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Closure Stylesheets is an extension to CSS that adds variables, functions, conditionals, and mixins to standard CSS. The tool also supports minification, linting, RTL flipping, and CSS class renaming.
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/*
 * Copyright 2009 Google Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.common.css.compiler.passes;

import com.google.common.base.Preconditions;
import com.google.common.collect.Lists;
import com.google.common.css.compiler.ast.CssAtRuleNode;
import com.google.common.css.compiler.ast.CssBlockNode;
import com.google.common.css.compiler.ast.CssCompilerPass;
import com.google.common.css.compiler.ast.CssComponentNode;
import com.google.common.css.compiler.ast.CssComponentNode.PrefixStyle;
import com.google.common.css.compiler.ast.CssLiteralNode;
import com.google.common.css.compiler.ast.CssNode;
import com.google.common.css.compiler.ast.CssStringNode;
import com.google.common.css.compiler.ast.CssUnknownAtRuleNode;
import com.google.common.css.compiler.ast.CssValueNode;
import com.google.common.css.compiler.ast.DefaultTreeVisitor;
import com.google.common.css.compiler.ast.ErrorManager;
import com.google.common.css.compiler.ast.GssError;
import com.google.common.css.compiler.ast.MutatingVisitController;

import java.util.List;

/**
 * A compiler pass that transforms each well-formed {@code @component} or
 * {@code @abstract_component} {@link CssUnknownAtRuleNode} into a {@link CssComponentNode}.
 *
 * The syntax for components is as follows:
 * 
 * {@literal @}(abstract_)?component LITERAL (extends LITERAL)? { ... }
 * 
 *
 */
public class CreateComponentNodes extends DefaultTreeVisitor
    implements CssCompilerPass {

  private static final String componentName = CssAtRuleNode.Type.COMPONENT.getCanonicalName();
  private static final String abstractComponentName =
      CssAtRuleNode.Type.ABSTRACT_COMPONENT.getCanonicalName();

  private final MutatingVisitController visitController;
  private final ErrorManager errorManager;

  public CreateComponentNodes(MutatingVisitController visitController, ErrorManager errorManager) {
    this.visitController = visitController;
    this.errorManager = errorManager;
  }

  @Override
  public void leaveUnknownAtRule(CssUnknownAtRuleNode node) {
    String name = node.getName().getValue();
    if (name.equals(componentName) || name.equals(abstractComponentName)) {
      if (!node.getType().hasBlock()) {
        reportError("@" + name + " without block", node);
        return;
      }
      List params = node.getParameters();
      CssNode nameNode;
      CssLiteralNode parentNode = null;
      int paramSize = params.size();
      CssComponentNode.PrefixStyle prefixStyle = PrefixStyle.LITERAL;
      if (paramSize == 0) {
        // Use a sentinel value in the name field to indicate that the component name
        // is implicit, and should be derived from the package name.
        prefixStyle = PrefixStyle.CASE_CONVERT;
        nameNode = new CssLiteralNode(
            CssComponentNode.IMPLICIT_NODE_NAME, node.getSourceCodeLocation());
      } else {
        nameNode = params.get(0);
        if (nameNode instanceof CssStringNode) {
          // CssValueNodes require that the name be a literal node, so if it's a
          // string convert it into a literal.
          prefixStyle = PrefixStyle.CASE_CONVERT;
          nameNode = new CssLiteralNode(((CssStringNode) nameNode).getValue(),
              nameNode.getSourceCodeLocation());
        } else if (!(nameNode instanceof CssLiteralNode)) {
          reportError("@" + name + " without a valid literal as name", node);
          return;
        }
        if (paramSize == 1) {
          // OK
        } else if (paramSize == 3) {
          CssNode extendNode = params.get(1);
          if (!(extendNode instanceof CssLiteralNode)
              || !((CssLiteralNode) extendNode).getValue().equals("extends")) {
            reportError("@" + name + " with invalid second parameter (expects 'extends')", node);
            return;
          }
          CssNode parentCssNode = params.get(2);
          if (!(parentCssNode instanceof CssLiteralNode)) {
            reportError("@" + name + " with invalid literal as parent name", node);
            return;
          }
          parentNode = (CssLiteralNode) parentCssNode;
        } else {
          reportError("@" + name + " with invalid number of parameters", node);
          return;
        }
      }
      Preconditions.checkState(node.getBlock() instanceof CssBlockNode);
      CssComponentNode comp = new CssComponentNode(
          (CssLiteralNode) nameNode,
          parentNode,
          name.equals(abstractComponentName),
          prefixStyle,
          (CssBlockNode) node.getBlock());
      comp.setComments(node.getComments());
      comp.setSourceCodeLocation(node.getSourceCodeLocation());
      visitController.replaceCurrentBlockChildWith(Lists.newArrayList((CssNode) comp), false);
    }
  }

  private void reportError(String message, CssNode node) {
    errorManager.report(new GssError(message, node.getSourceCodeLocation()));
    visitController.removeCurrentNode();
  }

  @Override
  public void runPass() {
    visitController.startVisit(this);
  }
}