use crate::position::{Alignment, Direction, Grid2d, Position2d};
use crate::tile::Tile;

const DEFAULT_BOARD_SIZE: usize = 19;

/// A board of tiles.
///
/// This is a fixed-size 2D grid of tiles, where each tile is either empty or
/// contains a single tile.
///
/// This struct is implement `Default` so that you can use [`Board::default()`]
/// to create a new board with a default size. To create a board with a custom
/// size, use [`Board::new()`].
#[derive(Debug, Clone, PartialEq, Eq)]
#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
pub struct Board {
    tiles: Vec<Option<Tile>>,
    width: usize,
    height: usize,
}

impl Board {
    /// Creates a new board with the given size.
    pub fn new(width: usize, height: usize) -> Self {
        Self {
            tiles: vec![None; width * height],
            width,
            height,
        }
    }

    /// Gets the tile at the given position, or `None` if there is no tile.
    ///
    /// # Panics
    /// Panics if the position is out of bounds.
    pub fn get(&self, x: usize, y: usize) -> Option<Tile> {
        self.tiles[y * self.width + x]
    }

    /// Sets the tile at the given position.
    ///
    /// If there was already a tile at the position, it is replaced.
    ///
    /// # Panics
    /// Panics if the position is out of bounds.
    pub fn set(&mut self, x: usize, y: usize, tile: Tile) {
        self.tiles[y * self.width + x] = Some(tile);
    }

    /// Removes and returns the tile at the given position.
    ///
    /// If there was no tile at the position, `None` is returned.
    ///
    /// # Panics
    /// Panics if the position is out of bounds.
    pub fn take(&mut self, x: usize, y: usize) -> Option<Tile> {
        self.tiles[y * self.width + x].take()
    }

    /// Gets the difference between this board and another.
    ///
    /// This returns a vector of positions at which the tiles differ.
    /// The order is not guaranteed.
    pub fn difference(&self, other: &Board) -> Vec<Position2d> {
        let mut diff = Vec::new();
        for y in 0..self.height {
            for x in 0..self.width {
                if self.get(x, y) != other.get(x, y) {
                    diff.push(Position2d::new(x, y));
                }
            }
        }
        diff
    }

    /// Gets all chains of tiles that are adjacent to the given positions.
    pub fn find_chains(&self, positions: &[Position2d]) -> Vec<Vec<Position2d>> {
        let mut chains = Vec::new();
        for &pos in positions {
            for &alignment in &[Alignment::Horizontal, Alignment::Vertical] {
                if let Some(start) = self.find_starting_tile(pos, alignment) {
                    if let Some(chain) = self.find_chain_in_direction(start, alignment) {
                        if chain.len() > 1 {
                            chains.push(chain);
                        }
                    }
                }
            }
        }
        chains
    }

    /// Tests whether the given positions form a unique chain.
    pub fn is_unique_chain(&self, positions: &Vec<Position2d>) -> bool {
        Self::is_aligned(positions, Alignment::Horizontal)
            && self.belong_to_same_chain(positions, Direction::Right)
            || Self::is_aligned(positions, Alignment::Vertical)
                && self.belong_to_same_chain(positions, Direction::Down)
    }

    /// Tests whether all the tiles can be placed contiguously.
    ///
    /// Such tiles can be placed in a single move and can reuse existing tiles
    /// on the board if the exact same tile present at the correct position in
    /// the `tiles` slice.
    ///
    /// # Examples
    /// ```
    /// use board_shared::board::Board;
    /// use board_shared::tile::{Digit, Tile};
    ///
    /// let mut board = Board::default();
    /// board.set(2, 0, Tile::Equals);
    /// let guess = vec![
    ///     Tile::Digit(Digit::new(7)),
    ///     Tile::Equals,
    ///     Tile::Digit(Digit::new(7)),
    /// ];
    ///
    /// // The equals sign is already on the board at the correct position,
    /// // so it can be reused.
    /// assert!(board.is_playable(&guess, (1, 0).into(), board_shared::position::Direction::Right));
    ///
    /// // The equals sign exist on the board but at the wrong position,
    /// // so it cannot be reused.
    /// assert!(!board.is_playable(&guess, (2, 0).into(), board_shared::position::Direction::Right));
    /// ```
    pub fn is_playable(
        &self,
        tiles: &[Tile],
        starting_from: Position2d,
        direction: Direction,
    ) -> bool {
        let mut pos = starting_from;
        let mut it = tiles.iter();
        while let Some(tile) = it.next() {
            if let Some(existing) = self.get(pos.x, pos.y) {
                if existing != *tile {
                    return false;
                }
            }
            if let Some(relative) = pos.relative(direction, self) {
                pos = relative;
            } else {
                return it.next().is_none();
            }
        }
        true
    }

    /// Finds the starting tile of a chain in the given direction.
    fn find_starting_tile(&self, pos: Position2d, alignment: Alignment) -> Option<Position2d> {
        self.get(pos.x, pos.y)?;
        let mut pos = pos;
        let direction = alignment.start();
        loop {
            if let Some(relative) = pos.relative(direction, self) {
                if self.get(relative.x, relative.y).is_some() {
                    pos = relative;
                } else {
                    return Some(pos);
                }
            } else {
                return Some(pos);
            }
        }
    }

    /// Finds a chain of tiles in the given direction.
    fn find_chain_in_direction(
        &self,
        pos: Position2d,
        direction: Alignment,
    ) -> Option<Vec<Position2d>> {
        let mut chain = Vec::new();
        let mut pos = pos;
        loop {
            chain.push(pos);
            if let Some(relative) = pos.relative(direction.end(), self) {
                if self.get(relative.x, relative.y).is_none() {
                    break;
                }
                pos = relative;
            } else {
                break;
            }
        }
        if chain.is_empty() {
            None
        } else {
            Some(chain)
        }
    }

    /// Tests whether the given positions are part of the same chain.
    fn belong_to_same_chain(&self, positions: &[Position2d], direction: Direction) -> bool {
        let mut it = positions.iter().copied().peekable();
        while let Some(mut pos) = it.next() {
            if let Some(&next) = it.peek() {
                while !pos.is_contiguous(next) {
                    if let Some(relative) = pos.relative(direction, self) {
                        pos = relative;
                        if self.get(pos.x, pos.y).is_none() {
                            return false;
                        }
                    } else {
                        return false;
                    }
                }
            }
        }
        true
    }

    /// Determines whether the given positions are contiguous.
    ///
    /// Contiguous means that the positions are adjacent in a straight line, either
    /// horizontally or vertically. This does not check the tiles at the positions.
    ///
    /// You may want to use [`Board::is_unique_chain`] to check if the positions are
    /// contiguous based on the current board state.
    pub fn is_contiguous(positions: &[Position2d]) -> Option<bool> {
        let mut it = positions.iter();
        let first = *it.next()?;
        let mut second = *it.next()?;

        let orientation = match (second.x.checked_sub(first.x), second.y.checked_sub(first.y)) {
            (Some(0), Some(1)) => (0, 1),
            (Some(1), Some(0)) => (1, 0),
            (_, _) => return Some(false),
        };

        for &pos in it {
            if pos.x != second.x + orientation.0 || pos.y != second.y + orientation.1 {
                return Some(false);
            }
            second = pos;
        }

        Some(true)
    }

    /// Determines whether the given positions are aligned.
    pub fn is_aligned(positions: &[Position2d], alignement: Alignment) -> bool {
        if let Some(&first) = positions.first() {
            positions
                .iter()
                .all(|&pos| alignement.is_aligned(first, pos))
        } else {
            true
        }
    }

    /// Determines whether the given positions have any alignment.
    pub fn has_alignment(positions: &[Position2d]) -> bool {
        Self::is_aligned(positions, Alignment::Horizontal)
            || Self::is_aligned(positions, Alignment::Vertical)
    }

    /// Gets a linear iterator over the tiles, row by row.
    ///
    /// # Example:
    /// ```
    /// use board_shared::board::Board;
    ///
    /// let board = Board::default();
    /// let placed_tiles = board.iter().filter(Option::is_some).count();
    /// assert_eq!(placed_tiles, 0);
    /// ```
    pub fn iter(&self) -> impl Iterator<Item = Option<Tile>> + '_ {
        self.tiles.iter().copied()
    }
}

impl Grid2d for Board {
    fn width(&self) -> usize {
        self.width
    }

    fn height(&self) -> usize {
        self.height
    }
}

impl Default for Board {
    fn default() -> Self {
        let size = DEFAULT_BOARD_SIZE * DEFAULT_BOARD_SIZE;
        let mut tiles = Vec::with_capacity(size);
        tiles.resize_with(size, || None);
        Self {
            tiles,
            width: DEFAULT_BOARD_SIZE,
            height: DEFAULT_BOARD_SIZE,
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::tile::Digit;

    fn positions(input: &[(usize, usize)]) -> Vec<Position2d> {
        input
            .iter()
            .map(|(x, y)| Position2d::new(*x, *y))
            .collect::<Vec<_>>()
    }

    #[test]
    fn test_is_contiguous() {
        assert_eq!(Board::is_contiguous(&[]), None);
        assert_eq!(Board::is_contiguous(&positions(&[(0, 0)])), None);
        assert_eq!(
            Board::is_contiguous(&positions(&[(0, 0), (0, 2)])),
            Some(false)
        );
        assert_eq!(
            Board::is_contiguous(&positions(&[(0, 0), (2, 0)])),
            Some(false)
        );
        assert_eq!(
            Board::is_contiguous(&positions(&[(0, 0), (0, 1), (0, 2)])),
            Some(true)
        );
        assert_eq!(
            Board::is_contiguous(&positions(&[(1, 0), (2, 0), (3, 0), (4, 0)])),
            Some(true)
        );
        assert_eq!(
            Board::is_contiguous(&positions(&[(0, 0), (0, 1), (1, 3)])),
            Some(false)
        );
        assert_eq!(
            Board::is_contiguous(&positions(&[(0, 0), (0, 1), (0, 2), (1, 2)])),
            Some(false)
        );
    }

    #[test]
    fn test_is_aligned() {
        assert!(Board::is_aligned(&[], Alignment::Horizontal));
        assert!(Board::is_aligned(&[], Alignment::Vertical));
        assert!(Board::is_aligned(
            &positions(&[(0, 0)]),
            Alignment::Vertical
        ));
        assert!(Board::is_aligned(
            &positions(&[(0, 0)]),
            Alignment::Horizontal
        ));
        assert!(Board::is_aligned(
            &positions(&[(0, 0), (0, 1)]),
            Alignment::Vertical
        ));
        assert!(Board::is_aligned(
            &positions(&[(0, 0), (1, 0)]),
            Alignment::Horizontal
        ));
        assert!(!Board::is_aligned(
            &positions(&[(0, 0), (1, 0)]),
            Alignment::Vertical
        ));
        assert!(!Board::is_aligned(
            &positions(&[(0, 0), (0, 1)]),
            Alignment::Horizontal
        ));
    }

    #[test]
    fn test_find_chains() {
        let mut board = Board::default();
        for x in 1..5 {
            board.set(x, 2, Tile::Equals);
        }
        assert_eq!(
            board.find_chains(&[Position2d::new(0, 0)]),
            Vec::<Vec<Position2d>>::new()
        );
        let expected = vec![vec![
            Position2d::new(1, 2),
            Position2d::new(2, 2),
            Position2d::new(3, 2),
            Position2d::new(4, 2),
        ]];
        assert_eq!(board.find_chains(&[Position2d::new(1, 2)]), expected);
        assert_eq!(board.find_chains(&[Position2d::new(2, 2)]), expected);
        assert_eq!(board.find_chains(&[Position2d::new(4, 2)]), expected);
    }

    #[test]
    fn test_is_unique_chain_empty_board() {
        let board = Board::default();
        assert_eq!(
            board.is_unique_chain(&vec![Position2d::new(0, 0), Position2d::new(0, 1),]),
            true
        );
        assert_eq!(
            board.is_unique_chain(&vec![
                Position2d::new(0, 0),
                Position2d::new(0, 1),
                Position2d::new(1, 1),
            ]),
            false
        );
        assert_eq!(board.is_unique_chain(&vec![]), true);
    }

    #[test]
    fn test_is_unique_chain_existing_board() {
        let mut board = Board::default();
        board.set(0, 1, Tile::Digit(Digit::new(2)));
        board.set(0, 2, Tile::Equals);
        board.set(1, 1, Tile::Equals);
        assert_eq!(board.is_unique_chain(&positions(&[(0, 0), (0, 3)])), true);
        assert_eq!(
            board.is_unique_chain(&positions(&[(0, 0), (0, 3), (0, 4)])),
            true
        );
        assert_eq!(board.is_unique_chain(&positions(&[(1, 1), (2, 1)])), true);
        assert_eq!(board.is_unique_chain(&positions(&[(1, 1), (3, 1)])), false);
    }

    #[test]
    fn test_is_playable() {
        let board = Board::new(5, 5);
        let tiles = vec![
            Tile::Digit(Digit::new(5)),
            Tile::Digit(Digit::new(6)),
            Tile::Digit(Digit::new(9)),
        ];
        assert_eq!(
            board.is_playable(&tiles, (1, 1).into(), Direction::Right),
            true
        );
        assert_eq!(
            board.is_playable(&tiles, (4, 0).into(), Direction::Down),
            true
        );
    }

    #[test]
    fn test_is_playable_constrained() {
        let board = Board::new(2, 2);
        let tiles = vec![Tile::Digit(Digit::new(7)), Tile::Digit(Digit::new(8))];
        assert_eq!(
            board.is_playable(&tiles, (0, 0).into(), Direction::Right),
            true
        );
        assert_eq!(
            board.is_playable(&tiles, (0, 0).into(), Direction::Down),
            true
        );
        assert_eq!(
            board.is_playable(&tiles, (1, 0).into(), Direction::Right),
            false
        );
    }

    #[test]
    fn test_is_playable_blocking_pieces() {
        let mut board = Board::new(3, 3);
        board.set(1, 1, Tile::Digit(Digit::new(1)));
        let tiles = vec![Tile::Digit(Digit::new(0)), Tile::Digit(Digit::new(0))];
        assert_eq!(
            board.is_playable(&tiles, (0, 0).into(), Direction::Right),
            true
        );
        assert_eq!(
            board.is_playable(&tiles, (0, 1).into(), Direction::Right),
            false
        );
        assert_eq!(
            board.is_playable(&tiles, (1, 0).into(), Direction::Down),
            false
        );
    }
}