// Copyright 2022 Christian Ulrich
//
// 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 <http://www.gnu.org/licenses/>.

use std::io;
use std::str::FromStr;

#[derive(Clone, Copy)]
enum Direction {
    Up,
    Right,
    Down,
    Left,
}

#[derive(Debug, Clone, Copy)]
pub struct RopeState<const LEN: usize> {
    knots: [(isize, isize); LEN],
}

pub struct RopeStates<const LEN: usize> {
    direction: Direction,
    count: isize,
    state: RopeState<LEN>,
}

impl FromStr for Direction {
    type Err = &'static str;

    fn from_str(s: &str) -> Result<Self, Self::Err> {
        match s {
            "U" => Ok(Self::Up),
            "R" => Ok(Self::Right),
            "D" => Ok(Self::Down),
            "L" => Ok(Self::Left),
            _ => Err("cannot parse direction"),
        }
    }
}

impl<const LEN: usize> RopeState<LEN> {
    pub fn new() -> Self {
        Self {
            knots: [(0, 0); LEN],
        }
    }

    pub fn head_pos(&self) -> (isize, isize) {
        self.knots[0]
    }

    pub fn tail_pos(&self) -> (isize, isize) {
        self.knots[LEN - 1]
    }

    fn adjust_knot(&mut self, index: usize) {
        let x_diff = self.knots[index - 1].0.abs_diff(self.knots[index].0);
        let y_diff = self.knots[index - 1].1.abs_diff(self.knots[index].1);
        let (move_x, move_y) = match (x_diff, y_diff) {
            (2, 0) => (true, false),
            (2, 1) => (true, true),
            (0, 2) => (false, true),
            (1, 2) => (true, true),
            (2, 2) => (true, true),
            _ => (false, false),
        };
        if move_x {
            match (self.knots[index - 1].0, self.knots[index].0) {
                (h, t) if { h > t } => self.knots[index].0 += 1,
                (h, t) if { h < t } => self.knots[index].0 -= 1,
                _ => (),
            }
        }
        if move_y {
            match (self.knots[index - 1].1, self.knots[index].1) {
                (h, t) if { h > t } => self.knots[index].1 += 1,
                (h, t) if { h < t } => self.knots[index].1 -= 1,
                _ => (),
            }
        }
    }

    fn move_head(&mut self, direction: Direction) -> Result<RopeState<LEN>, &'static str> {
        const ERROR: &'static str = "invalid move";
        match direction {
            Direction::Up => self.knots[0].1 = self.knots[0].1.checked_add(1).ok_or(ERROR)?,
            Direction::Right => self.knots[0].0 = self.knots[0].0.checked_add(1).ok_or(ERROR)?,
            Direction::Down => self.knots[0].1 = self.knots[0].1.checked_sub(1).ok_or(ERROR)?,
            Direction::Left => self.knots[0].0 = self.knots[0].0.checked_sub(1).ok_or(ERROR)?,
        }
        for i in 1..LEN {
            self.adjust_knot(i);
        }
        Ok(*self)
    }
}

impl<const LEN: usize> Iterator for RopeStates<LEN> {
    type Item = Result<RopeState<LEN>, &'static str>;

    fn next(&mut self) -> Option<Self::Item> {
        if self.count == 0 {
            let mut line = String::new();
            match io::stdin().read_line(&mut line) {
                Ok(_) if { line == "" || line == "\n" } => return None,
                Ok(_) => {
                    if let Some((left, right)) = line.trim().split_once(' ') {
                        if let (Ok(direction), Ok(count)) = (left.parse(), right.parse()) {
                            self.direction = direction;
                            self.count = count;
                        } else {
                            return Some(Err("input file contains invalid line"));
                        }
                    } else {
                        return Some(Err("input file contains invalid line"));
                    }
                }
                Err(_) => assert!(false),
            }
        }
        self.count -= 1;
        Some(self.state.move_head(self.direction))
    }
}

pub fn rope_states<const LEN: usize>() -> RopeStates<LEN> {
    RopeStates {
        direction: Direction::Up,
        count: 0,
        state: RopeState::new(),
    }
}