Avoid network requests while resolving versions when possible.

sdk/lib/_internal/pub/lib/src/solver/version_queue.dart

Index: sdk/lib/_internal/pub/lib/src/solver/version_queue.dart
diff --git a/sdk/lib/_internal/pub/lib/src/solver/version_queue.dart b/sdk/lib/_internal/pub/lib/src/solver/version_queue.dart
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index 0000000000000000000000000000000000000000..1adf948b4d78ecfb22aaa80bd8714a7fed1af366
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@@ -0,0 +1,92 @@
+// Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+library pub.solver.version_queue;
+
+import 'dart:async';
+import 'dart:collection' show Queue;
+
+import '../package.dart';
+import 'backtracking_solver.dart';
+
+/// A function that asynchronously returns a sequence of package IDs.
+typedef Future<Iterable<PackageId>> PackageIdGenerator();
+
+/// A prioritized, asynchronous queue of the possible versions that can be
+/// selected for one package.
+///
+/// If there is a locked version, that comes first, followed by other versions
+/// in descending order. This avoids requesting the list of versions until
+/// needed (i.e. after any locked version has been consumed) to avoid unneeded
+/// network requests.
+class VersionQueue {
+  /// The set of allowed versions that match [_constraint].
+  ///
+  /// If [_locked] is not `null`, this will initially be `null` until we
+  /// advance past the locked version.
+  Queue<PackageId> _allowed;
+
+  /// The callback that will generate the sequence of packages. This will be
+  /// called as lazily as possible.
+  final PackageIdGenerator _allowedGenerator;
+
+  /// The currently locked version of the package, or `null` if there is none,
+  /// or we have advanced past it.
+  PackageId _locked;
+
+  /// Gets the currently selected version.
+  PackageId get current {
+    if (_locked != null) return _locked;
+    return _allowed.first;
+  }
+
+  /// Creates a new [PrioritivedVersions] queue for versions of [dep] matching
+  /// [constraint] starting with (optionally) [locked].
+  ///
+  /// This is asynchronous so that [current] can always be accessed
+  /// synchronously. If there is no locked version, we need to get the list of
+  /// versions asynchronously before we can determine what the first one is.

[nweiz, 2013/11/26 21:00:43]

Update this documentation to reflect the new parameters.

[Bob Nystrom, 2013/11/26 22:16:10]

Done.

+  static Future<VersionQueue> create(PackageId locked,
+      PackageIdGenerator allowedGenerator) {
+    var versions = new VersionQueue._(locked, allowedGenerator);
+
+    // If there is a locked version, it's the current one so it's synchronously
+    // available now.
+    if (locked != null) return new Future.value(versions);
+
+    // Otherwise, the current version needs to be calculated before we can
+    // return.
+    return versions._calculateAllowed().then((_) => versions);
+  }
+
+  VersionQueue._(this._locked, this._allowedGenerator);
+
+  /// Tries to advance to the next possible version.
+  ///
+  /// Returns `true` if it moved to a new version (which can be accessed from
+  /// [current]. Returns `false` if there are no more versions.
+  Future<bool> advance() {
+    // If we have a locked version, consume it first.
+    if (_locked != null) {
+      // Advancing past the locked version, so need to load the others now
+      // so that [current] is available.
+      return _calculateAllowed().then((_) {
+        _locked = null;
+        return _allowed.isNotEmpty;
+      });
+    }
+
+    // Move to the next allowed version.
+    _allowed.removeFirst();
+    return new Future.value(_allowed.isNotEmpty);
+  }
+
+  /// Determines the list of allowed versions matching its constraint and places
+  /// them in [_allowed].
+  Future _calculateAllowed() {
+    return _allowedGenerator().then((allowed) {
+      _allowed = new Queue<PackageId>.from(allowed);
+    });
+  }
+}