Function inanis::engine::search::run_internal

fn run_internal<const ROOT: bool, const PV: bool, const DIAG: bool>(
    context: &mut SearchContext,
    depth: i8,
    ply: u16,
    alpha: i16,
    beta: i16,
    allow_null_move: bool,
    friendly_king_checked: bool
) -> i16

Entry point of the regular search, with generic ROOT parameter indicating if this is the root node where the moves filterigh might happen, and PV parameter determining if the current node is a PV (principal variation) node in the PVS framework. The implementation contains a typical alpha-beta approach, together with a bunch of reductions and prunings to optimize search. The most important parameter here, context, contains the current state of the search, board state, statistics, and is passed by reference to all nodes. Besides obvious parameters like depth, ply, alpha and beta, there’s also allow_null_move which prevents two null move checks in a row, and friendly_king_checked which is used to share friendly king check status between nodes (it’s always calculated one depth earlier, as it’s used as one of the LMR constraints). If DIAG is set to true, additional statistics will be gathered (with a small performance penalty).

Search steps for PV node:

• test of abort flag
• test of initial constraints: abort flag, forced depth
• test if the enemy king is checked
• test if there’s threefold repetition draw, fifty move rule draw or insufficient material draw
• switch to the quiescence search if the depth is equal to zero
• read from the transposition table, return score if possible or update alpha/beta (https://www.chessprogramming.org/Transposition_Table)
• main loop:
  • filter moves (if ROOT is set)
  • late move reduction (https://www.chessprogramming.org/Late_Move_Reductions)
  • PVS framework (https://www.chessprogramming.org/Principal_Variation_Search)
• test if stalemate draw is detected
• update transposition table

Search steps for non-PV node:

• test of abort flag
• test of initial constraints: abort flag, forced depth, max nodes count
• test if the enemy king is checked
• test if there’s threefold repetition draw, fifty move rule draw or insufficient material draw
• switch to the quiescence search if the depth is equal to zero
• read from the transposition table, return score if possible or update alpha/beta (https://www.chessprogramming.org/Transposition_Table)
• razoring (https://www.chessprogramming.org/Razoring)
• static null move pruning (https://www.chessprogramming.org/Reverse_Futility_Pruning)
• null move pruning (https://www.chessprogramming.org/Null_Move_Pruning)
• main loop:
  • filter moves (if ROOT is set)
  • late move pruning (https://www.chessprogramming.org/Futility_Pruning#MoveCountBasedPruning)
  • late move reduction (https://www.chessprogramming.org/Late_Move_Reductions)
  • PVS framework (https://www.chessprogramming.org/Principal_Variation_Search)
• test if stalemate draw is detected
• update transposition table