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Closure Compiler is a JavaScript optimizing compiler. It parses your JavaScript, analyzes it, removes dead code and rewrites and minimizes what's left. It also checks syntax, variable references, and types, and warns about common JavaScript pitfalls. It is used in many of Google's JavaScript apps, including Gmail, Google Web Search, Google Maps, and Google Docs.

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/*
 * Copyright 2007 The Closure Compiler Authors.
 *
 * 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.javascript.jscomp;

import static com.google.javascript.jscomp.SourceExcerptProvider.SourceExcerpt.LINE;

import com.google.common.base.Preconditions;
import com.google.javascript.jscomp.CheckLevel;
import com.google.javascript.jscomp.SourceExcerptProvider.ExcerptFormatter;
import com.google.javascript.jscomp.SourceExcerptProvider.SourceExcerpt;

/**
 * Lightweight message formatter. The format of messages this formatter
 * produces is very compact and to the point.
 *
 */
public class LightweightMessageFormatter extends AbstractMessageFormatter {
  private SourceExcerpt excerpt;
  private static final ExcerptFormatter excerptFormatter =
      new LineNumberingFormatter();

  /**
   * A constructor for when the client doesn't care about source information.
   */
  private LightweightMessageFormatter() {
    super(null);
    this.excerpt = LINE;
  }

  public LightweightMessageFormatter(SourceExcerptProvider source) {
    this(source, LINE);
  }

  public LightweightMessageFormatter(SourceExcerptProvider source,
      SourceExcerpt excerpt) {
    super(source);
    Preconditions.checkNotNull(source);
    this.excerpt = excerpt;
  }

  static LightweightMessageFormatter withoutSource() {
    return new LightweightMessageFormatter();
  }

  public String formatError(JSError error) {
    return format(error, false);
  }

  public String formatWarning(JSError warning) {
    return format(warning, true);
  }

  private String format(JSError error, boolean warning) {
    // extract source excerpt
    SourceExcerptProvider source = getSource();
    String sourceExcerpt = source == null ? null :
        excerpt.get(
            source, error.sourceName, error.lineNumber, excerptFormatter);

    // formatting the message
    StringBuilder b = new StringBuilder();
    if (error.sourceName != null) {
      b.append(error.sourceName);
      if (error.lineNumber > 0) {
        b.append(':');
        b.append(error.lineNumber);
      }
      b.append(": ");
    }

    b.append(getLevelName(warning ? CheckLevel.WARNING : CheckLevel.ERROR));
    b.append(" - ");

    b.append(error.description);
    b.append('\n');
    if (sourceExcerpt != null) {
      b.append(sourceExcerpt);
      b.append('\n');
      int charno = error.getCharno();

      // padding equal to the excerpt and arrow at the end
      if (excerpt.equals(LINE)
          && 0 <= charno && charno < sourceExcerpt.length()) {
        for (int i = 0; i < charno; i++) {
          char c = sourceExcerpt.charAt(i);
          if (Character.isWhitespace(c)) {
            b.append(c);
          } else {
            b.append(' ');
          }
        }
        b.append("^\n");
      }
    }
    return b.toString();
  }

  /**
   * Formats a region by appending line numbers in front, e.g.
   *    9| if (foo) {
   *   10|   alert('bar');
   *   11| }
   * and return line excerpt without any modification.
   */
  static class LineNumberingFormatter implements ExcerptFormatter {
    public String formatLine(String line, int lineNumber) {
      return line;
    }

    public String formatRegion(Region region) {
      if (region == null) {
        return null;
      }
      String code = region.getSourceExcerpt();
      if (code.length() == 0) {
        return null;
      }

      // max length of the number display
      int numberLength = Integer.toString(region.getEndingLineNumber())
          .length();

      // formatting
      StringBuilder builder = new StringBuilder(code.length() * 2);
      int start = 0;
      int end = code.indexOf('\n', start);
      int lineNumber = region.getBeginningLineNumber();
      while (start >= 0) {
        // line extraction
        String line;
        if (end < 0) {
          line = code.substring(start);
          if (line.length() == 0) {
            return builder.substring(0, builder.length() - 1);
          }
        } else {
          line = code.substring(start, end);
        }
        builder.append("  ");

        // nice spaces for the line number
        int spaces = numberLength - Integer.toString(lineNumber).length();
        builder.append(Strings.repeat(" ", spaces));
        builder.append(lineNumber);
        builder.append("| ");

        // end & update
        if (end < 0) {
          builder.append(line);
          start = -1;
        } else {
          builder.append(line);
          builder.append('\n');
          start = end + 1;
          end = code.indexOf('\n', start);
          lineNumber++;
        }
      }
      return builder.toString();
    }
  }
}