#![allow(clippy::cast_possible_truncation)]

use itertools::Itertools;
use lazy_static::lazy_static;
use pathfinding::prelude::{astar, idastar};
use rand::prelude::*;
use rand::rngs::OsRng;
use std::thread;
use std::time::Instant;

#[cfg(test)]
const SIDE: u8 = 3;
#[cfg(not(test))]
const SIDE: u8 = 4;
const LIMIT: usize = (SIDE * SIDE) as usize;

#[allow(clippy::derived_hash_with_manual_eq)]
#[derive(Clone, Debug, Hash)]
struct Game {
    positions: [u8; LIMIT], // Correct position of piece at every index
    hole_idx: u8,           // Current index of the hole
    weight: u8,             // Current sum of pieces Manhattan distances
}

impl PartialEq for Game {
    fn eq(&self, other: &Game) -> bool {
        self.hole_idx == other.hole_idx
            && self.weight == other.weight
            && self.positions == other.positions
    }
}

impl Eq for Game {}

lazy_static! {
    static ref GOAL: Game = Game {
        positions: {
            let mut p = [0u8; LIMIT];
            for (i, e) in p.iter_mut().enumerate() {
                *e = i as u8;
            }
            p
        },
        hole_idx: 0,
        weight: 0,
    };
    static ref SUCCESSORS: Vec<Vec<u8>> = (0..SIDE * SIDE)
        .map(|idx| (0..4)
            .filter_map(|dir| match dir {
                0 if idx % SIDE > 0 => Some(idx - 1),
                1 if idx >= SIDE => Some(idx - SIDE),
                2 if idx % SIDE < SIDE - 1 => Some(idx + 1),
                3 if idx < SIDE * SIDE - SIDE => Some(idx + SIDE),
                _ => None,
            })
            .collect::<Vec<_>>())
        .collect();
}

impl Game {
    /// Move the hole to the given index.
    fn switch(&self, idx: u8) -> Game {
        let mut g = self.clone();
        g.positions.swap(self.hole_idx as usize, idx as usize);
        g.hole_idx = idx;
        g.weight = g.weight
            + g.distance(self.hole_idx) // Distance of the moved piece at its new index
            - self.distance(idx); // Distance of the moved piece at its previous index
        g
    }

    #[inline]
    fn x(pos: u8) -> u8 {
        pos % SIDE
    }

    #[inline]
    fn y(pos: u8) -> u8 {
        pos / SIDE
    }

    // Compute the Manhattan distance between the piece at idx and its correct position.
    fn distance(&self, idx: u8) -> u8 {
        let (actual_x, actual_y) = (Self::x(idx), Self::y(idx));
        let (correct_x, correct_y) = (
            Self::x(self.positions[idx as usize]),
            Self::y(self.positions[idx as usize]),
        );
        actual_x.abs_diff(correct_x) + actual_y.abs_diff(correct_y)
    }

    fn solved(&self) -> bool {
        self.positions == GOAL.positions
    }

    // Here we try to illustrate that we can return an iterator without building a Vec.
    // However, since the successors are the current board with the hole moved one
    // position, we need to build a clone of the current board that will be reused in
    // this iterator.
    fn successors(&self) -> impl Iterator<Item = (Game, u8)> {
        let game = self.clone();
        SUCCESSORS[self.hole_idx as usize]
            .iter()
            .map(move |&n| (game.switch(n), 1))
    }

    fn is_solvable(&self) -> bool {
        let mut inversions = 0;
        for i in 0..LIMIT {
            let c = self.positions[i];
            if c != 0 {
                for j in i + 1..LIMIT {
                    let d = self.positions[j];
                    if d != 0 && d < c {
                        inversions ^= 1;
                    }
                }
            }
        }
        if SIDE % 2 == 1 {
            inversions == 0
        } else {
            Self::y(self.hole_idx) % 2 == inversions
        }
    }

    fn from_array(positions: [u8; LIMIT]) -> Game {
        let hole_idx = positions.iter().find_position(|&&n| n == 0).unwrap().0 as u8;
        let mut game = Game {
            positions,
            hole_idx,
            weight: 0,
        };
        game.weight = (0..u8::try_from(LIMIT).unwrap())
            .filter(|&n| n != game.hole_idx)
            .map(|n| game.distance(n))
            .sum();
        game
    }

    fn shuffled() -> Game {
        let mut rng = OsRng;
        let mut positions = Self::default().positions;
        loop {
            positions.shuffle(&mut rng);
            let game = Self::from_array(positions);
            if game.is_solvable() {
                return game;
            }
        }
    }
}

impl Default for Game {
    fn default() -> Game {
        GOAL.clone()
    }
}

#[test]
fn test() {
    // main() already contains checks.
    main();
}

fn main() {
    let b = Game::shuffled();
    println!("{b:?}");
    assert!(b.is_solvable());
    let start = Instant::now();
    let (astar_result, idastar_result) = thread::scope(|s| {
        let idastar_handle = s.spawn({
            || {
                let result = idastar(&b, Game::successors, |b| b.weight, Game::solved).unwrap();
                println!("idastar: {} moves in {:.3?}", result.1, start.elapsed(),);
                assert!(result.0.last().unwrap().weight == 0);
                result.1
            }
        });
        (
            {
                let result = astar(&b, Game::successors, |b| b.weight, Game::solved).unwrap();
                println!("astar: {} moves in {:.3?}", result.1, start.elapsed(),);
                assert!(result.0.last().unwrap().weight == 0);
                result.1
            },
            idastar_handle.join().unwrap(),
        )
    });
    println!("Total execution time: {:.3?}", start.elapsed());
    assert_eq!(idastar_result, astar_result);
    assert!(idastar_result >= b.weight);
}