[board] Implement a DiagramFormatter that writes a graphical diagram of a Position

Remove fmt::Display from Position and move display to a DiagramFormatter type that implements that trait and writes a position to a formatter.

board/src/position/diagram_formatter.rs

@@ -0,0 +1,61 @@
+// Eryn Wells <eryn@erynwells.me>
+
+use crate::{Position, Square};
+use std::fmt;
+use std::fmt::Write;
+
+pub struct DiagramFormatter<'a>(&'a Position);
+
+impl<'a> fmt::Display for DiagramFormatter<'a> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let mut output = String::new();
+
+        output.push_str("  +-----------------+\n");
+
+        for rank in (0..8).rev() {
+            write!(output, "{} | ", rank + 1)?;
+
+            for file in 0..8 {
+                let square = Square::from_rank_file(rank, file).unwrap();
+                match self.0.piece_on_square(square) {
+                    Some(placed_piece) => write!(output, "{} ", placed_piece.piece)?,
+                    None => output.push_str(". "),
+                }
+            }
+
+            output.push_str("|\n");
+        }
+
+        output.push_str("  +-----------------+\n");
+        output.push_str("    a b c d e f g h\n");
+
+        write!(f, "{}", output)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::piece::{Color, Piece};
+    use crate::Position;
+
+    #[test]
+    fn empty() {
+        let pos = Position::empty();
+        let diagram = DiagramFormatter(&pos);
+        println!("{}", diagram);
+    }
+
+    #[test]
+    fn one_king() {
+        let mut pos = Position::empty();
+        pos.place_piece(
+            &Piece::king(Color::Black),
+            &Square::from_algebraic_str("h3").expect("h3"),
+        )
+        .expect("Unable to place piece");
+
+        let diagram = DiagramFormatter(&pos);
+        println!("{}", diagram);
+    }
+}

board/src/position/mod.rs

@@ -1,5 +1,7 @@
+mod diagram_formatter;
 mod pieces;
 mod position;
 
+pub use diagram_formatter::DiagramFormatter;
 pub use pieces::Pieces;
 pub use position::Position;

board/src/position/position.rs

@@ -144,31 +144,6 @@ impl fmt::Debug for Position {
     }
 }
 
-impl fmt::Display for Position {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        let mut output = String::new();
-
-        output.push_str("   +-----------------+\n");
-
-        for rank in (0..8).rev() {
-            output.push_str("{rank} | ");
-
-            for file in 0..8 {
-                let square = Square::from_rank_file(rank, file);
-                // TODO: Get piece at square
-                output.push_str(". ");
-            }
-
-            output.push_str("|\n");
-        }
-
-        output.push_str("   +-----------------+\n");
-        output.push_str("     a b c d e f g h\n");
-
-        write!(f, "{}", output)
-    }
-}
-
 #[cfg(test)]
 mod tests {
     use super::*;