testEncodedLengthFails(newString(MIN_HIGH_SURROGATE), 0);
    testEncodedLengthFails("foobar" + newString(MIN_HIGH_SURROGATE), 6);
    testEncodedLengthFails(newString(MIN_LOW_SURROGATE), 0);
    testEncodedLengthFails("foobar" + newString(MIN_LOW_SURROGATE), 6);
    testEncodedLengthFails(newString(MIN_HIGH_SURROGATE, MIN_HIGH_SURROGATE), 0);
  }

    testEncodedLengthFails(newString(MIN_HIGH_SURROGATE), 0);
    testEncodedLengthFails("foobar" + newString(MIN_HIGH_SURROGATE), 6);
    testEncodedLengthFails(newString(MIN_LOW_SURROGATE), 0);
    testEncodedLengthFails("foobar" + newString(MIN_LOW_SURROGATE), 6);
    testEncodedLengthFails(newString(MIN_HIGH_SURROGATE, MIN_HIGH_SURROGATE), 0);
  }

   *   <li>If the first character was a high surrogate value, then an attempt is made to read the
   *       next character.
   *       <ol>
   *         <li><b>If the end of the sequence was reached, the negated value of the trailing high
   *             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>

    // center will be at either low or high.
    int low = requiredFrom;
    int high = requiredTo;
    while (high > low + 1) {
      int mid = (low + high) >>> 1;
      if (allRequired[mid] > centerFloor) {
        high = mid;
      } else if (allRequired[mid] < centerFloor) {
        low = mid;
      } else {

    assertThat(UnsignedLongs.compare(0xffffffffffffffffL, 0)).isGreaterThan(0);

    // both with high bit set
    assertThat(UnsignedLongs.compare(0xff1a618b7f65ea12L, 0xffffffffffffffffL)).isLessThan(0);
    assertThat(UnsignedLongs.compare(0xffffffffffffffffL, 0xff1a618b7f65ea12L)).isGreaterThan(0);

    // one with high bit set
    assertThat(UnsignedLongs.compare(0x5a4316b8c153ac4dL, 0xff1a618b7f65ea12L)).isLessThan(0);

  }

  static char randomHighSurrogate(Random random) {
    return (char)
        (Character.MIN_HIGH_SURROGATE
            + random.nextInt(Character.MAX_HIGH_SURROGATE - Character.MIN_HIGH_SURROGATE + 1));
  }

    assertThat(UnsignedLongs.compare(0xffffffffffffffffL, 0)).isGreaterThan(0);

    // both with high bit set
    assertThat(UnsignedLongs.compare(0xff1a618b7f65ea12L, 0xffffffffffffffffL)).isLessThan(0);
    assertThat(UnsignedLongs.compare(0xffffffffffffffffL, 0xff1a618b7f65ea12L)).isGreaterThan(0);

    // one with high bit set
    assertThat(UnsignedLongs.compare(0x5a4316b8c153ac4dL, 0xff1a618b7f65ea12L)).isLessThan(0);

 * <p>Summarizing the last two points: "equal yield equal <i>always</i>; unequal yield unequal
 * <i>often</i>." This is the most important characteristic of all hash functions.
 *
 * <h3>Desirable properties</h3>
 *
 * <p>A high-quality hash function strives for some subset of the following virtues:
 *
 * <ul>
 *   <li><b>collision-resistant:</b> while the definition above requires making at least <i>some</i>

    // Two differing invalid pairs.
    assertEquals("abc\uD8AB", Strings.commonPrefix("abc\uD8AB\uD8ABdef", "abc\uD8AB\uD8ACxyz"));
    // One orphan high surrogate.
    assertEquals("", Strings.commonPrefix("\uD8AB\uDCAB", "\uD8AB"));
    // Two orphan high surrogates.
    assertEquals("\uD8AB", Strings.commonPrefix("\uD8AB", "\uD8AB"));
  }

  public void testCommonSuffix() {

  static <K extends @Nullable Object> ObjectCountHashMap<K> create() {
    return new ObjectCountHashMap<K>();
  }

  /**
   * Creates a {@code ObjectCountHashMap} instance, with a high enough "initial capacity" that it
   * <i>should</i> hold {@code expectedSize} elements without growth.
   *
   * @param expectedSize the number of elements you expect to add to the returned set