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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 2010 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.annotations.VisibleForTesting;
import com.google.common.collect.Lists;
import com.google.common.css.compiler.ast.*;

import java.util.List;

/**
 * Check shorthand rule declarations that define positional values,
 * such as padding and margin, and eliminate duplicate values if possible.
 * For example, "margin: 1px 2px 3px 2px" will be shortened to just
 * "margin: 1px 2px 3px", since the final 2px is redundant.
 * 
 * Note: At present, this pass applies to border-width, but not border.
 *
 * @see Property#hasPositionalParameters()
 */
public class AbbreviatePositionalValues extends DefaultTreeVisitor
        implements CssCompilerPass {

    private final MutatingVisitController visitController;

    public AbbreviatePositionalValues(
            MutatingVisitController visitController) {
        this.visitController = visitController;
    }

    @Override
    public boolean enterDeclaration(CssDeclarationNode declaration) {
        Property property = declaration.getPropertyName().getProperty();
        if (property.hasPositionalParameters()) {
            CssPropertyValueNode valueNode = declaration.getPropertyValue();
            List newValues = abbreviateValues(valueNode.getChildren());
            if (newValues != null) {
                CssDeclarationNode newDeclaration = new CssDeclarationNode(declaration);
                CssPropertyValueNode newValuesNode = new CssPropertyValueNode(newValues);
                newDeclaration.setSourceCodeLocation(declaration.getSourceCodeLocation());
                newValuesNode.setSourceCodeLocation(valueNode.getSourceCodeLocation());
                newDeclaration.setPropertyValue(newValuesNode);
                List replacementList = Lists.newArrayList();
                replacementList.add(newDeclaration);
                visitController.replaceCurrentBlockChildWith(replacementList, false);
            }
        }
        return true;
    }

    /**
     * Attempt to shorten a CSS positional list containing values for
     * -top, -right, -bottom, and -left properties of a shorthand rule.
     * Returns null if the list is not shortenable.
     *
     * @param values List of values.
     * @return A shortened version of the input list, or null if not possible
     * to abbreviate.  The returned list is a "shallow" copy.
     */
    private List abbreviateValues(List values) {
        int numValues = values.size();
        if (numValues <= 1 || numValues > 4) {
            // Already abbreviated, or unexpected input.
            return null;
        }

        List mutableList = Lists.newArrayList(values);

        if (numValues == 4) {
            // Compare foo-left to foo-right.
            if (equalValues(values.get(3), values.get(1))) {
                numValues--;
                mutableList.remove(3);
            }
        }
        if (numValues == 3) {
            // Compare foo-bottom to foo-top.
            if (equalValues(values.get(2), values.get(0))) {
                numValues--;
                mutableList.remove(2);
            }
        }
        if (numValues == 2) {
            // Compare foo-{right,left} to foo-{top,bottom}.
            if (equalValues(values.get(1), values.get(0))) {
                numValues--;
                mutableList.remove(1);
            }
        }

        return numValues != values.size() ? mutableList : null;
    }

    /**
     * Compare 2 value nodes to see if they represent the same value for the
     * purposes of this compiler pass.  See {@link CssNode#equals} for an
     * explanation of why this is not defined elsewhere.
     *
     * @param v1 First value node.
     * @param v2 Second value node.
     * @return Whether the values are equivalent.
     */
    @VisibleForTesting
    static boolean equalValues(CssValueNode v1, CssValueNode v2) {
        if (v1.equals(v2)) {
            return true;
        }
        if (v1 instanceof CssNumericNode && v2 instanceof CssNumericNode) {
            CssNumericNode numeric1 = (CssNumericNode) v1;
            CssNumericNode numeric2 = (CssNumericNode) v2;
            return numeric1.getNumericPart().equals(numeric2.getNumericPart()) &&
                    numeric1.getUnit().equals(numeric2.getUnit());
        }
        if (v1 instanceof CssLiteralNode && v2 instanceof CssLiteralNode) {
            return v1.getValue().equals(v2.getValue());
        }
        if (v1 instanceof CssHexColorNode && v2 instanceof CssHexColorNode) {
            CssHexColorNode hex1 = (CssHexColorNode) v1;
            CssHexColorNode hex2 = (CssHexColorNode) v2;
            return hex1.toString().equals(hex2.toString());
        }
        return false;
    }

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