Editing Evaluation function (section)

===Handcrafted evaluation functions===
Historically in computer chess, the terms of an evaluation function are constructed (i.e. handcrafted) by the engine developer, as opposed to discovered through training [[Artificial neural network|neural networks]]. The general approach for constructing handcrafted evaluation functions is as a [[linear combination]] of various weighted terms determined to influence the value of a position. However, not all terms in a handcrafted evaluation function are linear. Some, such as king safety and pawn structure, are nonlinear. Each term may be considered to be composed of first order factors (those that depend only on the space and any piece on it), second order factors (the space in relation to other spaces), and nth-order factors (dependencies on history of the position). 

A handcrafted evaluation function typically has a material balance term that usually dominates the evaluation. The [[Chess piece relative value|conventional values]] used for material are Queen=9, Rook=5; Knight or Bishop=3; Pawn=1; the king is assigned an arbitrarily large value, usually larger than the total value of all the other pieces.<ref name="Shannon"/> In addition, it typically has a set of positional terms usually totaling no more than the value of a pawn, though in some positions the positional terms can get much larger, such as when checkmate is imminent. Handcrafted evaluation functions typically contain dozens to hundreds of individual terms. 

In practice, effective handcrafted evaluation functions are not created by expanding the list of evaluated parameters, but by careful tuning or training of the weights relative to each other, of a modest set of parameters such as those described above.  Toward this end, positions from various databases are employed, such as from master games, engine games, [[Lichess]] games, or even from self-play, as in [[reinforcement learning]]. 

====Example====
An example handcrafted evaluation function for [[chess]] might look like the following:
* c<sub>1</sub> * material + c<sub>2</sub> * mobility + c<sub>3</sub> * king safety + c<sub>4</sub> * center control + c<sub>5</sub> * pawn structure + c<sub>6</sub> * king tropism + ...

Each of the terms is a weight multiplied by a difference factor: the value of white's material or positional terms minus black's. 

* The material term is obtained by assigning a value in pawn-units to each of the pieces. 
* Mobility is the number of legal moves available to a player, or alternately the sum of the number of spaces attacked or defended by each piece, including spaces occupied by friendly or opposing pieces. Effective mobility, or the number of "safe" spaces a piece may move to, may also be taken into account. 
* King safety is a set of bonuses and penalties assessed for the location of the king and the configuration of pawns and pieces adjacent to or in front of the king, and opposing pieces bearing on spaces around the king. 
* Center control is derived from how many pawns and pieces occupy or bear on the four center spaces and sometimes the 12 spaces of the extended center. 
* Pawn structure is a set of penalties and bonuses for various strengths and weaknesses in pawn structure, such as penalties for doubled and isolated pawns. 
* King tropism is a bonus for closeness (or penalty for distance) of certain pieces, especially queens and knights, to the opposing king.