[explorer] Implement a reset command

Resets the board to an empty or starting state, or to a position specified by a FEN string.

explorer/src/main.rs

@@ -63,6 +63,17 @@ fn command_line() -> Command {
                 .arg(Arg::new("square").required(true))
                 .about("Show moves of a piece on a square"),
         )
+        .subcommand(
+            Command::new("reset")
+                .subcommand(Command::new("clear").about("Reset to a cleared board"))
+                .subcommand(Command::new("starting").about("Reset to the starting position"))
+                .subcommand(
+                    Command::new("fen")
+                        .arg(Arg::new("fen").required(true))
+                        .about("Reset to a position described by a FEN string"),
+                )
+                .about("Reset the board"),
+        )
         .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"))
@@ -171,6 +182,7 @@ fn respond(line: &str, state: &mut State) -> Result<CommandResult, String> {
             let starting_position = Position::starting();
             state.position = starting_position;
         }
+        Some(("reset", matches)) => do_reset_command(state, matches)?,
         Some((name, _matches)) => unimplemented!("{name}"),
         None => unreachable!("Subcommand required"),
     }
@@ -178,6 +190,21 @@ fn respond(line: &str, state: &mut State) -> Result<CommandResult, String> {
     Ok(result)
 }
 
+fn do_reset_command(state: &mut State, matches: &clap::ArgMatches) -> Result<(), String> {
+    match matches.subcommand() {
+        None | Some(("clear", _)) => state.position = Position::empty(),
+        Some(("starting", _)) => state.position = Position::starting(),
+        Some(("fen", matches)) => {
+            let fen = matches.get_one::<String>("fen").ok_or("Missing FEN")?;
+            let board = Board::from_fen_str(fen).map_err(|err| format!("{err}"))?;
+            state.position = Position::new(board);
+        }
+        Some((name, _matches)) => unimplemented!("{name}"),
+    }
+
+    Ok(())
+}
+
 fn main() -> Result<(), String> {
     let mut editor = DefaultEditor::new().map_err(|err| format!("Error: {err}"))?;