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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 2021 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.serialization;

import com.google.errorprone.annotations.NoAllocation;
import com.google.protobuf.ByteString;

/** Encoder/decoder into https://simonsapin.github.io/wtf-8/ */
final class Wtf8 {

  private Wtf8() {}

  private static final byte CONTINUATION_MASK = 0x3F;

  static ByteString encodeToWtf8(String s) {
    int length = s.length();
    ByteString.Output output = ByteString.newOutput(length);
    for (int i = 0; i < length; i++) {
      int codepoint = s.codePointAt(i);

      if (codepoint < 0x80) {
        output.write(codepoint);
      } else if (codepoint < 0x800) {
        output.write(0xC0 | (0x1f & (codepoint >>> 6)));
        output.write(0x80 | (CONTINUATION_MASK & codepoint));
      } else if (codepoint < 0x10000) {
        output.write(0xE0 | (0xf & (codepoint >>> 12)));
        output.write(0x80 | (CONTINUATION_MASK & (codepoint >>> 6)));
        output.write(0x80 | (CONTINUATION_MASK & codepoint));
      } else {
        // This codepoints takes two UTF-16 code units, so we need an extra increment.
        i++;
        output.write(0xF0 | (0x7 & (codepoint >>> 18)));
        output.write(0x80 | (CONTINUATION_MASK & (codepoint >>> 12)));
        output.write(0x80 | (CONTINUATION_MASK & (codepoint >>> 6)));
        output.write(0x80 | (CONTINUATION_MASK & codepoint));
      }
    }
    return output.toByteString();
  }

  static Decoder decoder(int maxLength) {
    return new Decoder(maxLength);
  }

  /**
   * Decodes strings from WTF8 bytes.
   *
   * This class is not threadsafe! In order to minimize allocations, and maximize performance, it
   * reuses buffers between decodings.
   */
  static class Decoder {
    private final int[] codepointBuffer;

    private Decoder(int maxLength) {
      this.codepointBuffer = new int[maxLength];
    }

    @NoAllocation
    String decode(ByteString encoded) {
      final int encodedByteCount = encoded.size();
      int codepointCount = 0;

      for (int i = 0; i < encodedByteCount; i++) {
        final byte b = encoded.byteAt(i);
        final int codepoint;

        if ((b & 0x80) == 0) {
          // 0xxx xxxx: 1 byte
          codepoint = b;
        } else if ((b & 0xE0) == 0xC0) {
          // 110x xxxx: 2 bytes
          int firstByte = 0x1F & b;
          int secondByte = encoded.byteAt(++i) & CONTINUATION_MASK;
          codepoint = (firstByte << 6) | secondByte;
        } else if ((b & 0xF0) == 0xE0) {
          // 1110 xxxx: 3 bytes
          int firstByte = 0xF & b;
          int secondByte = encoded.byteAt(++i) & CONTINUATION_MASK;
          int thirdByte = encoded.byteAt(++i) & CONTINUATION_MASK;
          codepoint = (firstByte << 12) | (secondByte << 6) | thirdByte;
        } else if ((b & 0xF8) == 0xF0) {
          // 1111 0xxx: 4 bytes
          int firstByte = 0x7 & b;
          int secondByte = encoded.byteAt(++i) & CONTINUATION_MASK;
          int thirdByte = encoded.byteAt(++i) & CONTINUATION_MASK;
          int fourthByte = encoded.byteAt(++i) & CONTINUATION_MASK;
          codepoint = (firstByte << 18) | (secondByte << 12) | (thirdByte << 6) | fourthByte;
        } else {
          throw new AssertionError();
        }

        this.codepointBuffer[codepointCount++] = codepoint;
      }

      return new String(this.codepointBuffer, 0, codepointCount);
    }
  }
}