Add selector specificity.

lib/src/ast/selector.dart

@@ -11,7 +11,7 @@ export 'selector/list.dart';
 export 'selector/namespaced_identifier.dart';
 export 'selector/placeholder.dart';
 export 'selector/pseudo.dart';
-export 'selector/simple.dart';
+export 'selector/simple.dart' hide baseSpecificity;
 export 'selector/type.dart';
 export 'selector/universal.dart';
 

lib/src/ast/selector/complex.dart

@@ -10,11 +10,34 @@ class ComplexSelector extends Selector {
   // Indices of [components] that are followed by line breaks.
   final List<int> lineBreaks;
 
+  int get minSpecificity {
+    if (_minSpecificity == null) _computeSpecificity();
+    return _minSpecificity;
+  }
+  int _minSpecificity;
+
+  int get maxSpecificity {
+    if (_maxSpecificity == null) _computeSpecificity();
+    return _maxSpecificity;
+  }
+  int _maxSpecificity;
+
   ComplexSelector(Iterable<ComplexSelectorComponent> components,
       {Iterable<int> lineBreaks})
       : components = new List.unmodifiable(components),
         lineBreaks = new List.unmodifiable(lineBreaks);
 
+  void _computeSpecificity() {
+    _minSpecificity = 0;
+    _maxSpecificity = 0;
+    for (var component in components) {
+      if (component is CompoundSelector) {
+        _minSpecificity += component.minSpecificity;
+        _maxSpecificity += component.maxSpecificity;
+      }
+    }
+  }
+
   String toString() => components.join(" ");
 }
 

lib/src/ast/selector/compound.dart

@@ -7,6 +7,18 @@ import '../selector.dart';
 class CompoundSelector extends Selector implements ComplexSelectorComponent {
   final List<SimpleSelector> components;
 
+  int get minSpecificity {
+    if (_minSpecificity == null) _computeSpecificity();
+    return _minSpecificity;
+  }
+  int _minSpecificity;
+
+  int get maxSpecificity {
+    if (_maxSpecificity == null) _computeSpecificity();
+    return _maxSpecificity;
+  }
+  int _maxSpecificity;
+
   CompoundSelector(Iterable<SimpleSelector> components)
       : components = new List.unmodifiable(components);
 
@@ -15,5 +27,14 @@ class CompoundSelector extends Selector implements ComplexSelectorComponent {
 
   bool isSuperselector(CompoundSelector selector);
 
+  void _computeSpecificity() {
+    _minSpecificity = 0;
+    _maxSpecificity = 0;
+    for (var simple in components) {
+      _minSpecificity += simple.minSpecificity;
+      _maxSpecificity += simple.maxSpecificity;
+    }
+  }
+
   String toString() => components.join("");
 }

lib/src/ast/selector/id.dart

@@ -2,11 +2,15 @@
 // MIT-style license that can be found in the LICENSE file or at
 // https://opensource.org/licenses/MIT.
 
+import 'dart:math' as math;
+
 import '../selector.dart';
 
 class IDSelector extends SimpleSelector {
   final String name;
 
+  int get minSpecificity => math.pow(super.minSpecificity, 2);
+
   IDSelector(this.name);
 
   List<SimpleSelector> unify(List<SimpleSelector> compound) {

lib/src/ast/selector/pseudo.dart

@@ -2,6 +2,8 @@
 // MIT-style license that can be found in the LICENSE file or at
 // https://opensource.org/licenses/MIT.
 
+import 'dart:math' as math;
+
 import 'package:charcode/charcode.dart';
 
 import '../selector.dart';
@@ -15,6 +17,18 @@ class PseudoSelector extends SimpleSelector {
 
   final SelectorList selector; 
 
+  int get minSpecificity {
+    if (_minSpecificity == null) _computeSpecificity();
+    return _minSpecificity;
+  }
+  int _minSpecificity;
+
+  int get maxSpecificity {
+    if (_maxSpecificity == null) _computeSpecificity();
+    return _maxSpecificity;
+  }
+  int _maxSpecificity;
+
   PseudoSelector(this.name, this.type, {this.argument, this.selector});
 
   List<SimpleSelector> unify(List<SimpleSelector> compound) {
@@ -40,6 +54,36 @@ class PseudoSelector extends SimpleSelector {
     return result;
   }
 
+  void _computeSpecificity() {
+    if (type == PseudoType.element) {
+      _minSpecificity = 1;
+      _maxSpecificity = 1;
+      return;
+    }
+
+    if (selector == null) {
+      _minSpecificity = super.minSpecificity;
+      _maxSpecificity = super.maxSpecificity;
+    }
+
+    if (name == 'not') {
+      _minSpecificity = 0;
+      _maxSpecificity = 0;
+      for (var complex in selector.members) {
+        _minSpecificity = math.max(_minSpecificity, complex.minSpecificity);
+        _maxSpecificity = math.max(_maxSpecificity, complex.maxSpecificity);
+      }
+    } else {
+      // This is higher than any selector's specificity can actually be.
+      _minSpecificity = math.pow(super.minSpecificity, 3);
+      _maxSpecificity = 0;
+      for (var complex in selector.members) {
+        _minSpecificity = math.min(_minSpecificity, complex.minSpecificity);
+        _maxSpecificity = math.max(_maxSpecificity, complex.maxSpecificity);
+      }
+    }
+  }
+
   // This intentionally uses identity for the selector list, if one is available.
   bool operator==(other) =>
       other is PseudoSelector &&

lib/src/ast/selector/simple.dart

@@ -5,6 +5,13 @@
 import '../selector.dart';
 
 abstract class SimpleSelector extends Selector {
+  // 1000 is the base used for calculating selector specificity.
+  //
+  // The spec says this should be "sufficiently high"; it's extremely unlikely
+  // that any single selector sequence will contain 1,000 simple selectors.
+  int get minSpecificity => 1000;
+  int get maxSpecificity => minSpecificity;
+
   List<SimpleSelector> unify(List<SimpleSelector> compound) {
     if (compound.contains(this)) return compound;
 

lib/src/ast/selector/type.dart

@@ -7,6 +7,8 @@ import '../selector.dart';
 class TypeSelector extends SimpleSelector {
   final NamespacedIdentifier name;
 
+  int get minSpecificity => 1;
+
   TypeSelector(this.name);
 
   List<SimpleSelector> unify(List<SimpleSelector> compound) {

lib/src/ast/selector/universal.dart

@@ -8,6 +8,8 @@ import '../selector.dart';
 class UniversalSelector extends SimpleSelector {
   final String namespace;
 
+  int get minSpecificity => 0;
+
   UniversalSelector({this.namespace});
 
   List<SimpleSelector> unify(List<SimpleSelector> compound) {