feat(web client): local heuristic bot

clients/web/src/game/trictrac/bot_local.rs

@@ -1,5 +1,4 @@
-use rand::prelude::IndexedRandom;
-use trictrac_store::{CheckerMove, Color, GameState, MoveRules, Stage, TurnStage};
+use trictrac_store::{Board, CheckerMove, Color, GameState, MoveRules, Stage, TurnStage};
 
 use super::types::{PlayerAction, PreGameRollState};
 
@@ -29,15 +28,65 @@ pub fn bot_decide(game: &GameState, pgr: Option<&PreGameRollState>) -> Option<Pl
         TurnStage::Move | TurnStage::HoldOrGoChoice => {
             let rules = MoveRules::new(&Color::Black, &game.board, game.dice);
             let sequences = rules.get_possible_moves_sequences(true, vec![]);
-            let mut rng = rand::rng();
-            let (m1, m2) = sequences
-                .choose(&mut rng)
-                .cloned()
-                .unwrap_or((CheckerMove::default(), CheckerMove::default()));
             // MoveRules with Color::Black mirrors the board internally, so
             // returned move coordinates are in mirrored (White) space — mirror back.
+            let (m1, m2) = sequences
+                .iter()
+                .max_by(|(m1a, m2a), (m1b, m2b)| {
+                    score_seq(&game.board, m1a, m2a)
+                        .partial_cmp(&score_seq(&game.board, m1b, m2b))
+                        .unwrap_or(std::cmp::Ordering::Equal)
+                })
+                .cloned()
+                .unwrap_or((CheckerMove::default(), CheckerMove::default()));
             Some(PlayerAction::Move(m1.mirror(), m2.mirror()))
         }
         _ => None,
     }
 }
+
+/// Score a candidate move sequence from the bot's (Black) perspective.
+/// `m1` and `m2` are in mirrored (White) space, as returned by MoveRules for Color::Black.
+fn score_seq(board: &Board, m1: &CheckerMove, m2: &CheckerMove) -> f32 {
+    let mut b = board.mirror();
+    let _ = b.move_checker(&Color::White, *m1);
+    let _ = b.move_checker(&Color::White, *m2);
+    evaluate(&b)
+}
+
+/// Evaluate a board position from White's perspective (call after mirroring for Black).
+fn evaluate(board: &Board) -> f32 {
+    let mut score = 0.0f32;
+
+    let white_fields = board.get_color_fields(Color::White);
+    let black_fields = board.get_color_fields(Color::Black);
+
+    // Quarter fill progress — quarters 1-6, 7-12, 19-24.
+    // Quarter 13-18 is skipped: field 13 is the opponent's rest corner so White can never fill it.
+    for &q in &[1usize, 7, 19] {
+        if board.is_quarter_filled(Color::White, q) {
+            score += 8.0;
+        } else {
+            let missing = board.get_quarter_filling_candidate(Color::White);
+            score += (6 - missing.len().min(6)) as f32 * 0.3;
+        }
+    }
+
+    // Singleton exposure: penalise a White singleton at field f only when there is at least
+    // one Black checker at a field g > f (opponent can potentially threaten it).
+    let max_black_field = black_fields.iter().map(|(f, _)| *f).max().unwrap_or(0);
+    for (f, count) in &white_fields {
+        if *count == 1 && *f < max_black_field {
+            score -= 0.5;
+        }
+    }
+
+    // Exit zone progress: reward checkers already in fields 19-24.
+    for (field, count) in &white_fields {
+        if *field >= 19 {
+            score += count.abs() as f32 * 0.3;
+        }
+    }
+
+    score
+}