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();
  }

   *             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>

   *             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>

    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);

   * for use with it (such as all-ASCII text) and for simple debugging text. When using this method,
   * consider the following:
   *
   * <ul>
   *   <li>it may split surrogate pairs
   *   <li>it may split characters and combining characters
   *   <li>it does not consider word boundaries
   *   <li>if truncating for display to users, there are other considerations that must be taken

    }
  }

  /**
   * 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) {