1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100

//! Defines the `Evaluator` trait.

use uci::SetOption;
use board::Board;
use moves::Move;
use value::*;


/// A trait used to statically evaluate positions.
///
/// An evaluation function is used to heuristically determine the
/// relative value of a position, i.e. the chances of winning. If we
/// could see to the end of the game in every line, the evaluation
/// would only have values of "loss", "draw", and "win". In practice,
/// however, we do not know the exact value of a position, so we must
/// make an approximation. Beginning chess players learn to do this
/// starting with the value of the pieces themselves. Computer
/// evaluation functions also use the value of the material as the
/// most significant aspect and then add other considerations.
///
/// Static evaluation is an evaluation that considers only the static
/// material and positional properties of the current position,
/// without analyzing any tactical variations. Therefore, if the
/// position has pending tactical threats, the static evaluation will
/// be grossly incorrect. To implement your own static evaluator, you
/// must define a type that implements the `Evaluator` trait.
pub trait Evaluator: Clone + SetOption + Send + 'static {
    /// Creates a new instance bound to a given position.
    ///
    /// When a new evaluator is created it is bound to a particular
    /// chess position (given by the `position` parameter), and for
    /// the moment, this is the only position that it can
    /// evaluate. The evaluator then can be re-bound to the next (or
    /// the previous) position in the current line of play by issuing
    /// a pair of calls to the `will_do_move` and `done_move` methods
    /// (or respectively, `will_undo_move` and `undone_move`
    /// methods). Thus, evaluator's state can be kept up-to-date,
    /// following the current line of play.
    fn new(position: &Board) -> Self;

    /// Evaluates the the position to which the evaluator is currently
    /// bound.
    ///
    /// `position` points to the position to which the evaluator is
    /// currently bound.
    ///
    /// The returned value must be between `VALUE_EVAL_MIN` and
    /// `VALUE_EVAL_MAX`.
    fn evaluate(&self, position: &Board) -> Value;

    /// Returns whether the position to which the evaluator is
    /// currently bound is "zugzwangy".
    ///
    /// `position` points to the position to which the evaluator is
    /// currently bound.
    ///
    /// In many endgame positions there is a relatively high
    /// probability of zugzwang occurring. For such positions, this
    /// method returns `true`. This is useful when we want to decide
    /// whether it is safe to try a null move.
    fn is_zugzwangy(&self, position: &Board) -> bool;

    /// Updates evaluator's state to keep up with a move that will be
    /// played.
    ///
    /// `position` points to the position to which the evaluator is
    /// currently bound (that is: before `m` is played).
    ///
    /// `m` is a legal move, or (if not in check) a "null move".
    #[inline]
    #[allow(unused_variables)]
    fn will_do_move(&mut self, position: &Board, m: Move) {}

    /// Updates evaluator's state to keep up with a move that was
    /// played.
    ///
    /// `position` points to the new position to which the evaluator
    /// is bound (that is: after `m` was played).
    #[inline]
    #[allow(unused_variables)]
    fn done_move(&mut self, position: &Board, m: Move) {}

    /// Updates evaluator's state to keep up with a move that will be
    /// taken back.
    ///
    /// `position` points to the position to which the evaluator is
    /// currently bound (that is: before `m` is taken back).
    #[inline]
    #[allow(unused_variables)]
    fn will_undo_move(&mut self, position: &Board, m: Move) {}

    /// Updates evaluator's state in accordance with a move that was
    /// taken back.
    ///
    /// `position` points to the new position to which the evaluator is
    /// bound (that is: after `m` was taken back).
    #[inline]
    #[allow(unused_variables)]
    fn undone_move(&mut self, position: &Board, m: Move) {}
}