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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.*;
import com.google.common.css.compiler.ast.CssComponentNode.PrefixStyle;

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 COMPONENT_NAME = CssAtRuleNode.Type.COMPONENT.getCanonicalName();
    private static final String ABSTRACT_COMPONENT_NAME =
            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(COMPONENT_NAME) || name.equals(ABSTRACT_COMPONENT_NAME)) {
            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(ABSTRACT_COMPONENT_NAME),
                    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);
    }
}