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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 2017 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 java.lang.Math.max;
import static java.lang.Math.min;
import static java.nio.charset.StandardCharsets.UTF_8;

import com.google.common.annotations.GwtIncompatible;
import com.google.debugging.sourcemap.proto.Mapping.OriginalMapping;
import com.google.gson.stream.JsonWriter;
import com.google.javascript.jscomp.LightweightMessageFormatter.LineNumberingFormatter;
import com.google.javascript.jscomp.SortingErrorManager.ErrorReportGenerator;
import com.google.javascript.jscomp.SortingErrorManager.ErrorWithLevel;
import com.google.javascript.jscomp.SourceExcerptProvider.SourceExcerpt;
import com.google.javascript.jscomp.base.format.SimpleFormat;
import com.google.javascript.rhino.Node;
import com.google.javascript.rhino.TokenUtil;
import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.OutputStreamWriter;
import java.io.PrintStream;

/**
 * An error report generator that prints error and warning data to the print stream as an array of
 * JSON objects.
 */
public class JsonErrorReportGenerator implements ErrorReportGenerator {
  private final PrintStream stream;
  private final SourceExcerptProvider sourceExcerptProvider;
  private static final LineNumberingFormatter excerptFormatter = new LineNumberingFormatter();

  /**
   * Creates an error manager.
   *
   * @param stream the stream on which the errors and warnings should be printed. This class does
   *     not close the stream
   * @param sourceExcerptProvider used to retrieve the source context which generated the error
   */
  public JsonErrorReportGenerator(PrintStream stream, SourceExcerptProvider sourceExcerptProvider) {
    this.stream = stream;
    this.sourceExcerptProvider = sourceExcerptProvider;
  }

  @Override
  @GwtIncompatible
  public void generateReport(SortingErrorManager manager) {
    ByteArrayOutputStream bufferedStream = new ByteArrayOutputStream();
    try (JsonWriter jsonWriter = new JsonWriter(new OutputStreamWriter(bufferedStream, UTF_8))) {
      jsonWriter.beginArray();
      for (ErrorWithLevel message : manager.getSortedDiagnostics()) {
        String sourceName = message.error.getSourceName();
        int lineNumber = message.error.getLineNumber();
        int charno = message.error.getCharno();

        jsonWriter.beginObject();
        jsonWriter.name("level").value(message.level == CheckLevel.ERROR ? "error" : "warning");
        jsonWriter.name("description").value(message.error.getDescription());
        jsonWriter.name("key").value(message.error.getType().key);
        jsonWriter.name("source").value(sourceName);
        jsonWriter.name("line").value(lineNumber);
        jsonWriter.name("column").value(charno);
        Node node = message.error.getNode();
        if (node != null) {
          jsonWriter.name("length").value(node.getLength());
        }

        // extract source excerpt
        String sourceExcerpt =
            SourceExcerpt.LINE.get(sourceExcerptProvider, sourceName, lineNumber, excerptFormatter);
        if (sourceExcerpt != null) {
          StringBuilder b = new StringBuilder(sourceExcerpt);
          b.append("\n");

          // padding equal to the excerpt and arrow at the end
          // charno == sourceExcerpt.length() means something is missing
          // at the end of the line
          if (0 <= charno && charno <= sourceExcerpt.length()) {
            for (int i = 0; i < charno; i++) {
              char c = sourceExcerpt.charAt(i);
              if (TokenUtil.isWhitespace(c)) {
                b.append(c);
              } else {
                b.append(' ');
              }
            }
            if (message.error.getNode() == null) {
              b.append("^");
            } else {
              int length =
                  max(1, min(message.error.getNode().getLength(), sourceExcerpt.length() - charno));
              for (int i = 0; i < length; i++) {
                b.append("^");
              }
            }
          }

          jsonWriter.name("context").value(b.toString());
        }

        OriginalMapping mapping =
            sourceExcerptProvider.getSourceMapping(
                sourceName, message.error.getLineNumber(), message.error.getCharno());

        if (mapping != null) {
          jsonWriter.name("originalLocation").beginObject();
          jsonWriter.name("source").value(mapping.getOriginalFile());
          jsonWriter.name("line").value(mapping.getLineNumber());
          jsonWriter.name("column").value(mapping.getColumnPosition());
          jsonWriter.endObject();
        }

        jsonWriter.endObject();
      }

      StringBuilder summaryBuilder = new StringBuilder();
      if (manager.getTypedPercent() > 0.0) {
        summaryBuilder.append(
            SimpleFormat.format(
                "%d error(s), %d warning(s), %.1f%% typed",
                manager.getErrorCount(), manager.getWarningCount(), manager.getTypedPercent()));
      } else {
        summaryBuilder.append(
            SimpleFormat.format(
                "%d error(s), %d warning(s)", manager.getErrorCount(), manager.getWarningCount()));
      }
      jsonWriter.beginObject();
      jsonWriter.name("level").value("info");
      jsonWriter.name("description").value(summaryBuilder.toString());
      jsonWriter.endObject();

      jsonWriter.endArray();
      jsonWriter.flush();
    } catch (IOException e) {
      throw new RuntimeException(e);
    }
    stream.append(bufferedStream.toString());
  }
}