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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;

import static java.lang.Math.max;

import com.google.common.collect.ImmutableMap;
import com.google.errorprone.annotations.Immutable;

/**
 * A decoder for strings encoded by the ReplaceStrings JS compiler pass. This class is immutable.
 */
@Immutable
public final class ReplacedStringsDecoder {

  // The default place holder for replaced string arguments.
  // Note: We're not allowing this to be changed here because it's present in
  // the replacement strings in the js_storage protocol buffer.
  public static final String ARGUMENT_PLACE_HOLDER = "`";

  private final ImmutableMap originalToNewNameMap;

  /** A null decoder that does no mapping. */
  public static final ReplacedStringsDecoder NULL_DECODER =
      new ReplacedStringsDecoder(VariableMap.fromMap(
          ImmutableMap.of())
      );

  public ReplacedStringsDecoder(VariableMap variableMap) {
    // VariableMap is not an immutable type, so we extract the map instead of directly using it.
    this.originalToNewNameMap = variableMap.getOriginalNameToNewNameMap();
  }

  /**
   * Decodes an encoded string from the JS Compiler ReplaceStrings pass.
   *
   * An original string with args might look like this:
   * 
  Error('Some ' + arg1 + ' error ' + arg2 + ' message.');
   * Which gets replaced with:
   *   Error('key' + '`' + arg1 + '`' + arg2);
   * Where ` is the argument place holder. The replacement mapping would be:
   *   key → 'Some ` error ` message.'
   * Where key is some arbitrary replacement string. An encoded string,
   * with args, from the client will look like:
   *   'key`arg1`arg2'
   *
   * @param encodedStr An encoded string.
   * @return The decoded string, or the encoded string if it fails to decode.
   * @see com.google.javascript.jscomp.ReplaceStrings
   */
  public String decode(String encodedStr) {
    String[] suppliedBits = encodedStr.split(ARGUMENT_PLACE_HOLDER, -1);
    String originalStr = originalToNewNameMap.get(suppliedBits[0]);
    if (originalStr == null) {
      return encodedStr; // Failed to decode.
    }
    String[] originalBits = originalStr.split(ARGUMENT_PLACE_HOLDER, -1);
    StringBuilder sb = new StringBuilder(originalBits[0]);
    for (int i = 1; i < max(originalBits.length, suppliedBits.length); i++) {
      // Replace missing bits with "-". Shouldn't happen except that we aren't
      // escaping the replacement token at the moment.
      sb.append(i < suppliedBits.length ? suppliedBits[i] : "-");
      sb.append(i < originalBits.length ? originalBits[i] : "-");
    }
    return sb.toString();
  }

}