private String[] diff_halfMatchI(String longtext, String shorttext, int i) {
    // Start with a 1/4 length substring at position i as a seed.
    String seed = longtext.substring(i, i + longtext.length() / 4);
    int j = -1;
    String best_common = "";
    String best_longtext_a = "", best_longtext_b = "";
    String best_shorttext_a = "", best_shorttext_b = "";
    while ((j = shorttext.indexOf(seed, j + 1)) != -1) {
      int prefixLength = diff_commonPrefix(longtext.substring(i),
                                           shorttext.substring(j));
      int suffixLength = diff_commonSuffix(longtext.substring(0, i),
                                           shorttext.substring(0, j));
      if (best_common.length() < suffixLength + prefixLength) {
        best_common = shorttext.substring(j - suffixLength, j)
            + shorttext.substring(j, j + prefixLength);
        best_longtext_a = longtext.substring(0, i - suffixLength);
        best_longtext_b = longtext.substring(i + prefixLength);
        best_shorttext_a = shorttext.substring(0, j - suffixLength);
        best_shorttext_b = shorttext.substring(j + prefixLength);
      }
    }
    if (best_common.length() * 2 >= longtext.length()) {
      return new String[]{best_longtext_a, best_longtext_b,
                          best_shorttext_a, best_shorttext_b, best_common};
    } else {
      return null;
    }
  }