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| -rw-r--r-- | engine/src/eval.rs | 67 |
1 files changed, 67 insertions, 0 deletions
diff --git a/engine/src/eval.rs b/engine/src/eval.rs index d1de95d..2abc9a3 100644 --- a/engine/src/eval.rs +++ b/engine/src/eval.rs @@ -6,6 +6,73 @@ use crate::transposition_table::TranspositionTableRef; const KING_WORTH: u32 = 2; +/// This is a compact way to store the evaluation of a position. Internally, it +/// is just a 16 bit integer being used as a fixed point number. +/// +/// ```txt +/// [ 1 bit | 1 bit | 8 bits | 6 bits ] +/// [ sign | mate? | whole | fraction ] +/// ``` +/// +/// The `mate?` field indicates whether or winning player has a force win. If +/// there's a force win, the last fourteen bits indicate how many ply it will +/// take to win. +#[derive(Clone, Copy, PartialEq, Eq, Hash)] +pub struct Evaluation(u16); + +impl Evaluation { + const LOSS: Self = Self(0b1100_0000_0000_0000); + const DRAW: Self = Self(0); + + fn new(eval: i32) -> Self { + let eval_u16 = eval.unsigned_abs() as u16; + let sign = if eval >= 0 { 0 } else { 1 }; + let sign = sign << 15; + + Self(sign | eval_u16) + } + + /// Returns `true` for -1, `false` for 1. + pub fn sign(self) -> bool { + ((self.0 >> 15) & 1) == 1 + } + + fn sign_f32(self) -> f32 { + if self.sign() { + -1.0 + } else { + 1.0 + } + } + + /// Returns `true` if the winning player can win by force. + pub fn is_force_win(self) -> bool { + ((self.0 >> 14) & 1) == 1 + } + /// Returns the evaluation * 64 + fn number_part(self) -> u16 { + // we just return the last 14 bits + self.0 & 0x3FFF + } + + /// If there is no force win, return the evaluation as an f32 + pub fn to_f32(self) -> Option<f32> { + if self.is_force_win() { + return None; + } + + Some(self.sign_f32() * (self.number_part() as f32 / 64.0)) + } + + pub fn force_win_in(self) -> Option<u16> { + if !self.is_force_win() { + return None; + } + + Some(self.number_part()) + } +} + fn eval_position(board: CheckersBitBoard) -> f32 { let light_pieces = board.pieces_bits() & !board.color_bits(); let dark_pieces = board.pieces_bits() & board.color_bits(); |