[jira] [Created] (LUCENE-8779) MinHashFilter generates invalid terms

["Adrien Grand (JIRA)" <ji...@apache.org>]

Adrien Grand created LUCENE-8779:
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             Summary: MinHashFilter generates invalid terms
                 Key: LUCENE-8779
                 URL: https://issues.apache.org/jira/browse/LUCENE-8779
             Project: Lucene - Core
          Issue Type: Improvement
            Reporter: Adrien Grand


This problem was reported at https://github.com/elastic/elasticsearch/issues/41556: MinHashFilter computes a hash and then folds its bits into the chars of the term. However this might generate invalid terms that eg. end with a character that is a high surrogate.

This doesn't trigger exceptions at index time because we are lenient with unmatched surrogates when converting to a binary term.

{code}
      } else {
        // surrogate pair
        // confirm valid high surrogate
        if (code < 0xDC00 && (i < end-1)) {
          int utf32 = (int) s.charAt(i+1);
          // confirm valid low surrogate and write pair
          if (utf32 >= 0xDC00 && utf32 <= 0xDFFF) { 
            utf32 = (code << 10) + utf32 + SURROGATE_OFFSET;
            i++;
            out[upto++] = (byte)(0xF0 | (utf32 >> 18));
            out[upto++] = (byte)(0x80 | ((utf32 >> 12) & 0x3F));
            out[upto++] = (byte)(0x80 | ((utf32 >> 6) & 0x3F));
            out[upto++] = (byte)(0x80 | (utf32 & 0x3F));
            continue;
          }
        }
        // replace unpaired surrogate or out-of-order low surrogate
        // with substitution character
        out[upto++] = (byte) 0xEF;
        out[upto++] = (byte) 0xBF;
        out[upto++] = (byte) 0xBD;
      }
{code}



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