From 634876822b48a9fabc5dd4ac9b7381c0406f6123 Mon Sep 17 00:00:00 2001
From: Eryn Wells <eryn@erynwells.me>
Date: Sun, 8 Jun 2025 16:49:55 -0700
Subject: [PATCH] [explorer] Add a load command

Loads a board position from a FEN string.

Plumb through setting the Zobrist state on an existing board. Rebuild the hash
when setting the state.
---
 board/src/board.rs       |  9 +++++++++
 explorer/src/main.rs     | 25 ++++++++++++++++++++++++-
 position/src/position.rs | 14 ++++++++++++++
 3 files changed, 47 insertions(+), 1 deletion(-)

diff --git a/board/src/board.rs b/board/src/board.rs
index a3afd64..6a93eef 100644
--- a/board/src/board.rs
+++ b/board/src/board.rs
@@ -306,6 +306,15 @@ impl Board {
             zobrist.set_hash_value(new_hash);
         }
     }
+
+    pub fn zobrist_state(&self) -> Option<Arc<ZobristState>> {
+        self.zobrist_hash.as_ref().map(ZobristHash::state)
+    }
+
+    pub fn set_zobrist_state(&mut self, state: Arc<ZobristState>) {
+        self.zobrist_hash = Some(ZobristHash::new(state));
+        self.recompute_zobrist_hash();
+    }
 }
 
 impl Board {
diff --git a/explorer/src/main.rs b/explorer/src/main.rs
index 9be313a..ea8ce26 100644
--- a/explorer/src/main.rs
+++ b/explorer/src/main.rs
@@ -47,6 +47,12 @@ fn command_line() -> Command {
         .subcommand_help_heading("COMMANDS")
         .help_template(PARSER_TEMPLATE)
         .subcommand(Command::new("fen").about("Print the current position as a FEN string"))
+        .subcommand(
+            Command::new("load")
+                .arg(Arg::new("fen").required(true))
+                .alias("l")
+                .about("Load a board position from a FEN string"),
+        )
         .subcommand(
             Command::new("make")
                 .arg(Arg::new("from").required(true))
@@ -75,7 +81,7 @@ fn command_line() -> Command {
         .subcommand(
             Command::new("movement")
                 .arg(Arg::new("square").required(true))
-                .about("Show moves of a piece on a square."),
+                .about("Show moves of a piece on a square"),
         )
         .subcommand(
             Command::new("reset")
@@ -112,6 +118,7 @@ fn respond(line: &str, state: &mut State) -> anyhow::Result<CommandResult> {
     let mut result = CommandResult::default();
 
     match matches.subcommand() {
+        Some(("load", matches)) => result = do_load_command(state, matches)?,
         Some(("print", _matches)) => {}
         Some(("quit", _matches)) => {
             result.should_continue = false;
@@ -184,6 +191,22 @@ fn respond(line: &str, state: &mut State) -> anyhow::Result<CommandResult> {
     Ok(result)
 }
 
+fn do_load_command(state: &mut State, matches: &clap::ArgMatches) -> anyhow::Result<CommandResult> {
+    let fen_string = matches
+        .get_one::<String>("fen")
+        .ok_or(CommandHandlingError::MissingArgument("fen"))?;
+
+    let mut board = Board::from_fen_str(fen_string.as_str())?;
+    board.set_zobrist_state(state.zobrist.clone());
+
+    state.position = Position::new(board);
+
+    Ok(CommandResult {
+        should_continue: true,
+        should_print_position: true,
+    })
+}
+
 fn do_reset_command(
     state: &mut State,
     matches: &clap::ArgMatches,
diff --git a/position/src/position.rs b/position/src/position.rs
index 332bf65..f7b7c4a 100644
--- a/position/src/position.rs
+++ b/position/src/position.rs
@@ -2,6 +2,7 @@
 
 mod captures;
 
+use crate::fen::{FromFenStr, FromFenStrError};
 use captures::CapturesList;
 use chessfriend_bitboard::BitBoard;
 use chessfriend_board::{
@@ -230,6 +231,10 @@ impl Position {
     pub fn zobrist_hash(&self) -> Option<u64> {
         self.board.zobrist_hash()
     }
+
+    pub fn set_zobrist_state(&mut self, state: Arc<ZobristState>) {
+        self.board.set_zobrist_state(state);
+    }
 }
 
 impl Position {
@@ -238,6 +243,15 @@ impl Position {
     }
 }
 
+impl FromFenStr for Position {
+    type Error = FromFenStrError;
+
+    fn from_fen_str(string: &str) -> Result<Self, Self::Error> {
+        let board = Board::from_fen_str(string)?;
+        Ok(Position::new(board))
+    }
+}
+
 impl ToFenStr for Position {
     type Error = <Board as ToFenStr>::Error;