[explorer] Add fen and make commands

Clean up the implementation of the place command.

Track state with a State struct that contains a position and a builder. The place
command will place a new piece and then regenerate the position.

The make command makes a move. The syntax is:

    make [color:w|b] [shape] [from square] [to square]

The fen command prints a FEN string representing the position.

explorer/src/main.rs

@@ -29,28 +29,31 @@ fn command_line() -> Command {
         {all-args}
     ";
 
-    // strip out name/version
-    const APPLET_TEMPLATE: &str = "\
-        {about-with-newline}\n\
-        {usage-heading}\n    {usage}\n\
-        \n\
-        {all-args}{after-help}\
-    ";
-
     Command::new("explorer")
         .multicall(true)
         .arg_required_else_help(true)
         .subcommand_required(true)
-        .subcommand_value_name("APPLET")
+        .subcommand_value_name("CMD")
-        .subcommand_help_heading("APPLETS")
+        .subcommand_help_heading("COMMANDS")
         .help_template(PARSER_TEMPLATE)
-        .subcommand(Command::new("print").about("Print the board"))
+        .subcommand(Command::new("fen").about("Print the current position as a FEN string"))
+        .subcommand(
+            Command::new("make")
+                .arg(Arg::new("piece").required(true))
+                .arg(Arg::new("from").required(true))
+                .arg(Arg::new("to").required(true))
+                .about("Make a move"),
+        )
         .subcommand(
             Command::new("place")
+                .arg(Arg::new("color").required(true))
                 .arg(Arg::new("piece").required(true))
-                .arg(Arg::new("square").required(true)),
+                .arg(Arg::new("square").required(true))
+                .about("Place a piece on the board"),
         )
-        .subcommand(Command::new("quit").about("Quit the program"))
+        .subcommand(Command::new("print").about("Print the board"))
+        .subcommand(Command::new("quit").alias("exit").about("Quit the program"))
+        .subcommand(Command::new("starting").about("Reset the board to the starting position"))
 }
 
 fn respond(line: &str, state: &mut State) -> Result<CommandResult, String> {
@@ -67,20 +70,72 @@ fn respond(line: &str, state: &mut State) -> Result<CommandResult, String> {
             result.should_continue = false;
             result.should_print_position = false;
         }
-        Some(("place", _matches)) => {
+        Some(("fen", _matches)) => {
-            let piece_arg = _matches.get_one::<String>("piece").unwrap();
+            println!(
-            let shape = match piece_arg.chars().nth(0) {
+                "{}",
-                Some(c) => Shape::try_from(c).map_err(|_| ()),
+                state
-                None => Err(()),
+                    .position
-            }
+                    .to_fen()
-            .map_err(|_| "Error: invalid piece specifier")?;
+                    .map_err(|_| "error: Unable to generate FEN for current position")?
+            );
 
-            let square_arg = _matches.get_one::<String>("square").unwrap();
+            result.should_print_position = false;
-            let square = Square::from_algebraic_str(square_arg)
+        }
+        Some(("make", matches)) => {
+            let shape = matches
+                .get_one::<String>("piece")
+                .ok_or("Missing piece descriptor")?;
+            let shape = Shape::try_from(shape).map_err(|_| "Invalid piece descriptor")?;
+
+            let from_square = Square::from_algebraic_str(
+                matches.get_one::<String>("from").ok_or("Missing square")?,
+            )
+            .map_err(|_| "Error: invalid square specifier")?;
+
+            let to_square = Square::from_algebraic_str(
+                matches.get_one::<String>("to").ok_or("Missing square")?,
+            )
+            .map_err(|_| "Error: invalid square specifier")?;
+
+            let mv = MoveBuilder::new(
+                Piece::new(state.position.player_to_move(), shape),
+                from_square,
+                to_square,
+            )
+            .build();
+
+            state.position = MakeMoveBuilder::new(&state.position)
+                .make(&mv)
+                .map_err(|err| format!("error: Cannot make move: {:?}", err))?
+                .build();
+            state.builder = PositionBuilder::from_position(&state.position);
+        }
+        Some(("place", matches)) => {
+            let color = matches
+                .get_one::<String>("color")
+                .ok_or("Missing color descriptor")?;
+            let color = Color::try_from(color).map_err(|_| "Invalid color descriptor")?;
+
+            let shape = matches
+                .get_one::<String>("piece")
+                .ok_or("Missing piece descriptor")?;
+            let shape = Shape::try_from(shape).map_err(|_| "Invalid piece descriptor")?;
+
+            let square = matches
+                .get_one::<String>("square")
+                .ok_or("Missing square")?;
+            let square = Square::from_algebraic_str(square)
                 .map_err(|_| "Error: invalid square specifier")?;
 
-            pos.place_piece(&Piece::new(Color::White, shape), &square)
+            let piece = PlacedPiece::new(Piece::new(color, shape), square);
-                .map_err(|_| "Error: Unable to place piece")?;
+
+            state.builder.place_piece(piece);
+            state.position = state.builder.build();
+        }
+        Some(("starting", _matches)) => {
+            let starting_position = Position::starting();
+            state.builder = PositionBuilder::from_position(&starting_position);
+            state.position = starting_position;
         }
         Some((name, _matches)) => unimplemented!("{name}"),
         None => unreachable!("Subcommand required"),
@@ -107,7 +162,7 @@ fn main() -> Result<(), String> {
             println!("{} to move.", state.position.player_to_move());
         }
 
-        let readline = editor.readline("? ");
+        let readline = editor.readline("\n? ");
         match readline {
             Ok(line) => {
                 let line = line.trim();