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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 2016 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.deps;

import com.google.common.annotations.VisibleForTesting;
import com.google.common.base.Throwables;
import com.google.common.cache.Cache;
import com.google.common.cache.CacheBuilder;
import com.google.common.collect.ImmutableList;
import com.google.common.hash.HashFunction;
import com.google.common.hash.Hashing;
import com.google.common.io.CharSource;
import com.google.common.util.concurrent.UncheckedExecutionException;
import com.google.javascript.jscomp.Compiler;
import com.google.javascript.jscomp.CompilerOptions;
import com.google.javascript.jscomp.CompilerOptions.LanguageMode;
import com.google.javascript.jscomp.PropertyRenamingPolicy;
import com.google.javascript.jscomp.SourceFile;
import com.google.javascript.jscomp.VariableRenamingPolicy;

import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.PrintStream;
import java.io.UnsupportedEncodingException;
import java.nio.charset.StandardCharsets;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;

import javax.annotation.concurrent.NotThreadSafe;

/**
 * {@link ClosureBundler} that transpiles its sources.
 */
@NotThreadSafe
public final class TranspilingClosureBundler extends ClosureBundler {

  private static final HashFunction HASH_FUNCTION = Hashing.goodFastHash(64);
  private static final int CACHE_SIZE = 100;

  // TODO(sdh): Not all transpilation requires the runtime, only inject if actually needed.
  private final String es6Runtime;
  private boolean needToBundleEs6Runtime = true;

  public TranspilingClosureBundler() {
    this(getEs6Runtime());
  }

  @VisibleForTesting
  TranspilingClosureBundler(String es6Runtime) {
    this.es6Runtime = es6Runtime;
  }

  /**
   * Cache recent transpilations, keyed by the hash code of the input
   * to avoid storing the whole input.
   */
  @VisibleForTesting final Cache cachedTranspilations =
      CacheBuilder.newBuilder().maximumSize(CACHE_SIZE).build();

  @Override
  public void appendTo(Appendable out, DependencyInfo info, CharSource content) throws IOException {
    if (needToBundleEs6Runtime) {
      // Piggyback on the first call to transformInput to include the ES6 runtime as well.
      super.appendTo(out, SimpleDependencyInfo.EMPTY, CharSource.wrap(es6Runtime));
      needToBundleEs6Runtime = false;
    }
    super.appendTo(out, info, content);
  }

  private static CompilerOptions getOptions() {
    CompilerOptions options = new CompilerOptions();
    options.setLanguageIn(LanguageMode.ECMASCRIPT6_STRICT);
    options.setLanguageOut(LanguageMode.ECMASCRIPT5);
    // Quoting keyword properties is only needed in ES3, so basically only in IE8.
    // But we set it explicitly here because the way the test bundler works, it invokes
    // the compiler without giving information about the browser, so we have to quote
    // every time to be safe :-/
    options.setQuoteKeywordProperties(true);
    options.setSkipNonTranspilationPasses(true);
    options.setVariableRenaming(VariableRenamingPolicy.OFF);
    options.setPropertyRenaming(PropertyRenamingPolicy.OFF);
    options.setWrapGoogModulesForWhitespaceOnly(false);
    options.setPrettyPrint(true);
    return options;
  }

  @Override
  protected String transformInput(final String js, final String path) {
    try {
      // Don't use built-in hashCode to decrease the likelihood of a collision.
      long hashCode = HASH_FUNCTION.hashString(js, StandardCharsets.UTF_8).asLong();
      return cachedTranspilations.get(
          hashCode,
          new Callable() {
            @Override
            public String call() {
              // Neither the compiler nor the options is thread safe, so they can't be
              // saved as instance state.
              ByteArrayOutputStream baos = new ByteArrayOutputStream();
              Compiler compiler = new Compiler(new PrintStream(baos));
              // Threads can't be used in small unit tests.
              compiler.disableThreads();
              SourceFile externs = SourceFile.fromCode("externs", "function Symbol() {}");
              SourceFile sourceFile = SourceFile.fromCode(path, js);
              compiler.compile(
                  ImmutableList.of(externs),
                  ImmutableList.of(sourceFile),
                  getOptions());
              if (compiler.getErrorManager().getErrorCount() > 0) {
                String message;
                try {
                  message = baos.toString(StandardCharsets.UTF_8.name());
                } catch (UnsupportedEncodingException e) {
                  throw new RuntimeException(e);
                }
                throw new IllegalStateException(message);
              }
              return compiler.toSource();
            }
          });
    } catch (ExecutionException | UncheckedExecutionException e) {
      // IllegalStateExceptions thrown from the callable above will end up here as
      // UncheckedExecutionExceptions, per the contract of Cache#get. Throw the underlying
      // IllegalStateException so that the compiler error message is at the top of the stack trace.
      if (e.getCause() instanceof IllegalStateException) {
        throw (IllegalStateException) e.getCause();
      } else {
        throw Throwables.propagate(e);
      }
    }
  }

  /** Generates the runtime by requesting the "es6_runtime" library from the compiler. */
  private static String getEs6Runtime() {
    CompilerOptions options = getOptions();
    options.setLanguageOut(LanguageMode.ECMASCRIPT3); // change .delete to ['delete']
    options.setForceLibraryInjection(ImmutableList.of("es6_runtime"));
    Compiler compiler = new Compiler();
    // Threads can't be used in small unit tests.
    compiler.disableThreads();
    SourceFile externs = SourceFile.fromCode("externs", "function Symbol() {}");
    SourceFile sourceFile = SourceFile.fromCode("source", "");
    compiler.compile(ImmutableList.of(externs), ImmutableList.of(sourceFile), options);
    return compiler.toSource();
  }
}