**Use case**

I want to estimate better parameters for fulltext (`ngrambf_v1`) index.

**Describe the solution you'd like**

Implement it as SQL function:

`ngrams(string, N)`

that returns array of FixedString.

**Describe alternatives you've considered**

It can be done with less efficient and more cumbersome way:

```
WITH 3 AS n, extractAll(text, '.') AS chars 
SELECT 
  arrayMap(x -> arrayStringConcat(x), 
    arrayFilter(x -> length(x) = n, 
      arrayMap(x, i -> arraySlice(chars, i, n), 
        chars, arrayEnumerate(chars)))) AS ngrams
```

**Additional context**

`ngramsUTF8` can also be implemented to extract ngrams of Unicode code points.
++*pos;
        }
        else if (!escaped && str[*pos] == '\\')
        {
            escaped = true;
            ++*pos;
        }
        else if (isASCII(str[*pos]) && !isAlphaNumericASCII(str[*pos]))
        {
            if (!bad_token && !token.empty())
                return true;
            token.clear();
            bad_token = false;
            escaped = false;
            ++*pos;
        }
        else
        {
            const size_t sz = UTF8::seqLength(static_cast<UInt8>(str[*pos]));
            for (size_t j = 0; j < sz; ++j)
            {
                token += str[*pos];
                ++*pos;
            }
            escaped = false;
        }
    }
    return !bad_token && !token.empty();
}
MergeTreeIndexPtr bloomFilterIndexCreator(