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//! Defines the `Qsearch` trait.

use uci::SetOption;
use value::*;
use depth::*;
use move_generator::MoveGenerator;


/// Parameters describing a quiescence search.
///
/// **Important note:** `lower_bound` and `upper_bound` fields
/// together give the interval within which an as precise as possible
/// evaluation is required. If during the search is determined that
/// the exact evaluation is outside of this interval, the search may
/// return a value that is closer to the the interval bounds than the
/// exact evaluation, but always staying on the correct side of the
/// interval (i.e. "fail-soft").
pub struct QsearchParams<'a, T: MoveGenerator + 'a> {
    /// A mutable reference to the root position for the search.
    ///
    /// **Important note:** The search routine may use this reference
    /// to do and undo moves, but when the search is finished, all
    /// played moves must be taken back so that the board is restored
    /// to its original state.
    pub position: &'a mut T,

    /// The requested search depth.
    ///
    /// This is the depth at which the main search stops and the
    /// quiescence search takes on. It should be between `DEPTH_MIN`
    /// and `0`. The quiescence search implementation may decide to
    /// perform less thorough analysis when `depth` is smaller than
    /// zero.
    pub depth: Depth,

    /// The lower bound for the new search.
    ///
    /// Should be no lesser than `VALUE_MIN`.
    pub lower_bound: Value,

    /// The upper bound for the new search.
    ///
    /// Should be greater than `lower_bound`, but no greater than
    /// `VALUE_MAX`.
    pub upper_bound: Value,

    /// Position's static evaluation, or `VALUE_UNKNOWN`.
    ///
    /// Saves the re-calculation if position's static evaluation is
    /// already available.
    pub static_eval: Value,
}


/// A trait for quiescence searches' results.
pub trait QsearchResult: Clone {
    /// Creates a new instance.
    ///
    /// * `value` -- the calculated evaluation for the position. Must
    ///   be between `VALUE_EVAL_MIN` and `VALUE_EVAL_MAX`.
    ///
    /// * `searched_nodes` -- the number of positions searched to
    ///   calculate the evaluation.
    fn new(value: Value, searched_nodes: u64) -> Self;

    /// Returns the calculated evaluation for the position.
    ///
    /// Will always be between `VALUE_EVAL_MIN` and `VALUE_EVAL_MAX`.
    fn value(&self) -> Value;

    /// Retruns the number of positions searched to calculate the evaluation.
    fn searched_nodes(&self) -> u64;
}


/// A trait for performing quiescence searches.
///
/// Quiescence search is a restricted search which considers only a
/// limited set of moves (for example: winning captures, pawn
/// promotions to queen, check evasions). The goal is to statically
/// evaluate only "quiet" positions (positions where there are no
/// winning tactical moves to be made). Although this search can
/// cheaply and correctly resolve many simple tactical issues, it is
/// completely blind to the more complex ones. To implement your own
/// quiescence search routine, you must define a type that implements
/// the `Qsearch` trait.
pub trait Qsearch: SetOption + Send {
    /// The type of move generator that the implementation works with.
    type MoveGenerator: MoveGenerator;

    /// The type of result object that the search produces.
    type QsearchResult: QsearchResult;

    /// Performs a quiescence search and returns a result object.
    fn qsearch(params: QsearchParams<Self::MoveGenerator>) -> Self::QsearchResult;
}