use indicatif::{ProgressIterator, ProgressStyle};

use crate::{
    board::{Board, Winner},
    piece::Piece,
};

#[derive(Clone, Debug)]
pub struct Move {
    /// `i` position of move
    i: usize,

    /// `j` position of move
    j: usize,

    /// [`Board`] state after move is made
    board: Board,

    /// Current winner of the match
    winner: Winner,

    /// Index of this move's parent
    parent: Option<usize>,

    /// Indices of this Move's Children
    children: Vec<usize>,

    /// Value of this move
    value: i64,

    color: Piece,

    lazy_children: bool,
}

impl Move {
    pub const fn coords(&self) -> (usize, usize) {
        (self.i, self.j)
    }

    fn compute_self_value(&self, agent_color: Piece) -> i64 {
        if self.winner == Winner::Player(!agent_color) {
            // if this board results in the opponent winning, MAJORLY negatively weigh this move
            // NOTE! this branch isn't completely deleted because if so, the bot wouldn't make a move.
            // We shouldn't prune branches because we still need to always react to the opponent's moves
            return i64::MIN;
        } else if self.winner == Winner::Player(agent_color) {
            // results in a win for the agent
            return i64::MAX;
        } else if self.winner == Winner::Tie {
            // idk what a Tie should be valued?
            return 0;
        }

        let mut self_value = self.board.net_score(agent_color) as i64;
        let corner_v_agent = Board::sides()
            .filter(|&(i, j)| self.board.get_piece(i, j, agent_color))
            .count() as i64;
        let corner_v_not_agent = Board::sides()
            .filter(|&(i, j)| self.board.get_piece(i, j, !agent_color))
            .count() as i64;

        // make net-corner capture important
        self_value += (corner_v_agent - corner_v_not_agent) * 4;

        self_value
    }
}

pub struct FutureMoves {
    /// Arena containing all [`Move`]
    arena: Vec<Move>,

    /// Index of the [`Move`] tree's root node
    current_root: Option<usize>,

    /// Current generated depth of the Arena
    current_depth: usize,

    /// Target depth of children to generate
    max_depth: usize,

    /// How many deep should the lazy children status expire?
    lazy_expire: usize,

    /// Color w.r.t
    agent_color: Piece,
}

impl FutureMoves {
    pub const fn new(agent_color: Piece, max_depth: usize, lazy_expire: usize) -> Self {
        Self {
            arena: Vec::new(),
            current_root: None,
            current_depth: 0,
            max_depth,
            agent_color,
            lazy_expire,
        }
    }

    pub fn len(&self) -> usize {
        self.arena.len()
    }

    pub fn is_empty(&self) -> bool {
        self.len() == 0
    }

    /// Generate children for all children of `nodes`
    /// only `pub` for the sake of benchmarking
    pub fn extend_layers(&mut self) {
        let mut next_nodes: Vec<usize> = (0..self.arena.len())
            // we want to select all nodes that don't have children, or are lazy (need to maybe be regenerated)
            .filter(|&idx| {
                let got = &self.arena[idx];
                got.lazy_children || got.children.is_empty()
            })
            .filter(|&idx| self.is_connected_to_root(idx)) // put here so this will not extend needlessly before prunes
            .collect();

        for i in self.current_depth..=self.max_depth {
            next_nodes = next_nodes
                .into_iter()
                .progress_with_style(
                    ProgressStyle::with_template(&format!(
                        "Generating children (depth: {}/{}): ({{pos}}/{{len}}) {{per_sec}}",
                        i, self.max_depth
                    ))
                    .unwrap(),
                )
                .flat_map(|node_idx| {
                    self.generate_children(
                        node_idx,
                        if self.arena[node_idx].lazy_children {
                            self.depth_of(node_idx) - 1
                        } else {
                            i
                        } > self.lazy_expire,
                    )
                })
                .flatten()
                .collect();
        }
        self.current_depth = self.max_depth;
    }

    /// Determines if a [`Move`] at index `idx` is connected to `self.current_root`
    /// Returns `false` if `self.current_root` is None
    fn is_connected_to_root(&self, idx: usize) -> bool {
        if let Some(root) = self.current_root {
            let mut current = Some(idx);
            while let Some(parent_idx) = current {
                if parent_idx == root {
                    return true;
                }
                current = self.arena[parent_idx].parent;
            }
        }

        false
    }

    /// Creates children for a parent (`parent`), returns an iterator it's children's indexes
    fn generate_children(
        &mut self,
        parent_idx: usize,
        lazy_children: bool,
    ) -> Option<impl Iterator<Item = usize>> {
        let parent = &self.arena[parent_idx];

        // early-exit if a winner for the parent already exists
        if parent.winner != Winner::None {
            return None;
        }

        let new_color = !parent.color;
        let mut new: Vec<Move> =
            // use [`Board::all_positions`] here instead of [`Board::possible_moves`]
            // because we use [`Board::what_if`] later and we want to reduce calls to [`Board::propegate_from_dry`]
            Board::all_positions()
                .flat_map(|(i, j)| parent.board.what_if(i, j, new_color).map(move |x| (i, j, x)))
                .map(|(i, j, new_board)| Move {
                    i,
                    j,
                    board: new_board,
                    winner: new_board.game_winner(),
                    parent: Some(parent_idx),
                    children: Vec::new(),
                    value: 0,
                    color: new_color,
                    lazy_children,
                }).collect();

        // negative, because we want the max value to be at the first index
        new.sort_by_key(|x| -x.compute_self_value(self.agent_color));

        // keep the TOP_K children `self.agent_color`-color moves
        const TOP_K_CHILDREN: usize = 1;

        // we want to keep only the best move of the agent
        if lazy_children && new_color == self.agent_color && new.len() > TOP_K_CHILDREN {
            new.drain(TOP_K_CHILDREN..);
        }

        let start_idx = self.arena.len();
        if parent.lazy_children && !lazy_children {
            self.arena[parent_idx].lazy_children = false;
            // this move's children are being regenerated after lazy child expiration, don't append first node
            if new.len() > TOP_K_CHILDREN {
                self.arena.extend(new.drain(TOP_K_CHILDREN..));
            } else {
                // nothing will be appended
                // even though it was sorted the first time around
                // there's still no more than one element (which is already in the arena)
                return None;
            }
        } else {
            self.arena.extend(new);
        }
        let new_indices = start_idx..self.arena.len();

        self.arena[parent_idx].children.extend(new_indices.clone());

        Some(new_indices)
    }

    /// Given an index from `self.arena`, what depth is it at? 1-indexed (ROOT IS AT INDEX 1)
    fn depth_of(&self, node_idx: usize) -> usize {
        let mut depth = 0;
        let mut current = Some(node_idx);
        while let Some(parent_idx) = current {
            depth += 1;
            current = self.arena[parent_idx].parent;
        }

        depth
    }

    fn compute_values(&mut self, indexes: impl Iterator<Item = usize>) {
        // PERF! pre-organize all indexes based on what depth they're at
        // previously, I did a lookup map based on if a node was visited, still resulted in a full
        // O(n) iteration each depth
        let mut by_depth: Vec<Vec<usize>> = (0..=self.max_depth + 2).map(|_| Vec::new()).collect();
        for idx in indexes {
            let depth = self.depth_of(idx);
            // -1 because `depth_of` is one-indexed
            by_depth[depth - 1].push(idx);
        }

        for (depth, nodes) in by_depth.into_iter().enumerate().rev() {
            for idx in nodes {
                let node = &self.arena[idx];
                let self_value = node.compute_self_value(self.agent_color) / (depth + 1) as i64;

                let children_value = node
                    .children
                    .iter()
                    .rev() // rev then reverse so we get an index starting from the back
                    .enumerate()
                    // since children are sorted by value, we should weight the first one more
                    .map(|(i, &child)| self.arena[child].value * (i as i64 + 1))
                    .sum::<i64>()
                    .checked_div(node.children.len() as i64)
                    .unwrap_or(0);

                self.arena[idx].value = self_value + children_value;
            }
        }
    }

    pub fn best_move(&self) -> Option<(usize, usize)> {
        self.current_root
            .and_then(|x| {
                self.arena[x]
                    .children
                    .iter()
                    .max_by_key(|&&idx| self.arena[idx].value)
            })
            .inspect(|&&x| {
                assert_eq!(
                    self.arena[x].color, self.agent_color,
                    "selected move color should be the same as the color of the agent"
                );
            })
            .map(|&x| self.arena[x].coords())
    }

    /// Updates `FutureMoves` based on the current state of the board
    /// The board is supposed to be after the opposing move
    pub fn update(&mut self, board: &Board) {
        let curr_board = self
            .arena
            .iter()
            .enumerate()
            .find(|(_, m)| {
                &m.board == board && (m.parent == self.current_root) && self.current_root.is_some()
            })
            .map(|(idx, _)| idx);

        if let Some(curr_board_idx) = curr_board {
            self.update_root_idx(curr_board_idx);
        } else {
            self.create_root_raw(*board);
            self.update_root_idx(0);
        }
    }

    pub fn create_root_raw(&mut self, board: Board) {
        self.arena.clear();
        self.arena.push(Move {
            i: 0,
            j: 0,
            board,
            winner: board.game_winner(),
            parent: None,
            children: Vec::new(),
            value: 0,
            color: !self.agent_color,
            lazy_children: false,
        });
    }

    /// Update the root based on the coordinate of the move
    /// Returns a boolean, `true` if the operation was successful, false if not
    #[must_use = "You must check if the root was properly set"]
    pub fn update_root_coord(&mut self, i: usize, j: usize) -> bool {
        self.arena
            .iter()
            .enumerate()
            .find(|(_, node)| {
                node.parent == self.current_root
                    && self.current_root.is_some()
                    && node.coords() == (i, j)
            })
            .map(|x| x.0)
            .inspect(|&root| self.update_root_idx(root))
            .is_some()
    }

    fn update_root_idx_raw(&mut self, idx: usize) {
        self.current_root = Some(idx);
        self.current_depth -= self.depth_of(idx) - 1;
    }

    fn update_root_idx(&mut self, idx: usize) {
        self.update_root_idx_raw(idx);

        self.refocus_tree();
        self.extend_layers();
        self.compute_values(0..self.arena.len());
    }

    /// Rebuilds the Arena based on `self.current_root`, prunes unrelated nodes
    fn refocus_tree(&mut self) {
        let Some(root) = self.current_root else {
            return;
        };

        // make sure `root` doesn't reference another node
        self.arena[root].parent = None;

        let mut retain = vec![false; self.arena.len()];

        // stack is going to be AT MAXIMUM, the size of the array,
        // so lets just pre-allocate it
        let mut stack: Vec<usize> = Vec::with_capacity(self.arena.len());
        stack.push(root);

        // traverse children of the current root
        while let Some(idx) = stack.pop() {
            retain[idx] = true;
            stack.extend(self.arena[idx].children.iter());
        }

        let mut index_map = vec![None; self.arena.len()];

        self.arena = retain
            .into_iter()
            .enumerate() // old_idx
            .zip(self.arena.drain(..))
            .flat_map(|((old_idx, keep), node)| keep.then_some((old_idx, node))) // filter out unrelated nodes
            .enumerate() // new_idx
            .map(|(new_idx, (old_idx, mut node))| {
                index_map[old_idx] = Some(new_idx);

                if let Some(parent) = node.parent.as_mut() {
                    if let Some(new_parent) = index_map[*parent] {
                        *parent = new_parent;
                    } else {
                        // make sure we don't have dangling parents
                        node.parent = None;
                    }
                }

                node.children.retain_mut(|c| {
                    if let Some(new_c) = index_map[*c] {
                        *c = new_c;
                        true
                    } else {
                        false
                    }
                });

                node
            })
            .collect();

        self.current_root = index_map[root];
    }
}