brutus tweaks

src/agents/brutus.rs

@@ -2,6 +2,7 @@ use std::{cmp::Ordering, iter};
 
 use anyhow::Result;
 use itertools::Itertools;
+use rand::{seq::SliceRandom, thread_rng};
 use rayon::iter::{ParallelBridge, ParallelIterator};
 
 use super::{Agent, Chase, MaximizerAgent, MinimizerAgent};
@@ -27,14 +28,20 @@ impl Brutus {
 			.collect_vec();
 
 		if symbols.is_empty() {
-			return Chase::new(self.player).step_max(board);
+			return if minimize {
+				Chase::new(self.player).step_min(board)
+			} else {
+				Chase::new(self.player).step_max(board)
+			};
 		}
 
 		// Number of free slots
 		let n_free = board.get_board().iter().filter(|x| x.is_none()).count();
 
 		// Number of slots we need to fill with numbers
-		let n_fill = n_free - symbols.len();
+		// Add one if we have two or fewer symbols available, so that we can
+		// account for one unused symbol while keeping a reasonable runtime
+		let n_fill = n_free - symbols.len() + if symbols.len() <= 2 { 1 } else { 0 };
 
 		let mut items = iter::repeat(None)
 			.take(n_fill)
@@ -70,7 +77,7 @@ impl Brutus {
 			.collect::<Vec<_>>();
 
 		if minimize {
-			// Sort from smallest midpoint to biggest midpoint
+			// Sort from smallest to biggest midpoint
 			items.sort_by(|a, b| a.1.partial_cmp(&b.1).unwrap_or(Ordering::Equal));
 		} else {
 			items.sort_by(|a, b| b.1.partial_cmp(&a.1).unwrap_or(Ordering::Equal));
@@ -88,15 +95,27 @@ impl Brutus {
 
 		let (t, _) = items.first().unwrap();
 
-		for (i, s) in t.get_board().iter().enumerate() {
-			if let Some(s) = s {
-				if s.is_op() && board.get_board()[i].is_none() {
-					return Ok(PlayerAction { pos: i, symb: *s });
+		let mut symbols = symbols.clone();
+		symbols.shuffle(&mut thread_rng());
+
+		// Place a random unused symbol
+		for target_s in symbols {
+			for (i, s) in t.get_board().iter().enumerate() {
+				if let Some(s) = s {
+					if board.get_board()[i].is_none() && target_s == *s {
+						return Ok(PlayerAction { pos: i, symb: *s });
+					}
 				}
 			}
 		}
 
-		unreachable!()
+		// Final escape hatch, if we didn't decide to place any symbols
+		// (which is possible, since we add one to free_spots above!)
+		if minimize {
+			Chase::new(self.player).step_min(board)
+		} else {
+			Chase::new(self.player).step_max(board)
+		}
 	}
 }