user/src/com/google/gwt/resources/gss/CssPrinter.java

/*
 * Copyright 2014 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.gwt.resources.gss;

import com.google.gwt.core.ext.Generator;
import com.google.gwt.resources.gss.ast.CssDotPathNode;
import com.google.gwt.resources.gss.ast.CssJavaExpressionNode;
import com.google.gwt.resources.gss.ast.CssRuntimeConditionalRuleNode;
import com.google.gwt.thirdparty.common.css.compiler.ast.CssAtRuleNode.Type;
import com.google.gwt.thirdparty.common.css.compiler.ast.CssConditionalBlockNode;
import com.google.gwt.thirdparty.common.css.compiler.ast.CssConditionalRuleNode;
import com.google.gwt.thirdparty.common.css.compiler.ast.CssNode;
import com.google.gwt.thirdparty.common.css.compiler.ast.CssRootNode;
import com.google.gwt.thirdparty.common.css.compiler.ast.CssTree;
import com.google.gwt.thirdparty.common.css.compiler.ast.CssValueNode;
import com.google.gwt.thirdparty.common.css.compiler.passes.CompactPrinter;

import java.util.Stack;

/**
 * Visitor that converts the AST to a {@code String} that can be evaluated as a Java expression.
 *
 * <p>For example, the following GSS code
 * <pre>
 *   @if(eval("com.foo.bar()")) {
 *     .foo {
 *       padding: 5px;
 *     }
 *   }
 *   {@literal @}else {
 *     .foo {
 *       padding: 15px;
 *     }
 *   }
 *   .bar {
 *     width:10px;
 *   }
 * }
 * </pre>
 * will be translated to
 * {@code "(com.foo.bar() ? (\".foo{padding:5px}\") : (\".foo{padding:15px}\")) + (\".bar{width:10px}\")"}
 */
public class CssPrinter extends CompactPrinter {
  /**
   * This value is used by {@link #concat} to help create a more balanced AST
   * tree by producing parenthetical expressions.
   */
  private static final int CONCAT_EXPRESSION_LIMIT = 20;
  private static final String CONTATENATION = " + ";
  private static final String COLON = " : ";
  private static final String LEFT_PARENTHESIS = "(";
  private static final String RIGHT_PARENTHESIS = ")";
  private static final String DOUBLE_QUOTE = "\"";
  private static final String CONTATENATION_BLOCK = ") + (";
  private static final String CONDITIONAL_OPERATOR = ") ? (";

  private final Stack<Boolean> elseNodeFound = new Stack<Boolean>();

  private StringBuilder masterStringBuilder;
  private String css;
  private int concatenationNumber;

  public CssPrinter(CssTree tree) {
    super(tree);
  }

  public CssPrinter(CssNode node) {
    super(node);
  }

  @Override
  public boolean enterTree(CssRootNode root) {
    masterStringBuilder.append(LEFT_PARENTHESIS);
    return super.enterTree(root);
  }

  @Override
  public String getCompactPrintedString() {
    return css;
  }

  @Override
  public void leaveTree(CssRootNode root) {
    masterStringBuilder.append(flushInternalStringBuilder()).append(RIGHT_PARENTHESIS);
    super.leaveTree(root);
  }

  @Override
  public void runPass() {
    masterStringBuilder = new StringBuilder();
    concatenationNumber = 0;

    super.runPass();

    css = masterStringBuilder
        .toString()
        // remove empty string concatenation : '+ ("")'
        .replaceAll(" \\+ \\(\"\"\\)", "")
        // remove possible empty string concatenation '("") + ' at the  beginning
        .replaceAll("^\\(\"\"\\) \\+ ", "");
  }

  @Override
  public boolean enterConditionalBlock(CssConditionalBlockNode node) {
    masterStringBuilder.append(flushInternalStringBuilder());

    masterStringBuilder.append(CONTATENATION_BLOCK);

    elseNodeFound.push(false);

    return true;
  }

  @Override
  public void leaveConditionalBlock(CssConditionalBlockNode block) {
    if (!elseNodeFound.pop()) {
      masterStringBuilder.append(DOUBLE_QUOTE).append(DOUBLE_QUOTE);
    }
    masterStringBuilder.append(CONTATENATION_BLOCK);

    // Reset concatenation counter
    concatenationNumber = 0;
  }

  @Override
  public boolean enterConditionalRule(CssConditionalRuleNode node) {
    if (node.getType() == Type.ELSE) {
      elseNodeFound.pop();
      elseNodeFound.push(true);

      masterStringBuilder.append(LEFT_PARENTHESIS);
    } else {
      CssRuntimeConditionalRuleNode conditionalRuleNode = (CssRuntimeConditionalRuleNode) node;

      masterStringBuilder.append(LEFT_PARENTHESIS);
      masterStringBuilder.append(conditionalRuleNode.getRuntimeCondition().getValue());
      masterStringBuilder.append(CONDITIONAL_OPERATOR);

      // Reset concatenation counter
      concatenationNumber = 0;
    }

    return true;
  }

  @Override
  public void leaveConditionalRule(CssConditionalRuleNode node) {
    masterStringBuilder.append(flushInternalStringBuilder()).append(RIGHT_PARENTHESIS);

    if (node.getType() != Type.ELSE) {
      masterStringBuilder.append(COLON);
    }
  }

  @Override
  protected void appendValueNode(CssValueNode node) {
    if (node instanceof CssJavaExpressionNode || node instanceof CssDotPathNode) {
      concat(LEFT_PARENTHESIS + node.getValue() + RIGHT_PARENTHESIS);
    } else {
      super.appendValueNode(node);
    }
  }

  private void concat(String stringToAppend) {
    masterStringBuilder.append(flushInternalStringBuilder());

    appendConcatOperation();

    masterStringBuilder.append(stringToAppend);

    appendConcatOperation();
  }

  private void appendConcatOperation() {
    // Avoid long string concatenation chain
    if (concatenationNumber >= CONCAT_EXPRESSION_LIMIT) {
      masterStringBuilder.append(CONTATENATION_BLOCK);
      concatenationNumber = 0;
    } else {
      masterStringBuilder.append(CONTATENATION);
      concatenationNumber++;
    }
  }

  /**
   * Read what the internal StringBuilder used by the CompactPrinter has already built. Escape it.
   * and reset the internal StringBuilder
   *
   * @return
   */
  private String flushInternalStringBuilder() {
    // NOTE(flan): Note that you have to be careful where you do this. Internally,
    // the compact printer sometimes deletes characters from the end of the stringBuilder to save
    // space. I believe that you'll be safe because, if there's nothing in the buffer, there is
    // nothing to delete, but you may have some unnecessary characters in the output. you may
    // want to call that out explicitly in the code.
    String content = DOUBLE_QUOTE + Generator.escape(getOutputBuffer()) + DOUBLE_QUOTE;
    resetBuffer();

    return content;
  }
}