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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.nio.charset.StandardCharsets.UTF_8;

import com.google.common.annotations.GwtIncompatible;
import com.google.common.annotations.VisibleForTesting;
import com.google.common.collect.ImmutableSet;
import com.google.common.io.BaseEncoding;
import java.net.URI;
import java.net.URISyntaxException;
import java.nio.file.FileSystems;
import javax.annotation.Nullable;

/** Utility class for resolving source maps and files referenced in source maps. */
@GwtIncompatible("Accesses the file system")
public class SourceMapResolver {
  private static final String BASE64_URL_PREFIX = "data:";
  private static final String BASE64_START = "base64,";

  // For now only accept UTF-8. We could use the actual charset information in the future.
  private static final ImmutableSet ACCEPTED_MEDIA_TYPES = ImmutableSet.of(
      "application/json;charset=utf-8;",
      // default is UTF-8 if unspecified.
      "application/json;");

  /**
   * For a given //# sourceMappingUrl, this locates the appropriate sourcemap on disk. This is use
   * for sourcemap merging (--apply_input_source_maps) and for error resolution.
   *
   * @param parseInlineSourceMaps Whether to parse Base64 encoded source maps.
   */
  static SourceFile extractSourceMap(
      SourceFile jsFile, String sourceMapURL, boolean parseInlineSourceMaps) {
    if (parseInlineSourceMaps && sourceMapURL.startsWith(BASE64_URL_PREFIX)) {
      String extractedString = extractBase64String(sourceMapURL);
      if (extractedString != null) {
        return SourceFile.fromCode(jsFile.getName() + ".inline.map", extractedString);
      }
      return null;
    }
    // TODO(tdeegan): Handle absolute urls here.  The compiler needs to come up with a scheme for
    // properly resolving absolute urls from http:// or the root /some/abs/path/... See b/62544959.
    if (isAbsolute(sourceMapURL)) {
      return null;
    }
    // If not absolute, its relative.
    // TODO(tdeegan): Handle urls relative to //# sourceURL. See the sourcemap spec.
    return getRelativePath(jsFile.getName(), sourceMapURL);
  }

  /**
   * Based on https://developer.mozilla.org/en-US/docs/Web/HTTP/Basics_of_HTTP/Data_URIs
   * @param url
   * @return String or null.
   */
  @Nullable
  private static String extractBase64String(String url) {
    if (url.startsWith(BASE64_URL_PREFIX) && url.contains(BASE64_START)) {
      int base64StartIndex = url.indexOf(BASE64_START);
      String mediaType = url.substring(BASE64_URL_PREFIX.length(), base64StartIndex);
      if (ACCEPTED_MEDIA_TYPES.contains(mediaType)) {
        byte[] data = BaseEncoding.base64().decode(
                url.substring(base64StartIndex + BASE64_START.length()));
        return new String(data, UTF_8);
      }
    }
    return null;
  }

  @VisibleForTesting
  private static boolean isAbsolute(String url) {
    try {
      return new URI(url).isAbsolute() || url.startsWith("/");
    } catch (URISyntaxException e) {
      throw new RuntimeException("Sourcemap url was invalid: " + url, e);
    }
  }

  /**
   * Returns the relative path, resolved relative to the base path, where the base path is
   * interpreted as a filename rather than a directory. E.g.: getRelativeTo("../foo/bar.js",
   * "baz/bam/qux.js") --> "baz/foo/bar.js"
   */
  @Nullable
  static SourceFile getRelativePath(String baseFilePath, String relativePath) {
    return SourceFile.fromPath(
        FileSystems.getDefault().getPath(baseFilePath).resolveSibling(relativePath).normalize(),
        UTF_8);
  }
}