utf8Length += (0x7f - c) >>> 31; // branch free!
      } else {
        utf8Length += 2;
        // We can't use Character.isSurrogate(c) here and below because of GWT.
        if (MIN_SURROGATE <= c && c <= MAX_SURROGATE) {
          // Check that we have a well-formed surrogate pair.
          if (Character.codePointAt(sequence, i) == c) {
            throw new IllegalArgumentException(unpairedSurrogateMsg(i));
          }

      MAX_LOW_SURROGATE, MAX_HIGH_SURROGATE, MIN_LOW_SURROGATE, MIN_HIGH_SURROGATE,
    };
    for (char surrogate : surrogates) {
      builder.add(newString(surrogate));
      builder.add(newString(surrogate, 'n'));
      builder.add(newString('n', surrogate));
      builder.add(newString(surrogate, surrogate));
    }
    builder.add(newString(MIN_LOW_SURROGATE, MAX_HIGH_SURROGATE));
    ILL_FORMED_STRINGS = builder.build();
  }

    assertEquals("abc", Strings.commonPrefix(new StringBuilder("abcdef"), "abcxyz"));

    // Identical valid surrogate pairs.
    assertEquals(
        "abc\uD8AB\uDCAB", Strings.commonPrefix("abc\uD8AB\uDCABdef", "abc\uD8AB\uDCABxyz"));
    // Differing valid surrogate pairs.
    assertEquals("abc", Strings.commonPrefix("abc\uD8AB\uDCABdef", "abc\uD8AB\uDCACxyz"));
    // One invalid pair.

    return new String(array);
  }

  /**
   * Returns the longest string {@code prefix} such that {@code a.toString().startsWith(prefix) &&
   * b.toString().startsWith(prefix)}, taking care not to split surrogate pairs. If {@code a} and
   * {@code b} have no common prefix, returns the empty string.
   *
   * @since 11.0
   */
  public static String commonPrefix(CharSequence a, CharSequence b) {
    checkNotNull(a);

    }
  }

  /**
   * This verifies that putUnencodedChars(String) and hashUnencodedChars(String) are equivalent,
   * even for funny strings composed by (possibly unmatched, and mostly illegal) surrogate
   * characters. (But doesn't test that they do the right thing - just their consistency).
   */
  private static void assertHashStringWithSurrogatesEquivalence(
      HashFunction hashFunction, Random random) {

 *
 * <p>Supplementary characters are <a
 * href="https://docs.oracle.com/javase/8/docs/api/java/lang/Character.html#supplementary">encoded
 * into a {@code String} using surrogate pairs</a>, and a {@code CharMatcher} treats these just as
 * two separate characters. {@link #countIn} counts each supplementary character as 2 {@code char}s.
 *