Idea is to do more futility pruning if previous move has bad histories and less if it has good histories.
passed STC
https://tests.stockfishchess.org/tests/view/
61e3757fbabab931824e0db7
LLR: 2.96 (-2.94,2.94) <0.00,2.50>
Total: 156816 W: 42282 L: 41777 D: 72757
Ptnml(0-2): 737, 17775, 40913, 18212, 771
passed LTC
https://tests.stockfishchess.org/tests/view/
61e43496928632f7813a5535
LLR: 2.95 (-2.94,2.94) <0.50,3.00>
Total: 349968 W: 94612 L: 93604 D: 161752
Ptnml(0-2): 300, 35934, 101550, 36858, 342
closes https://github.com/official-stockfish/Stockfish/pull/3903
bench
4720954
// The depth condition is important for mate finding.
if ( !ss->ttPv
&& depth < 9
- && eval - futility_margin(depth, improving) >= beta
+ && eval - futility_margin(depth, improving) - (ss-1)->statScore / 256 >= beta
+ && eval >= beta
&& eval < 15000) // 50% larger than VALUE_KNOWN_WIN, but smaller than TB wins.
return eval;