Percentiles in HSQLDB

create AGGREGATE FUNCTION PERCENTILES(IN val float, IN flag_return boolean, INOUT buffer float ARRAY, INOUT counter INT)
   RETURNS varchar2
   CONTAINS SQL
   BEGIN ATOMIC
     declare i number default 1;
     declare stats varchar2(2000) default '';
     declare asum float default 0;  
     declare p0, p50, p90, p99, p100, pavg number default 0;
     declare sorted_buffer float ARRAY;
     IF flag_return THEN
       --call println('PERCENTILES return');
       set sorted_buffer = SORT_ARRAY(buffer);  
       call println('PERCENTILES sorted');
       set p50 = sorted_buffer[1];
       set p90 = sorted_buffer[1];
       set p99 = sorted_buffer[1];
       while i <= counter do
       begin atomic
  --call println('PERCENTILES selection '||i);
           set asum = asum + sorted_buffer[i];
           if abs(i - round(counter / 2, 0)) < 0.001 then
               set p50 = round(sorted_buffer[i], 2);
           end if;        
           if abs(i - round(counter * 9 / 10, 0)) < 0.001 then
               set p90 = round(sorted_buffer[i], 2);
           end if;
           if abs(i - round(counter  * 99 / 100, 0)) < 0.001 then
               set p99 = round(sorted_buffer[i], 2);
           end if;
  set i = i+1;        
       end;
       end while;
       set p0 = round(sorted_buffer[1],2);
       set p100 = round(sorted_buffer[counter],2);
       set pavg = round(asum / counter, 2);
       --call println('PERCENTILES preparing result');
       set stats = 'min='|| p0 || '|p50='|| p50 || '|p90='||p90 || '|p99='||p99 || '|max='||p100 || '|avg='|| pavg;
       RETURN stats;
     ELSE      
       IF val IS NULL THEN RETURN NULL; END IF;
       IF counter IS NULL THEN SET counter = 0; END IF;
       SET counter = counter + 1;
       IF counter = 1 THEN SET buffer = ARRAY[val];
       ELSE SET buffer[counter] = val; END IF;
       RETURN NULL;
     END IF;
   END

for (select appid, percentiles(mem_pct) as mem_stats, avg(mem_mb) as mem,
percentiles(cpu_pct) as cpu_stats, avg(cpu_cnt) as cpu,
    percentiles(gc_millis) as gc_stats from T_VM_STAT group by appid) do
  begin atomic
set r = REGEXP_SUBSTRING_ARRAY(mem_stats, '((?<==)[0-9]+[\.\,]*[0-9]*)');
set (p0,p50,p90,p99,p100,pavg)=(to_number(r[1]), to_number(r[2]), to_number(r[3]), to_number(r[4]), to_number(r[5]), to_number(r[6]));