/* Copyright 2023 Christophe Parent (christopheparent@thatspaceandtime.org)
 *
 * This file is part of OOX.
 *
 * OOX is free software: you can redistribute it and/or modify it under the terms of the GNU
 * General Public License as published by the Free Software Foundation, either version 3 of the
 * License, or (at your option) any later version.
 *
 * OOX is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even
 * the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General
 * Public License for more details.
 *
 * You should have received a copy of the GNU General Public License along with OOX. If not, see
 * <https://www.gnu.org/licenses/>.
 */

use std::collections::HashMap;
use std::error::Error;
use std::fmt::Display;
use std::fmt::Formatter;
use std::fmt::Result as FmtResult;
use std::fmt::Write;
use std::io::stdin;
use std::process;

const BOARD_SIZE: u32 = 3;
const FIRST_COLUMN_LABEL: u32 = 0x61; // LATIN SMALL LETTER A in UTF-8
const LAST_COLUMN_LABEL: u32 = FIRST_COLUMN_LABEL + BOARD_SIZE - 1;
const FIRST_ROW_LABEL: u32 = 0x31; // DIGIT ONE in UTF-8
const LAST_ROW_LABEL: u32 = FIRST_ROW_LABEL + BOARD_SIZE - 1;

enum Player {
    P1,
    P2,
}

impl Player {
    fn next(&mut self) {
        *self = match self {
            Player::P1 => Player::P2,
            Player::P2 => Player::P1,
        };
    }
}

impl Display for Player {
    fn fmt(&self, formatter: &mut Formatter) -> FmtResult {
        write!(
            formatter,
            "{}",
            match self {
                Player::P1 => "1",
                Player::P2 => "2",
            }
        )
    }
}

struct Coordinates {
    row: u32,
    column: u32,
}

#[derive(Clone, Copy, PartialEq)]
enum Mark {
    X,
    O,
}

enum Status {
    SpaceAlreadyTaken,
    InProgress,
    Won,
    Draw,
}

fn draw(board: &[Option<Mark>]) -> FmtResult {
    println!();
    for row_index in 0..BOARD_SIZE as usize {
        let mut row = (BOARD_SIZE as usize - row_index).to_string();
        for column_index in 0..BOARD_SIZE as usize {
            write!(
                row,
                " {}",
                match board[row_index * BOARD_SIZE as usize + column_index] {
                    Some(mark) => match mark {
                        Mark::X => "X",
                        Mark::O => "O",
                    },
                    None => "•",
                }
            )?;
        }
        println!("{}", row);
    }
    let mut footer_row = String::from(" ");
    for x in FIRST_COLUMN_LABEL..=LAST_COLUMN_LABEL {
        write!(footer_row, " {}", char::from_u32(x).unwrap_or(' '))?;
    }
    println!("{}\n", footer_row);
    Ok(())
}

fn get_input() -> Result<Coordinates, String> {
    let mut input = String::new();
    if let Err(error) = stdin().read_line(&mut input) {
        return Err(error.to_string());
    }
    let mut chars = input.trim().chars();
    let coordinates = Coordinates {
        column: match chars.next() {
            Some(x) if x as u32 >= FIRST_COLUMN_LABEL && x as u32 <= LAST_COLUMN_LABEL => {
                x as u32 - FIRST_COLUMN_LABEL
            }
            _ => return Err("That input is invalid.".to_string()),
        },
        row: match chars.next() {
            Some(x) if x as u32 >= FIRST_ROW_LABEL && x as u32 <= LAST_ROW_LABEL => {
                BOARD_SIZE as u32 - 1 - (x as u32 - FIRST_ROW_LABEL)
            }
            _ => return Err("That input is invalid.".to_string()),
        },
    };
    match chars.next() {
        Some(_) => Err("That input is invalid.".to_string()),
        _ => Ok(coordinates),
    }
}

fn process_logic(
    current_player: &Player,
    board: &mut [Option<Mark>],
    lines: &mut HashMap<u32, (u32, Option<Mark>)>,
    input_coordinates: Coordinates,
) -> Result<Status, String> {
    let current_index =
        match usize::try_from(input_coordinates.row * BOARD_SIZE + input_coordinates.column) {
            Ok(x) => x,
            Err(error) => return Err(error.to_string()),
        };
    if board[current_index] != None {
        return Ok(Status::SpaceAlreadyTaken);
    }
    let current_mark = match current_player {
        Player::P1 => Mark::X,
        Player::P2 => Mark::O,
    };
    board[current_index] = Some(current_mark);

    // The matching lines are the lines where the current space is located.
    let mut matching_lines = vec![input_coordinates.row, BOARD_SIZE + input_coordinates.column];
    if input_coordinates.row == input_coordinates.column {
        matching_lines.push(BOARD_SIZE * 2);
    }
    if input_coordinates.row + input_coordinates.column == BOARD_SIZE - 1 {
        matching_lines.push(BOARD_SIZE * 2 + 1);
    }
    for line in matching_lines {
        if let Some((number_free_spaces, mark_option)) = lines.get(&line) {
            // A line with two different marks is removed from the hash map.
            if mark_option != &None && mark_option != &Some(current_mark) {
                lines.remove(&line);
            // Otherwise, a line with more than one free space is filled with the current mark.
            } else if number_free_spaces > &1 {
                lines.insert(line, (number_free_spaces - 1, Some(current_mark)));
            // Otherwise, ie. when a line only has one free space,
            // that last free space is filled and the game is won.
            } else {
                return Ok(Status::Won);
            }
        };
    }
    // The game is a draw if the board is entirely filled up and there is no winner.
    if board.iter().all(|x| x.is_some()) {
        return Ok(Status::Draw);
    }
    Ok(Status::InProgress)
}

fn main() -> Result<(), Box<dyn Error>> {
    println!("OOX");
    println!("A Tic‐tac‐toe game");

    let mut current_player = Player::P1;
    let mut board = [None; (BOARD_SIZE * BOARD_SIZE) as usize];
    // Lines are rows, columns, and diagonals that can be a win.
    // A hash map is used to keep track of what lines are still up for grabs and worth checking.
    // The keys are 0-indexed integers, starting with the rows in order, followed by the columns
    // in order, followed by the top-left-to-bottom-right and top-right-to-bottom-left diagonals.
    let mut lines = (0..BOARD_SIZE * 2 + 2)
        .map(|i| (i, (BOARD_SIZE, None)))
        .collect::<HashMap<u32, (u32, Option<Mark>)>>();

    loop {
        if let Err(error) = draw(&board) {
            println!("{}", error);
            process::exit(1);
        }

        println!("Waiting for Player {}...", current_player);
        let input_coordinates = match get_input() {
            Ok(x) => x,
            Err(error) => {
                println!("{}", error);
                continue;
            }
        };

        match process_logic(&current_player, &mut board, &mut lines, input_coordinates) {
            Ok(status) => match status {
                Status::SpaceAlreadyTaken => println!("That space is already taken."),
                Status::InProgress => current_player.next(),
                Status::Won => {
                    if let Err(error) = draw(&board) {
                        println!("{}", error);
                        process::exit(1);
                    }
                    println!("Player {} won the game!", current_player);
                    break;
                }
                Status::Draw => {
                    if let Err(error) = draw(&board) {
                        println!("{}", error);
                        process::exit(1);
                    }
                    println!("The game ended in a draw!");
                    break;
                }
            },
            Err(error) => {
                println!("{}", error);
                process::exit(1);
            }
        };
    }

    println!("\nGAME OVER");
    Ok(())
}