Make CssNodes directly mutable.

This shaves off a significant amount of time, since now we don't have to
do what is essentially a full copy of the CSS tree. We may want to use
interfaces to provide an immutable view of the CSS tree for use outside
of the perform visitor.

lib/src/ast/css/at_rule.dart

@@ -5,29 +5,21 @@
 import 'package:source_span/source_span.dart';
 
 import '../../visitor/css.dart';
-import '../parent.dart';
 import 'node.dart';
 import 'value.dart';
 
-class CssAtRule implements CssNode, Parent<CssNode, CssAtRule> {
+class CssAtRule extends CssParentNode {
   final String name;
 
   final CssValue<String> value;
 
-  final List<CssNode> children;
-
   final FileSpan span;
 
-  // TODO: validate that children contains only at-rule and declaration nodes?
-  CssAtRule(this.name, {this.value, Iterable<CssNode> children, this.span})
-      : children = children == null ? null : new List.unmodifiable(children);
+  CssAtRule(this.name, {this.value, this.span});
 
   /*=T*/ accept/*<T>*/(CssVisitor/*<T>*/ visitor) =>
       visitor.visitAtRule(this);
 
-  CssAtRule withChildren(Iterable<CssNode> children) =>
-      new CssAtRule(name, value: value, children: children, span: span);
-
   String toString() {
     var buffer = new StringBuffer("@$name");
     if (value != null) buffer.write(" $value");

lib/src/ast/css/comment.dart

@@ -7,7 +7,7 @@ import 'package:source_span/source_span.dart';
 import '../../visitor/css.dart';
 import 'node.dart';
 
-class CssComment implements CssNode {
+class CssComment extends CssNode {
   final String text;
 
   final FileSpan span;

lib/src/ast/css/declaration.dart

@@ -9,7 +9,7 @@ import '../../visitor/css.dart';
 import 'node.dart';
 import 'value.dart';
 
-class CssDeclaration implements CssNode {
+class CssDeclaration extends CssNode {
   final CssValue<String> name;
 
   final CssValue<Value> value;

lib/src/ast/css/media_rule.dart

@@ -5,27 +5,18 @@
 import 'package:source_span/source_span.dart';
 
 import '../../visitor/css.dart';
-import '../parent.dart';
 import 'node.dart';
 
-class CssMediaRule implements CssNode, Parent<CssNode, CssMediaRule> {
+class CssMediaRule extends CssParentNode {
   final List<CssMediaQuery> queries;
 
-  final List<CssNode> children;
-
   final FileSpan span;
 
   // TODO: validate that children contains only at-rule and style rule nodes?
-  CssMediaRule(Iterable<CssMediaQuery> queries, Iterable<CssNode> children,
-      {this.span})
-      : queries = new List.unmodifiable(queries),
-        children = new List.unmodifiable(children);
+  CssMediaRule(this.queries, {this.span});
 
   /*=T*/ accept/*<T>*/(CssVisitor/*<T>*/ visitor) =>
       visitor.visitMediaRule(this);
 
-  CssMediaRule withChildren(Iterable<CssNode> children) =>
-      new CssMediaRule(queries, children, span: span);
-
   String toString() => "@media ${queries.join(", ")} {${children.join(" ")}}";
 }

lib/src/ast/css/node.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:collection';
+
 import '../../visitor/css.dart';
 import '../node.dart';
 
@@ -15,5 +17,39 @@ export 'stylesheet.dart';
 export 'value.dart';
 
 abstract class CssNode extends AstNode {
+  CssParentNode get parent => _parent;
+  CssParentNode _parent;
+
+  int _indexInParent;
+
   /*=T*/ accept/*<T>*/(CssVisitor/*<T>*/ visitor);
+
+  void remove() {
+    if (_parent == null) {
+      throw new StateError("Can't remove a node without a parent.");
+    }
+
+    _parent._children.removeAt(_indexInParent);
+    for (var i = _indexInParent; i < _parent._children.length; i++) {
+      _parent._children[i]._indexInParent--;
+    }
+  }
+}
+
+abstract class CssParentNode extends CssNode {
+  final List<CssNode> children;
+  final List<CssNode> _children;
+
+  CssParentNode() : this._([]);
+
+  CssParentNode._(List<CssNode> children)
+      : _children = children,
+        children = new UnmodifiableListView<CssNode>(children);
+
+  void addChild(CssNode child) {
+    // TODO: validate that children are valid?
+    child._parent = this;
+    child._indexInParent = _children.length;
+    _children.add(child);
+  }
 }

lib/src/ast/css/style_rule.dart

@@ -5,27 +5,19 @@
 import 'package:source_span/source_span.dart';
 
 import '../../visitor/css.dart';
-import '../parent.dart';
 import '../selector.dart';
 import 'node.dart';
 import 'value.dart';
 
-class CssStyleRule implements CssNode, Parent<CssNode, CssStyleRule> {
+class CssStyleRule extends CssParentNode {
   final CssValue<SelectorList> selector;
 
-  final List<CssNode> children;
-
   final FileSpan span;
 
-  // TODO: validate that children contains only at-rule and declaration nodes?
-  CssStyleRule(this.selector, Iterable<CssNode> children, {this.span})
-      : children = new List.unmodifiable(children);
+  CssStyleRule(this.selector, {this.span});
 
   /*=T*/ accept/*<T>*/(CssVisitor/*<T>*/ visitor) =>
       visitor.visitStyleRule(this);
 
-  CssStyleRule withChildren(Iterable<CssNode> children) =>
-      new CssStyleRule(selector, children, span: span);
-
   String toString() => "$selector {${children.join(" ")}}";
 }

lib/src/ast/css/stylesheet.dart

@@ -5,22 +5,15 @@
 import 'package:source_span/source_span.dart';
 
 import '../../visitor/css.dart';
-import '../parent.dart';
 import 'node.dart';
 
-class CssStylesheet implements CssNode, Parent<CssNode, CssStylesheet> {
-  final List<CssNode> children;
-
+class CssStylesheet extends CssParentNode {
   final FileSpan span;
 
-  CssStylesheet(Iterable<CssNode> children, {this.span})
-      : children = new List.unmodifiable(children);
+  CssStylesheet({this.span});
 
   /*=T*/ accept/*<T>*/(CssVisitor/*<T>*/ visitor) =>
       visitor.visitStylesheet(this);
 
-  CssStylesheet withChildren(Iterable<CssNode> children) =>
-      new CssStylesheet(children, span: span);
-
   String toString() => children.join(" ");
 }

lib/src/ast/css/value.dart

@@ -4,17 +4,14 @@
 
 import 'package:source_span/source_span.dart';
 
-import '../../visitor/css.dart';
-import 'node.dart';
+import '../node.dart';
 
-class CssValue<T> implements CssNode {
+class CssValue<T> implements AstNode {
   final T value;
 
   final FileSpan span;
 
   CssValue(this.value, {this.span});
 
-  /*=T*/ accept/*<T>*/(CssVisitor/*<T>*/ visitor) => visitor.visitValue(this);
-
   String toString() => value.toString();
 }

lib/src/ast/parent.dart

@@ -1,11 +0,0 @@
-// Copyright 2016 Google Inc. Use of this source code is governed by an
-// MIT-style license that can be found in the LICENSE file or at
-// https://opensource.org/licenses/MIT.
-
-import 'node.dart';
-
-abstract class Parent<T extends AstNode, N extends T> extends AstNode {
-  List<T> get children;
-
-  N withChildren(Iterable<T> children);
-}

lib/src/ast/sass/at_rule.dart

@@ -5,11 +5,10 @@
 import 'package:source_span/source_span.dart';
 
 import '../../visitor/sass/statement.dart';
-import '../parent.dart';
 import 'expression/interpolation.dart';
 import 'statement.dart';
 
-class AtRule implements Statement, Parent<Statement, AtRule> {
+class AtRule implements Statement {
   final String name;
 
   final InterpolationExpression value;
@@ -26,9 +25,6 @@ class AtRule implements Statement, Parent<Statement, AtRule> {
   /*=T*/ accept/*<T>*/(StatementVisitor/*<T>*/ visitor) =>
       visitor.visitAtRule(this);
 
-  AtRule withChildren(Iterable<Statement> children) =>
-      new AtRule(name, value: value, children: children, span: span);
-
   String toString() {
     var buffer = new StringBuffer("@$name");
     if (value != null) buffer.write(" $value");

lib/src/ast/sass/media_rule.dart

@@ -5,10 +5,9 @@
 import 'package:source_span/source_span.dart';
 
 import '../../visitor/sass/statement.dart';
-import '../parent.dart';
 import 'statement.dart';
 
-class MediaRule implements Statement, Parent<Statement, MediaRule> {
+class MediaRule implements Statement {
   final List<MediaQuery> queries;
 
   final List<Statement> children;
@@ -23,8 +22,5 @@ class MediaRule implements Statement, Parent<Statement, MediaRule> {
   /*=T*/ accept/*<T>*/(StatementVisitor/*<T>*/ visitor) =>
       visitor.visitMediaRule(this);
 
-  MediaRule withChildren(Iterable<Statement> children) =>
-      new MediaRule(queries, children, span: span);
-
   String toString() => "@media ${queries.join(", ")} {${children.join(" ")}}";
 }

lib/src/ast/sass/style_rule.dart

@@ -5,11 +5,10 @@
 import 'package:source_span/source_span.dart';
 
 import '../../visitor/sass/statement.dart';
-import '../parent.dart';
 import 'expression/interpolation.dart';
 import 'statement.dart';
 
-class StyleRule implements Statement, Parent<Statement, StyleRule> {
+class StyleRule implements Statement {
   final InterpolationExpression selector;
 
   final List<Statement> children;
@@ -24,8 +23,5 @@ class StyleRule implements Statement, Parent<Statement, StyleRule> {
   /*=T*/ accept/*<T>*/(StatementVisitor/*<T>*/ visitor) =>
       visitor.visitStyleRule(this);
 
-  StyleRule withChildren(Iterable<Statement> children) =>
-      new StyleRule(selector, children, span: span);
-
   String toString() => "$selector {${children.join(" ")}}";
 }

lib/src/ast/sass/stylesheet.dart

@@ -5,10 +5,9 @@
 import 'package:source_span/source_span.dart';
 
 import '../../visitor/sass/statement.dart';
-import '../parent.dart';
 import 'statement.dart';
 
-class Stylesheet implements Statement, Parent<Statement, Stylesheet> {
+class Stylesheet implements Statement {
   final List<Statement> children;
 
   final FileSpan span;
@@ -19,8 +18,5 @@ class Stylesheet implements Statement, Parent<Statement, Stylesheet> {
   /*=T*/ accept/*<T>*/(StatementVisitor/*<T>*/ visitor) =>
       visitor.visitStylesheet(this);
 
-  Stylesheet withChildren(Iterable<Statement> children) =>
-      new Stylesheet(children, span: span);
-
   String toString() => children.join(" ");
 }

lib/src/mutable_node.dart

@@ -1,66 +0,0 @@
-// Copyright 2016 Google Inc. Use of this source code is governed by an
-// MIT-style license that can be found in the LICENSE file or at
-// https://opensource.org/licenses/MIT.
-
-import 'dart:collection';
-
-import 'ast/node.dart';
-import 'ast/parent.dart';
-import 'utils.dart';
-
-class MutableNode<T extends AstNode, N extends T> {
-  N get node => _node;
-
-  void set node(N value) {
-    _node = value;
-    _checkNode();
-  }
-
-  N _node;
-
-  final MutableNode<T, T> parent;
-
-  LinkedListValue<MutableNode<T, N>> _linkedListValue;
-
-  Iterable<MutableNode<T, T>> get children =>
-      _children.map((value) => value.value);
-  final _children = new LinkedList<LinkedListValue<MutableNode<T, T>>>();
-
-  MutableNode.root(this._node) : parent = null {
-    _checkNode();
-  }
-
-  MutableNode._(this.parent, this._node) {
-    _checkNode();
-  }
-
-  MutableNode<T, T/*=S*/> add/*<S extends T>*/(T/*=S*/ child) {
-    var node = new MutableNode<T, T/*=S*/>._(this, child);
-    node._linkedListValue = new LinkedListValue(node);
-    _children.add(node._linkedListValue);
-    return node;
-  }
-
-  void remove() {
-    _linkedListValue.unlink();
-  }
-
-  void _checkNode() {
-    if (node is! Parent) return;
-    var parent = node as Parent;
-    if (parent.children == null) return;
-    if (parent.children.isEmpty) return;
-
-    throw new StateError("A mutable node can't have immutable children.");
-  }
-
-  N build() {
-    if (children.isEmpty) return node;
-    if (node is Parent<T, N>) {
-      return (node as Parent<T, N>)
-          .withChildren(children.map((entry) => entry.build()));
-    }
-
-    throw new StateError("Non-Parent $node may not have children.");
-  }
-}

lib/src/visitor/css.dart

@@ -3,7 +3,6 @@
 // https://opensource.org/licenses/MIT.
 
 import '../ast/css/node.dart';
-import '../value.dart';
 import 'value.dart';
 
 abstract class CssVisitor<T> extends ValueVisitor<T> {
@@ -38,6 +37,4 @@ abstract class CssVisitor<T> extends ValueVisitor<T> {
     }
     return null;
   }
-
-  T visitValue(CssValue node) => node is Value ? node.accept(this) : null;
 }

lib/src/visitor/css/serialize.dart

@@ -8,6 +8,7 @@ import '../../ast/css/node.dart';
 import '../../util/character.dart';
 import '../../value.dart';
 import '../css.dart';
+import '../../parser.dart';
 
 String toCss(CssNode node) {
   var visitor = new _SerializeCssVisitor();

lib/src/visitor/sass/statement/perform.dart

@@ -7,7 +7,6 @@ import '../../../ast/sass/expression.dart';
 import '../../../ast/sass/statement.dart';
 import '../../../ast/selector.dart';
 import '../../../environment.dart';
-import '../../../mutable_node.dart';
 import '../../../parser.dart';
 import '../../../value.dart';
 import '../expression/perform.dart';
@@ -27,11 +26,11 @@ class PerformVisitor extends StatementVisitor {
   /// This will always have an empty list of children.
   CssMediaRule _mediaRule;
 
-  /// A mutable view of the root stylesheet node.
-  MutableNode<CssNode, CssStylesheet> _root;
+  /// The root stylesheet node.
+  CssStylesheet _root;
 
-  /// A mutable view of the current parent node in the output CSS tree.
-  MutableNode<CssNode, CssNode> _parent;
+  /// The current parent node in the output CSS tree.
+  CssParentNode _parent;
   
   PerformVisitor() : this._(new Environment());
 
@@ -42,16 +41,15 @@ class PerformVisitor extends StatementVisitor {
   void visit(Statement node) => node.accept(this);
 
   CssStylesheet visitStylesheet(Stylesheet node) {
-    _root = new MutableNode<CssNode, CssStylesheet>.root(
-        new CssStylesheet(const [], span: node.span));
+    _root = new CssStylesheet(span: node.span);
     _parent = _root;
     super.visitStylesheet(node);
-    return _root.build();
+    return _root;
   }
 
   void visitComment(Comment node) {
     if (node.isSilent) return;
-    _parent.add(new CssComment(node.text, span: node.span));
+    _addChild(new CssComment(node.text, span: node.span));
   }
 
   void visitDeclaration(Declaration node) {
@@ -66,7 +64,7 @@ class PerformVisitor extends StatementVisitor {
       return;
     }
 
-    _parent.add(new CssDeclaration(name, cssValue, span: node.span));
+    _addChild(new CssDeclaration(name, cssValue, span: node.span));
   }
 
   void visitAtRule(AtRule node) {
@@ -75,7 +73,7 @@ class PerformVisitor extends StatementVisitor {
         : _performInterpolation(node.value, trim: true);
 
     if (node.children == null) {
-      _parent.add(new CssAtRule(node.name, value: value, span: node.span));
+      _addChild(new CssAtRule(node.name, value: value, span: node.span));
     }
 
     _withParent(
@@ -85,24 +83,25 @@ class PerformVisitor extends StatementVisitor {
   }
 
   void visitMediaRule(MediaRule node) {
-    var queries = node.queries.map(_visitMediaQuery);
-    if (_mediaRule != null) {
-      queries = _mergeMediaQueries(_mediaRule.queries, queries);
-    }
+    var queryIterable = node.queries.map(_visitMediaQuery);
+    var queries = _mediaRule == null
+        ? new List<CssMediaQuery>.unmodifiable(queryIterable)
+        : _mergeMediaQueries(_mediaRule.queries, queryIterable);
     if (queries.isEmpty) return;
 
-    var rule = new CssMediaRule(queries, const [], span: node.span);
-    _withParent(rule, () {
-      _withMediaRule(rule, () => super.visitMediaRule(node));
-      if (_parent.children.isEmpty) _parent.remove();
-    }, through: (node) => node is CssStyleRule || node is CssMediaRule);
+    var rule = new CssMediaRule(queries, span: node.span);
+    _withParent(
+        rule,
+        () => _withMediaRule(rule, () => super.visitMediaRule(node)),
+        through: (node) => node is CssStyleRule || node is CssMediaRule,
+        removeIfEmpty: true);
   }
 
   List<CssMediaQuery> _mergeMediaQueries(
       Iterable<CssMediaQuery> queries1, Iterable<CssMediaQuery> queries2) {
-    return queries1.expand/*<CssMediaQuery>*/((query1) {
+    return new List.unmodifiable(queries1.expand/*<CssMediaQuery>*/((query1) {
       return queries2.map((query2) => query1.merge(query2));
-    }).where((query) => query != null).toList();
+    }).where((query) => query != null));
   }
 
   CssMediaQuery _visitMediaQuery(MediaQuery query) {
@@ -136,11 +135,12 @@ class PerformVisitor extends StatementVisitor {
         new Parser(selectorText.value).parseSelector(),
         span: node.selector.span);
 
-    var rule = new CssStyleRule(selector, const [], span: node.span);
-    _withParent(rule, () {
-      _withStyleRule(rule, () => super.visitStyleRule(node));
-      if (_parent.children.isEmpty) _parent.remove();
-    }, through: (node) => node is CssStyleRule);
+    var rule = new CssStyleRule(selector, span: node.span);
+    _withParent(
+        rule,
+        () => _withStyleRule(rule, () => super.visitStyleRule(node)),
+        through: (node) => node is CssStyleRule,
+        removeIfEmpty: true);
   }
 
   void visitVariableDeclaration(VariableDeclaration node) {
@@ -159,20 +159,27 @@ class PerformVisitor extends StatementVisitor {
   CssValue<Value> _performExpression(Expression expression) =>
       new CssValue(expression.accept(_expressionVisitor));
 
-  /*=T*/ _withParent/*<S extends CssNode, T>*/(CssNode/*=S*/ node,
-      /*=T*/ callback(), {bool through(CssNode node)}) {
+  void _addChild(CssNode node) {
+    _parent.addChild(node);
+  }
+
+  /*=T*/ _withParent/*<S extends CssParentNode, T>*/(
+      /*=S*/ node, /*=T*/ callback(),
+      {bool through(CssNode node), bool removeIfEmpty: false}) {
     var oldParent = _parent;
 
     // Go up through parents that match [through].
     var parent = _parent;
     if (through != null) {
-      while (through(parent.node)) {
+      while (through(parent)) {
         parent = parent.parent;
       }
     }
 
-    _parent = parent.add(node);
+    parent.addChild(node);
+    _parent = node;
     var result = _environment.scope(callback);
+    if (removeIfEmpty && node.children.isEmpty) node.remove();
     _parent = oldParent;
 
     return result;