), eagerError: true);
   }
 

lib/src/shortest_path.dart

@@ -10,11 +10,6 @@ import 'dart:collection';
 /// If [start] `==` [target], an empty [List] is returned and [edges] is never
 /// called.
 ///
-/// [start], [target] and all values returned by [edges] must not be `null`.
-/// If asserts are enabled, an [AssertionError] is raised if these conditions
-/// are not met. If asserts are not enabled, violations result in undefined
-/// behavior.
-///
 /// If [equals] is provided, it is used to compare nodes in the graph. If
 /// [equals] is omitted, the node's own [Object.==] is used instead.
 ///
@@ -24,12 +19,12 @@ import 'dart:collection';
 ///
 /// If you supply one of [equals] or [hashCode], you should generally also to
 /// supply the other.
-Iterable<T> shortestPath<T>(
+Iterable<T>? shortestPath<T extends Object>(
   T start,
   T target,
   Iterable<T> Function(T) edges, {
-  bool Function(T, T) equals,
-  int Function(T) hashCode,
+  bool Function(T, T)? equals,
+  int Function(T)? hashCode,
 }) =>
     _shortestPaths<T>(
       start,
@@ -58,11 +53,11 @@ Iterable<T> shortestPath<T>(
 ///
 /// If you supply one of [equals] or [hashCode], you should generally also to
 /// supply the other.
-Map<T, Iterable<T>> shortestPaths<T>(
+Map<T, Iterable<T>> shortestPaths<T extends Object>(
   T start,
   Iterable<T> Function(T) edges, {
-  bool Function(T, T) equals,
-  int Function(T) hashCode,
+  bool Function(T, T)? equals,
+  int Function(T)? hashCode,
 }) =>
     _shortestPaths<T>(
       start,
@@ -71,37 +66,35 @@ Map<T, Iterable<T>> shortestPaths<T>(
       hashCode: hashCode,
     );
 
-Map<T, Iterable<T>> _shortestPaths<T>(
+Map<T, Iterable<T>> _shortestPaths<T extends Object>(
   T start,
   Iterable<T> Function(T) edges, {
-  T target,
-  bool Function(T, T) equals,
-  int Function(T) hashCode,
+  T? target,
+  bool Function(T, T)? equals,
+  int Function(T)? hashCode,
 }) {
-  assert(start != null, '`start` cannot be null');
-  assert(edges != null, '`edges` cannot be null');
-
   final distances = HashMap<T, _Tail<T>>(equals: equals, hashCode: hashCode);
   distances[start] = _Tail<T>();
 
-  equals ??= _defaultEquals;
-  if (equals(start, target)) {
+  final nonNullEquals = equals ??= _defaultEquals;
+  final isTarget =
+      target == null ? _neverTarget : (T node) => nonNullEquals(node, target);
+  if (isTarget(start)) {
     return distances;
   }
 
   final toVisit = ListQueue<T>()..add(start);
 
   while (toVisit.isNotEmpty) {
     final current = toVisit.removeFirst();
-    final currentPath = distances[current];
+    final currentPath = distances[current]!;
 
     for (var edge in edges(current)) {
-      assert(edge != null, '`edges` cannot return null values.');
       final existingPath = distances[edge];
 
       if (existingPath == null) {
         distances[edge] = currentPath.append(edge);
-        if (equals(edge, target)) {
+        if (isTarget(edge)) {
           return distances;
         }
         toVisit.add(edge);
@@ -113,6 +106,7 @@ Map<T, Iterable<T>> _shortestPaths<T>(
 }
 
 bool _defaultEquals(Object a, Object b) => a == b;
+bool _neverTarget(Object _) => false;
 
 /// An immutable iterable that can efficiently return a copy with a value
 /// appended.
@@ -123,9 +117,9 @@ bool _defaultEquals(Object a, Object b) => a == b;
 /// space because it copies all the values to a new list and uses that
 /// iterator in order to avoid stack overflows for large paths. This copy is
 /// cached for subsequent calls.
-class _Tail<T> extends Iterable<T> {
-  final T /*?*/ tail;
-  final _Tail<T> /*?*/ head;
+class _Tail<T extends Object> extends Iterable<T> {
+  final T? tail;
+  final _Tail<T>? head;
   @override
   final int length;
   _Tail()
@@ -135,19 +129,16 @@ class _Tail<T> extends Iterable<T> {
   _Tail._(this.tail, this.head, this.length);
   _Tail<T> append(T value) => _Tail._(value, this, length + 1);
 
-  Iterator<T> /*?*/ _iterator;
-
   @override
-  Iterator<T> get iterator {
-    if (_iterator == null) {
-      var /*_Tail<T>?*/ next = this;
-      var values = List<T>.generate(length, (_) {
-        var val = next.tail;
-        next = next.head;
-        return val;
-      });
-      _iterator = values.reversed.iterator;
-    }
-    return _iterator;
-  }
+  Iterator<T> get iterator => _asIterable.iterator;
+
+  late final _asIterable = () {
+    _Tail<T>? next = this;
+    var reversed = List.generate(length, (_) {
+      var val = next!.tail;
+      next = next!.head;
+      return val as T;
+    });
+    return reversed.reversed;
+  }();
 }

lib/src/strongly_connected_components.dart

@@ -29,35 +29,34 @@ import 'dart:math' show min;
 ///
 /// If you supply one of [equals] or [hashCode], you should generally also to
 /// supply the other.
-List<List<T>> stronglyConnectedComponents<T>(
+List<List<T>> stronglyConnectedComponents<T extends Object>(
   Iterable<T> nodes,
   Iterable<T> Function(T) edges, {
-  bool Function(T, T) equals,
-  int Function(T) hashCode,
+  bool Function(T, T)? equals,
+  int Function(T)? hashCode,
 }) {
   final result = <List<T>>[];
   final lowLinks = HashMap<T, int>(equals: equals, hashCode: hashCode);
   final indexes = HashMap<T, int>(equals: equals, hashCode: hashCode);
   final onStack = HashSet<T>(equals: equals, hashCode: hashCode);
 
-  equals ??= _defaultEquals;
+  final nonNullEquals = equals ?? _defaultEquals;
 
   var index = 0;
   var lastVisited = Queue<T>();
 
   void strongConnect(T node) {
     indexes[node] = index;
-    lowLinks[node] = index;
+    var lowLink = lowLinks[node] = index;
     index++;
     lastVisited.addLast(node);
     onStack.add(node);
-    // ignore: omit_local_variable_types
-    for (final T next in edges(node) ?? const []) {
+    for (final next in edges(node)) {
       if (!indexes.containsKey(next)) {
         strongConnect(next);
-        lowLinks[node] = min(lowLinks[node], lowLinks[next]);
+        lowLink = lowLinks[node] = min(lowLink, lowLinks[next]!);
       } else if (onStack.contains(next)) {
-        lowLinks[node] = min(lowLinks[node], indexes[next]);
+        lowLink = lowLinks[node] = min(lowLink, indexes[next]!);
       }
     }
     if (lowLinks[node] == indexes[node]) {
@@ -67,7 +66,7 @@ List<List<T>> stronglyConnectedComponents<T>(
         next = lastVisited.removeLast();
         onStack.remove(next);
         component.add(next);
-      } while (!equals(next, node));
+      } while (!nonNullEquals(next, node));
       result.add(component);
     }
   }

pubspec.yaml

@@ -4,13 +4,13 @@ description: Graph algorithms that operate on graphs in any representation
 homepage: https://github.com/dart-lang/graphs
 
 environment:
-  sdk: '>=2.9.0 <3.0.0'
+  sdk: '>=2.12.0-0 <3.0.0'
 
 dev_dependencies:
-  pedantic: ^1.3.0
-  test: ^1.5.1
+  pedantic: ^1.10.0
+  test: ^1.16.0
 
   # For examples
-  analyzer: '>=0.35.0 <0.42.0'
-  path: ^1.1.0
-  pool: ^1.3.0
+  path: ^1.8.0
+  pool: ^1.5.0
+  analyzer: ^1.0.0