Add support for reporting how an accessed member related to the context type and extract feature computation

Change-Id: Id5cd64ac5aaa8b5f502f978e4dab5f9502efb47c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/138560
Reviewed-by: Jaime Wren <[email protected]>
Commit-Queue: Brian Wilkerson <[email protected]>

Author: bwilkerson

pkg/analysis_server/tool/completion_metrics/features.dart

@@ -0,0 +1,67 @@
+// Copyright (c) 2020, 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.
+
+/// Utility methods to compute the value of the features used for code
+/// completion.
+import 'package:analyzer/dart/element/element.dart' show ClassElement;
+
+// TODO(brianwilkerson) Move this file to `lib` so that it can be used by the
+//  completion contributors.
+
+/// Return the inheritance distance between the [subclass] and the [superclass].
+/// We define the inheritance distance between two types to be zero if the two
+/// types are the same and the minimum number of edges that must be traversed in
+/// the type graph to get from the subtype to the supertype if the two types are
+/// not the same. Return `-1` if the [subclass] is not a subclass of the
+/// [superclass].
+int inheritanceDistance(ClassElement subclass, ClassElement superclass) {
+  return _inheritanceDistance(subclass, superclass, {});
+}
+
+/// Return the value of the inheritance distance feature for a member defined in
+/// the [superclass] that is being accessed through an expression whose static
+/// type is the [subclass].
+double inheritanceDistanceFeature(
+    ClassElement subclass, ClassElement superclass) {
+  var distance = _inheritanceDistance(subclass, superclass, {});
+  if (distance < 0) {
+    return 0;
+  }
+  return 1 / (distance + 1);
+}
+
+/// Return the inheritance distance between the [subclass] and the [superclass].
+/// The set of [visited] elements is used to guard against cycles in the type
+/// graph.
+///
+/// This is the implementation of [inheritanceDistance].
+int _inheritanceDistance(
+    ClassElement subclass, ClassElement superclass, Set<ClassElement> visited) {
+  if (subclass == null) {
+    return -1;
+  } else if (subclass == superclass) {
+    return 0;
+  } else if (!visited.add(subclass)) {
+    return -1;
+  }
+  var minDepth =
+      _inheritanceDistance(subclass.supertype?.element, superclass, visited);
+  for (var mixin in subclass.mixins) {
+    var depth = _inheritanceDistance(mixin.element, superclass, visited);
+    if (minDepth < 0 || depth < minDepth) {
+      minDepth = depth;
+    }
+  }
+  for (var interface in subclass.interfaces) {
+    var depth = _inheritanceDistance(interface.element, superclass, visited);
+    if (minDepth < 0 || (depth >= 0 && depth < minDepth)) {
+      minDepth = depth;
+    }
+  }
+  visited.remove(subclass);
+  if (minDepth < 0) {
+    return minDepth;
+  }
+  return minDepth + 1;
+}