//Four Line Dropper Frontend/Backend - A webapp that allows one to play a game of Four Line Dropper
//Copyright (C) 2022 Stephen Seo
//
//This program 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.
//
//This program 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 this program. If not, see <https://www.gnu.org/licenses/>.
use std::collections::BTreeMap;

use crate::constants::{AI_EASY_MAX_CHOICES, AI_NORMAL_MAX_CHOICES, COLS, ROWS};
use crate::game_logic::check_win_draw;
use crate::random_helper::get_seeded_random;
use crate::state::{board_deep_clone, BoardState, BoardType, Turn};

const AI_THIRD_MAX_UTILITY: f64 = 0.89;

#[allow(dead_code)]
#[derive(Copy, Clone, Debug, PartialEq, Eq)]
pub enum AIDifficulty {
    Easy,
    Normal,
    Hard,
}

#[derive(Copy, Clone, Debug, PartialEq, Eq)]
pub enum SlotChoice {
    Slot0,
    Slot1,
    Slot2,
    Slot3,
    Slot4,
    Slot5,
    Slot6,
    Invalid,
}

impl From<SlotChoice> for usize {
    fn from(slot_choice: SlotChoice) -> Self {
        match slot_choice {
            SlotChoice::Slot0 => 0,
            SlotChoice::Slot1 => 1,
            SlotChoice::Slot2 => 2,
            SlotChoice::Slot3 => 3,
            SlotChoice::Slot4 => 4,
            SlotChoice::Slot5 => 5,
            SlotChoice::Slot6 => 6,
            SlotChoice::Invalid => (ROWS * COLS) as usize,
        }
    }
}

impl From<usize> for SlotChoice {
    fn from(idx: usize) -> Self {
        if idx >= (ROWS * COLS) as usize {
            return SlotChoice::Invalid;
        }

        match idx % (COLS as usize) {
            0 => SlotChoice::Slot0,
            1 => SlotChoice::Slot1,
            2 => SlotChoice::Slot2,
            3 => SlotChoice::Slot3,
            4 => SlotChoice::Slot4,
            5 => SlotChoice::Slot5,
            6 => SlotChoice::Slot6,
            _ => SlotChoice::Invalid,
        }
    }
}

pub fn get_ai_choice(
    difficulty: AIDifficulty,
    player: Turn,
    board: &BoardType,
) -> Result<SlotChoice, String> {
    let mut rng = get_seeded_random()?;

    let mut utilities = Vec::with_capacity(COLS as usize);
    for i in 0..(COLS as usize) {
        let slot = i.into();
        if slot == SlotChoice::Invalid {
            return Err("Internal error: get_ai_choice() iterated to SlotChoice::Invalid".into());
        }
        if let Some(utility) = get_utility_for_slot(player, slot, board) {
            utilities.push((i, utility));
        }
    }

    if utilities.is_empty() {
        return Err("All slots are full".into());
    }

    // shuffle utilities for the cases where there are equivalent utilities
    if utilities.len() > 1 {
        for i in 1..utilities.len() {
            utilities.swap(i, rng.rand_range(0..((i + 1) as u32)) as usize);
        }
    }

    let mut pick_some_of_choices = |amount: usize| -> Result<SlotChoice, String> {
        let mut maximums: BTreeMap<i64, usize> = BTreeMap::new();
        for (idx, utility) in &utilities {
            // f64 cannot be used as Key since it doesn't implement Ord.
            // Use i64 as a substitute, noting that the map stores in ascending
            // order.
            let mut utility_value = (utility * 10000.0) as i64;
            while maximums.contains_key(&utility_value) {
                utility_value += rng.rand_range(0..7) as i64 - 3;
            }
            maximums.insert(utility_value, *idx);
        }

        // don't pick from more items than what exists
        let mod_amount = if maximums.len() < amount {
            maximums.len()
        } else {
            amount
        };

        // don't use random if only 1 item is to be picked
        let random_number: usize = if mod_amount > 1 {
            rng.rand_u32() as usize % mod_amount
        } else {
            0
        };

        let rand_idx = maximums.len() - 1 - random_number;
        // turns the map into a vector of (key, value), then pick out of the
        // last few values by index the "value" which is the SlotChoice.
        // This is done because BTreeMap stores keys in ascending order.
        Ok((*maximums.iter().collect::<Vec<(&i64, &usize)>>()[rand_idx].1).into())
    };

    match difficulty {
        AIDifficulty::Easy => pick_some_of_choices(AI_EASY_MAX_CHOICES),
        AIDifficulty::Normal => pick_some_of_choices(AI_NORMAL_MAX_CHOICES),
        AIDifficulty::Hard => {
            // only pick the best option all the time
            let mut max = -1.0f64;
            let mut max_idx: usize = 0;
            for (idx, utility) in utilities {
                if utility > max {
                    max = utility;
                    max_idx = idx;
                }
            }
            Ok(max_idx.into())
        }
    }
}

/// Returns a value between 0.0 and 1.0 where 1.0 is highest utility
/// "None" indicates it is impossible to place at the given slot
fn get_utility_for_slot(player: Turn, slot: SlotChoice, board: &BoardType) -> Option<f64> {
    // get idx of location where dropped token will reside in
    let mut idx: usize = slot.into();
    if board[idx].get() != BoardState::Empty {
        // slot is full, cannot place in slot
        return None;
    }
    while idx < ((ROWS - 1) * COLS) as usize
        && board[idx + COLS as usize].get() == BoardState::Empty
    {
        idx += COLS as usize;
    }

    // check if placing a token here is a win
    if get_block_amount(player.get_opposite(), idx, 3, board) {
        return Some(1.0);
    }

    // check if placing a token here blocks a win
    if get_block_amount(player, idx, 3, board) {
        return Some(0.9);
    }

    let mut utility: f64 = 0.5;

    // check if placing a token here connects 2 pieces
    if get_block_amount(player.get_opposite(), idx, 2, board) {
        utility *= 1.22;
        if utility >= AI_THIRD_MAX_UTILITY {
            utility = AI_THIRD_MAX_UTILITY;
        }
    }

    // check if placing a token here blocks 2 pieces
    if get_block_amount(player, idx, 2, board) {
        utility *= 1.11;
        if utility >= AI_THIRD_MAX_UTILITY {
            utility = AI_THIRD_MAX_UTILITY;
        }
    }

    // check if placing a token here connects 1 piece
    if get_block_amount(player.get_opposite(), idx, 1, board) {
        utility *= 1.05;
        if utility >= AI_THIRD_MAX_UTILITY {
            utility = AI_THIRD_MAX_UTILITY;
        }
    }

    // check if placing a token here allows the opposing player to win
    if idx >= COLS as usize {
        let cloned_board = board_deep_clone(board);
        cloned_board[idx].replace(player.into());
        cloned_board[idx - (COLS as usize)].replace(player.get_opposite().into());
        if let Some((state, _)) = check_win_draw(&cloned_board) {
            if state == player.get_opposite().into() {
                utility *= 0.1;
            }
        }
    }

    Some(utility)
}

/// Returns true if placing a token at idx will block the opposite player that
/// has "amount" in a line (horizontally, vertically, and diagonally).
fn get_block_amount(player: Turn, idx: usize, amount: usize, board: &BoardType) -> bool {
    let opposite = player.get_opposite();

    // setup for checks
    let mut count = 0;
    let mut temp_idx = idx;

    // check left
    while temp_idx % (COLS as usize) > 0 {
        temp_idx -= 1;
        if board[temp_idx].get() == opposite.into() {
            count += 1;
            if count >= amount {
                return true;
            }
        } else {
            break;
        }
    }

    // check right
    // count = 0; // don't reset count, since horizontal may pass through idx
    temp_idx = idx;
    while temp_idx % (COLS as usize) < (COLS - 1) as usize {
        temp_idx += 1;
        if board[temp_idx].get() == opposite.into() {
            count += 1;
            if count >= amount {
                return true;
            }
        } else {
            break;
        }
    }

    // check down
    count = 0;
    temp_idx = idx;
    while temp_idx / (COLS as usize) < (ROWS - 1) as usize {
        temp_idx += COLS as usize;
        if board[temp_idx].get() == opposite.into() {
            count += 1;
            if count >= amount {
                return true;
            }
        } else {
            break;
        }
    }

    // check diagonal left down
    count = 0;
    temp_idx = idx;
    while temp_idx % (COLS as usize) > 0 && temp_idx / (COLS as usize) < (ROWS - 1) as usize {
        temp_idx = temp_idx - 1 + COLS as usize;
        if board[temp_idx].get() == opposite.into() {
            count += 1;
            if count >= amount {
                return true;
            }
        } else {
            break;
        }
    }

    // check diagonal right up
    // count = 0; // don't reset count as diagonal may pass through idx
    temp_idx = idx;
    while temp_idx % (COLS as usize) < (COLS - 1) as usize && temp_idx / (COLS as usize) > 0 {
        temp_idx = temp_idx + 1 - COLS as usize;
        if board[temp_idx].get() == opposite.into() {
            count += 1;
            if count >= amount {
                return true;
            }
        } else {
            break;
        }
    }

    // check diagonal right down
    count = 0;
    temp_idx = idx;
    while temp_idx % (COLS as usize) < (COLS - 1) as usize
        && temp_idx / (COLS as usize) < (ROWS - 1) as usize
    {
        temp_idx = temp_idx + 1 + COLS as usize;
        if board[temp_idx].get() == opposite.into() {
            count += 1;
            if count >= amount {
                return true;
            }
        } else {
            break;
        }
    }

    // check diagonal left up
    // count = 0; // don't reset count as diagonal may pass through idx
    temp_idx = idx;
    while temp_idx % (COLS as usize) > 0 && temp_idx / (COLS as usize) > 0 {
        temp_idx = temp_idx - 1 - COLS as usize;
        if board[temp_idx].get() == opposite.into() {
            count += 1;
            if count >= amount {
                return true;
            }
        } else {
            break;
        }
    }

    // exhausted all possible potential wins, therefore does not block a win
    false
}

#[cfg(test)]
mod tests {
    use crate::state::new_empty_board;

    use super::*;

    #[test]
    fn test_get_block_amount() {
        let board = new_empty_board();
        board[51].set(BoardState::Cyan);
        board[52].set(BoardState::Cyan);
        board[53].set(BoardState::Cyan);
        assert!(!get_block_amount(Turn::MagentaPlayer, 50, 4, &board));
        assert!(get_block_amount(Turn::MagentaPlayer, 50, 3, &board));
        assert!(get_block_amount(Turn::MagentaPlayer, 50, 2, &board));
        assert!(!get_block_amount(Turn::MagentaPlayer, 54, 4, &board));
        assert!(get_block_amount(Turn::MagentaPlayer, 54, 3, &board));
        assert!(get_block_amount(Turn::MagentaPlayer, 54, 2, &board));
    }
}