King safety is an important concept as when the king is going to be mated the game is over. King safety takes care of good king locations, intact pawn shelters, number of attacker and defenders around the king, material mating potential etc. Overall this is a fairly complex sub evaluation and to have it fairly accurate is important for the playing strength. A decision whether an engine should sacrifice material in order to be able to launch a mating attack against the opponents king comes mainly from this concept. And a wrong decision is usually fatal.
To get a better understanding what terms are important I first ran some statistics. I analyzed a few million positions where the king safety term kicked in and recorder the location of the kings.
|Mid game probability the king is on a given square (black side is mirrored)|
In my evaluation I increase or decrease a counter for attacking or defending pieces. Each piece type is handled individually. A defending queen gets no bonus, a defending bishop does. The valid counter range after adding all attackers and subtracting the defenders is between 0 and 50. Now I wanted to know how this counter is distributed. I expected a uniform distribution, but it looked quiet differently.
Obviously there are some values much more likely than others. Probably because some attack / defend combinations are much more likely than others but overall the range between 10 and 30 is the most interesting one. I intend later to give tuned values higher focus if they relate to a counter value that occurs frequently.
Finally I tuned the king safety evaluation where I assigned each counter value an evaluation value. If plotted as a curve the evaluation values form a somewhat exponential function.
|Evaluation per Counter|
The final regression test got me a somewhat nice improvement in strength over iCE 2. So at least it was worth it.