Some CSS query find elements very slowly

[821938089]

Example document : https://www.123duw.com/dudu-38/51311/
CSS query : dl > dt:nth-of-type(2) ~ dd > a
In jsoup, It took about 4 minutes.

In Chrome, it took only 50ms to complete.

jsoup version : 1.16.1

[jhy]

What platform are you on? I tried this and on my M1, it took 7.1 seconds (first run, no hotspot etc). Certainly it would be good to improve that, but I wonder if something else is impacting the speed?

[821938089]

I am running on Android platform and my phone chip is Snapdragon 660.

[Snipped large images]

[821938089]

This amount of operations looks like an exponential increase.

You can try how long this document takes: https://www.123duw.com/dudu-32/50731/

[jhy]

OK, surprised that it performs so differently on Android. I guess the combination of the high query count and the memory allocation in nth-of-type is causing the difference in.

Yes, I believe the ~ (preceding sibling) combinator is causing the query count to go exponential. Similar to repeatedly backtracking to try every variant. If we can rein that in, would expect a massive performance improvement.

[jhy]

Thanks for reporting this! I have optimized the selector by changing the query evaluation order of the any preceding sibling operator, and by memoizing the results. On my M1 this moves the execution time from ~ 7 seconds to 0.01 seconds.

I also improved the memory consumption (by removing ephemeral calls to children(), which allocates a new Elements list on each hit).

So I expect you will see a good improvement; if you could test with a snapshot version and report back that would be great!

Future work would be to improve in what order the And evaluator is executed, so simpler expressions (like a tag test) occur before slower expressions like nth-of-type.

[821938089]

Yes, I see a huge improvement. The execution time has been reduced from 4 min 30 s to 125 ms.

Test Result

[jhy]

I made the memoization a bit more generic, so that it applies to each of the Structural Evaluators. And made it more correct so that reused Evaluators have a ThreadLocal memo.

c57e683