[board] Implement a custom Debug for Position

Newlines help a lot!

board/src/position.rs

@@ -4,8 +4,9 @@ use crate::bitboard::BitBoard;
 use crate::piece::{Piece, PiecePlacementError};
 use crate::square::Square;
 use std::fmt;
+use std::fmt::Write;
 
-#[derive(Clone, Debug, Eq, Hash, PartialEq)]
+#[derive(Clone, Eq, Hash, PartialEq)]
 pub struct Position {
     /// Composite bitboards for all the pieces of a particular color.
     pieces_per_color: [BitBoard; 2],
@@ -87,6 +88,32 @@ impl Position {
     fn bitboard_for_piece_mut(&mut self, piece: &Piece) -> &mut BitBoard {
         &mut self.pieces_per_type[piece.color as usize][piece.shape as usize]
     }
+
+impl fmt::Debug for Position {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let mut output = String::new();
+
+        output.push_str("Position {\n");
+        write!(output, "    color_to_move: {:?},\n", self.color_to_move)?;
+
+        output.push_str("    pieces_per_color: [\n");
+        for bb in self.pieces_per_color {
+            write!(output, "        {bb:?},\n")?;
+        }
+        output.push_str("    ],\n");
+
+        output.push_str("    pieces_per_type: [\n");
+        for color_bbs in self.pieces_per_type {
+            output.push_str("        [\n");
+            for bb in color_bbs {
+                write!(output, "            {bb:?},\n")?;
+            }
+            output.push_str("        ],\n");
+        }
+        output.push_str("    ],\n");
+
+        write!(f, "{}", output)
+    }
 }
 
 impl fmt::Display for Position {