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();
  }

    pos: Int,
    limit: Int,
  ): List<Int> {
    val result = mutableListOf<Int>()
    var i = pos
    while (i < limit) {
      val c = this[i]
      result +=
        when {
          c.isSurrogate() -> {
            val low = (if (i + 1 < limit) this[i + 1] else '\u0000')
            if (c.isLowSurrogate() || !low.isLowSurrogate()) {
              '?'.code
            } else {
              i++

   *             surrogate is returned.</b>
   *         <li><b>If the next character was a valid low surrogate, the code point value of the
   *             high/low surrogate pair is returned.</b>
   *         <li>If the next character was not a low surrogate value, then {@link
   *             IllegalArgumentException} is thrown.
   *       </ol>

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

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

    // Build up a range of surrogate pair characters to test
    int min = Character.MIN_SUPPLEMENTARY_CODE_POINT;
    int max = Character.MAX_CODE_POINT;
    int range = max - min;
    int s1 = min + (1 * range) / 4;
    int s2 = min + (2 * range) / 4;
    int s3 = min + (3 * range) / 4;
    char[] dst = new char[12];

    // Put surrogate pairs at odd indices so they can be split easily

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

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

    assertStringHash(0xb5a4be05, "ABCDefGHI\u0799", UTF_8);
    assertStringHash(0xfc5ba834, "毎月１日,毎週月曜日", UTF_8);
    assertStringHash(0x8a5c3699, "surrogate pair: \uD83D\uDCB0", UTF_8);

    assertStringHash(0, "", UTF_16LE);
    assertStringHash(0x288418e4, "k", UTF_16LE);
    assertStringHash(0x5a0cb7c3, "hell", UTF_16LE);
    assertStringHash(0xd7c31989, "hello", UTF_16LE);

    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);

    assertStringHash(0xb5a4be05, "ABCDefGHI\u0799", UTF_8);
    assertStringHash(0xfc5ba834, "毎月１日,毎週月曜日", UTF_8);
    assertStringHash(0x8a5c3699, "surrogate pair: \uD83D\uDCB0", UTF_8);

    assertStringHash(0, "", UTF_16LE);
    assertStringHash(0x288418e4, "k", UTF_16LE);
    assertStringHash(0x5a0cb7c3, "hell", UTF_16LE);
    assertStringHash(0xd7c31989, "hello", UTF_16LE);

        }
      } else {
        // and everything else is replaced with FFFD.
        assertEscaping(xmlEscaper, "\uFFFD", ch);
      }
    }

    // Test _all_ allowed characters (including surrogate values).
    for (char ch = 0x20; ch <= 0xFFFD; ch++) {
      // There are a small number of cases to consider, so just do it manually.
      if (ch == '&') {
        assertEscaping(xmlEscaper, "&amp;", ch);