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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 2008 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.css.compiler.ast.CssAtRuleNode.Type;
import com.google.common.css.compiler.ast.*;

import java.util.Set;

/**
 * A compiler pass that eliminates the conditional blocks for which the boolean
 * expression does not evaluate to true.
 */
public class EliminateConditionalNodes extends DefaultTreeVisitor
        implements CssCompilerPass {

    private final MutatingVisitController visitController;

    private final Set trueConditions;

    public EliminateConditionalNodes(MutatingVisitController visitController,
                                     Set trueConditions) {
        this.visitController = visitController;
        this.trueConditions = trueConditions;
    }

    @Override
    public boolean enterConditionalBlock(CssConditionalBlockNode block) {

        for (CssConditionalRuleNode currentCondtional : block.childIterable()) {

            if (currentCondtional.getType() == Type.ELSE) {
                // TODO(dgajda): Optimize by using immutable children list or an
                //     iterable.
                visitController.replaceCurrentBlockChildWith(
                        currentCondtional.getBlock().getChildren(), true);
                return true;
            }

            BooleanExpressionEvaluator evaluator = new BooleanExpressionEvaluator(
                    currentCondtional.getCondition(), trueConditions);
            CssBooleanExpressionNode result = evaluator.evaluate();
            if (CssBooleanExpressionNode.Type.TRUE_CONSTANT.equals(
                    result.getValue())) {
                visitController.replaceCurrentBlockChildWith(
                        currentCondtional.getBlock().getChildren(), true);
                return true;
            }
        }

        // No ELSE was encountered and nothing was true.
        visitController.removeCurrentNode();
        return true;
    }

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