From c14297a483f7905d61e6f22068d33b199916257a Mon Sep 17 00:00:00 2001
From: Shawn Xu <shawn@shawnxu.org>
Date: Mon, 27 May 2024 01:21:32 -0700
Subject: [PATCH] Tune Fail Low Bonus

Fractional bonus idea is from @Ergodice on
[discord](https://discord.com/channels/435943710472011776/735707599353151579/1244039134499180614).
Values are tuned for 149k games at LTC.

SPSA tune:
https://tests.stockfishchess.org/tests/view/6652d5d5a86388d5e27da9d6

Failed STC:
LLR: -2.95 (-2.94,2.94) <0.00,2.00>
Total: 67424 W: 17364 L: 17528 D: 32532
Ptnml(0-2): 238, 8043, 17299, 7909, 223
https://tests.stockfishchess.org/tests/view/66551e1ba86388d5e27db9f9

Passed LTC:
LLR: 2.94 (-2.94,2.94) <0.00,2.00>
Total: 146910 W: 37141 L: 36695 D: 73074
Ptnml(0-2): 84, 16201, 40441, 16643, 86
https://tests.stockfishchess.org/tests/view/66559949a86388d5e27dcc5d

Passed VLTC:
LLR: 2.95 (-2.94,2.94) <0.50,2.50>
Total: 27248 W: 6924 L: 6633 D: 13691
Ptnml(0-2): 5, 2744, 7835, 3035, 5
https://tests.stockfishchess.org/tests/view/66563f4da86388d5e27dd27a

closes https://github.com/official-stockfish/Stockfish/pull/5299

Bench: 1390709
---
 src/search.cpp | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/src/search.cpp b/src/search.cpp
index c074e342..425782eb 100644
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -1341,18 +1341,19 @@ moves_loop:  // When in check, search starts here
     // Bonus for prior countermove that caused the fail low
     else if (!priorCapture && prevSq != SQ_NONE)
     {
-        int bonus = (depth > 4) + (depth > 5) + (PvNode || cutNode) + ((ss - 1)->statScore < -14144)
-                  + ((ss - 1)->moveCount > 9) + (!ss->inCheck && bestValue <= ss->staticEval - 115)
-                  + (!(ss - 1)->inCheck && bestValue <= -(ss - 1)->staticEval - 81);
+        int bonus = (54 * (depth > 4) + 62 * (depth > 5) + 115 * (PvNode || cutNode)
+                     + 186 * ((ss - 1)->statScore < -14144) + 121 * ((ss - 1)->moveCount > 9)
+                     + 64 * (!ss->inCheck && bestValue <= ss->staticEval - 115)
+                     + 137 * (!(ss - 1)->inCheck && bestValue <= -(ss - 1)->staticEval - 81));
         update_continuation_histories(ss - 1, pos.piece_on(prevSq), prevSq,
-                                      stat_bonus(depth) * bonus);
+                                      stat_bonus(depth) * bonus / 100);
         thisThread->mainHistory[~us][((ss - 1)->currentMove).from_to()]
-          << stat_bonus(depth) * bonus / 2;
+          << stat_bonus(depth) * bonus / 200;
 
 
         if (type_of(pos.piece_on(prevSq)) != PAWN && ((ss - 1)->currentMove).type_of() != PROMOTION)
             thisThread->pawnHistory[pawn_structure_index(pos)][pos.piece_on(prevSq)][prevSq]
-              << stat_bonus(depth) * bonus * 4;
+              << stat_bonus(depth) * bonus / 25;
     }
 
     if (PvNode)