King danger: retire attacked-by-bishop defense

In a recent commit, "Introduce king flank defenders," a term was introduced
by Michael Chaly (@Vizvezdenec) to reduce king danger based on king defenders,
i.e., friendly attacks on our King Flank and Camp. This is a powerful idea
and broadly applicable to all of our pieces.

An earlier, but narrower, version of a similar idea was already coded into
king danger, with a term reducing king danger simply if we had a bishop and
king attacking the same square -- there is also a similar term for knights,
but roughly three times larger. I had attempted to tweak this term's coefficient
fairly recently, in a series of tests in early September which increased this
coefficient.  All failed STC with significantly negative scores.

Now that the king flank defenders term has been introduced, it appears that
the bishop-defense term can be simplified away without compensation or
significant Elo loss.

Where do we go from here? This PR is a natural follow-up to "Introduce king
flank defenders," which proposed simplification with existing and overlapping
terms, such as this one.  That PR also mentioned that the coefficient it
introduced appeared arbitrary, so perhaps this PR can facilitate a tweak to
increase king flank defenders' coefficient.

Additionally, this pull request is extremely similar to https://github.com/official-stockfish/Stockfish/pull/1821,
which was (coincidentally) merged a year ago, to the day (November 23, 2018).
That patch also simplified away a linear king danger tropism term, which was
soon after replaced with a quadratic term by @Vizvezdenec (which would not have
passed without the simplification).  @Vizvezdenec, again by coincidence, has
recently been trying to implement a quadratic term, this time for defenders
rather than attackers.  This history of this evaluation code suggests that
this simplification might be enough to help a patch for quadratic king-flank
defenders pass.

Bench: 4959670

STC:
LLR: 2.94 (-2.94,2.94) [-3.00,1.00]
Total: 22209 W: 4920 L: 4800 D: 12489
https://tests.stockfishchess.org/tests/view/5dd444d914339111b9b6bed7

LTC:
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 152107 W: 24658 L: 24743 D: 102706
https://tests.stockfishchess.org/tests/view/5dd4be31f531e81cf278ea9d

Interesting discussion on Github about this pull request:
https://github.com/official-stockfish/Stockfish/pull/2424

---

This pull request was opened less than one week before the holiday of
Thanksgiving here in the United States.  In keeping with the holiday
tradition of expressing gratitude, I would like to thank our generous
CPU donors, talented forum contributors, innovative developers, speedy
fishtest approvers, and especially our hardworking server maintainers
(@ppigazzini and @tomtor). Thank you all for a year of great Stockfish
progress!

diff --git a/src/evaluate.cpp b/src/evaluate.cpp
index eb5719bdad01326d9d6cdbebf44afc3c9bdf32c7..4c876820d060e58862c6e132c6010cb2809585e5 100644 (file)
--- a/src/evaluate.cpp
+++ b/src/evaluate.cpp
@@ -454,7 +454,6 @@ namespace {
                  +       mg_value(mobility[Them] - mobility[Us])
                  - 873 * !pos.count<QUEEN>(Them)
                  - 100 * bool(attackedBy[Us][KNIGHT] & attackedBy[Us][KING])
                  +       mg_value(mobility[Them] - mobility[Us])
                  - 873 * !pos.count<QUEEN>(Them)
                  - 100 * bool(attackedBy[Us][KNIGHT] & attackedBy[Us][KING])

-                 -  35 * bool(attackedBy[Us][BISHOP] & attackedBy[Us][KING])

                  -   6 * mg_value(score) / 8
                  -   7;
 
                  -   6 * mg_value(score) / 8
                  -   7;