othello/src/board.rs

use crate::{misc::split_from, piece::Piece};
use std::fmt;

pub const BOARD_SIZE: usize = 8;

#[derive(Copy, Clone)]
pub struct Board {
    board: [[Option<Piece>; BOARD_SIZE]; BOARD_SIZE],
}

impl fmt::Display for Board {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        let horiz_sep_line = "-".repeat(BOARD_SIZE * 2 + 1);
        for i in 0..BOARD_SIZE {
            writeln!(f, "{}", horiz_sep_line)?;

            write!(f, "|")?;
            for j in 0..BOARD_SIZE {
                write!(
                    f,
                    "{}|",
                    self.get(i, j).as_ref().map(Piece::text).unwrap_or(" ")
                )?;
            }
            writeln!(f)?;
        }

        // put a line at the bottom of the board too
        writeln!(f, "{}", horiz_sep_line)?;

        Ok(())
    }
}

impl Board {
    pub const fn new() -> Self {
        Self {
            board: [[None; BOARD_SIZE]; BOARD_SIZE],
        }
    }

    pub const fn starting_pos(mut self) -> Self {
        self.place_unchecked(3, 3, Piece::White);
        self.place_unchecked(4, 3, Piece::Black);
        self.place_unchecked(3, 4, Piece::Black);
        self.place_unchecked(4, 4, Piece::White);
        self
    }

    /// Returns a mutable reference to a place on the [`Board`]
    /// at (i, j)
    pub const fn get_mut(&mut self, i: usize, j: usize) -> &mut Option<Piece> {
        &mut self.board[i][j]
    }

    /// Returns a reference to a place on the [`Board`]
    /// at (i, j)
    pub const fn get(&self, i: usize, j: usize) -> &Option<Piece> {
        &self.board[i][j]
    }

    const fn place_unchecked(&mut self, i: usize, j: usize, piece: Piece) {
        *self.get_mut(i, j) = Some(piece);
    }

    pub fn what_if(&self, i: usize, j: usize, piece: Piece) -> Option<(Self, usize)> {
        let mut self_copy = *self;
        if self_copy.get(i, j).is_some() {
            return None;
        }
        self_copy.place_unchecked(i, j, piece);
        let how_many_prop = self_copy.propegate_from(i, j);
        Some((self_copy, how_many_prop))
    }

    pub fn place(&mut self, i: usize, j: usize, piece: Piece) -> Result<(), String> {
        let what_if_result = self.what_if(i, j, piece);
        if let Some(what_if_result) = what_if_result {
            if what_if_result.1 > 0 {
                *self = what_if_result.0;
                return Ok(());
            }
        }
        Err("move would not propegate".to_string())
    }

    /// Propegate piece captures originating from (i, j)
    /// DO NOT USE THIS ALONE, this should be called as a part of
    /// [`Board::place`] or [`Board::place_and_prop_unchecked`]
    fn propegate_from(&mut self, i: usize, j: usize) -> usize {
        // returns if that place is empty
        let Some(starting_color) = *self.get(i, j) else {
            return 0;
        };

        // Create all chains from the piece being propegated from in `i` and `j` coordinates
        let mut chains: Vec<Vec<(usize, usize)>> = split_from(0, BOARD_SIZE - 1, i)
            .into_iter()
            .map(|range| range.into_iter().map(|i| (i, j)).collect())
            .collect();

        chains.extend(
            split_from(0, BOARD_SIZE - 1, j)
                .into_iter()
                .map(|range| range.into_iter().map(|j| (i, j)).collect()),
        );

        // TODO! add diagonals to `chains` here

        let mut captured: usize = 0;

        for chain in chains {
            for (chain_length, &(new_i, new_j)) in chain.iter().enumerate() {
                let Some(piece) = self.get(new_i, new_j) else {
                    break;
                };

                if piece == &starting_color {
                    if chain_length > 0 {
                        // fill all opposite colors with this color
                        let Some(history) = chain.get(..chain_length) else {
                            break;
                        };
                        captured += history.len();

                        // fill all opposite colors with this color
                        for &(i_o, j_o) in history {
                            self.place_unchecked(i_o, j_o, starting_color);
                        }
                    }
                    // either the other pieces were replaced, or this was an invalid chain,
                    // in both cases, the loop needs to be breaked
                    break;
                }
            }
        }
        captured
    }
}

#[cfg(test)]
mod test {
    use super::*;

    #[test]
    fn place_and_get() {
        let mut board = Board::new();
        assert_eq!(board.get(0, 0), &None);
        assert_eq!(
            board.place(0, 0, Piece::Black),
            Ok(()),
            "placing black on (0, 0)"
        );
        assert_eq!(board.get(0, 0), &Some(Piece::Black));
    }

    #[test]
    fn place_and_capture_simple() {
        let mut board = Board::new();
        assert_eq!(
            board.place(0, 0, Piece::Black),
            Ok(()),
            "first move of piece black to (0, 0)"
        );
        assert_eq!(
            board.place(0, 1, Piece::White),
            Ok(()),
            "white move to (0, 1)"
        );
        assert_eq!(
            board.place(0, 2, Piece::Black),
            Ok(()),
            "black counter, capturing white"
        );
        assert_eq!(board.get(0, 1), &Some(Piece::Black));
    }

    #[test]
    fn failed_capture() {
        let mut board = Board::new();

        assert_eq!(board.place(0, 0, Piece::Black), Ok(()));

        assert_eq!(board.place(0, 2, Piece::White), Ok(()));

        assert_eq!(board.place(0, 3, Piece::Black), Ok(()));

        assert_eq!(
            board.get(0, 1),
            &None,
            "(0, 1) was overridden even though it's an empty space"
        );
    }

    #[test]
    fn long_capture_horiz() {
        let mut board = Board::new();

        assert_eq!(board.place(0, 0, Piece::Black), Ok(()));

        for j in 1..=6 {
            assert_eq!(board.place(0, j, Piece::White), Ok(()));
        }

        assert_eq!(board.place(0, 7, Piece::Black), Ok(()));

        for j in 2..=6 {
            assert_eq!(
                board.get(0, j),
                &Some(Piece::Black),
                "should be black at: ({}, {})",
                0,
                j
            );
        }
    }

    #[test]
    fn long_capture_vert() {
        let mut board = Board::new();

        assert_eq!(board.place(0, 0, Piece::Black), Ok(()));

        for i in 1..=6 {
            assert_eq!(board.place(i, 0, Piece::White), Ok(()));
        }

        assert_eq!(board.place(7, 0, Piece::Black), Ok(()));

        for i in 2..=6 {
            assert_eq!(
                board.get(i, 0),
                &Some(Piece::Black),
                "should be black at: ({}, {})",
                i,
                0
            );
        }
    }
}