aoc-rs-2021/src/day_17/model.rs

use std::ops::RangeBounds;

use itertools::Itertools;

#[derive(PartialEq, Eq, Debug)]
pub struct TargetArea(pub(crate) (isize, isize), pub(crate) (isize, isize));

#[derive(PartialEq, Eq)]
pub enum State {
    EnRoute,
    InTargetArea,
    Overshot,
}

impl TargetArea {
    pub fn get_position_state(&self, position: (isize, isize)) -> State {
        let x_bounds_en_route = self.get_bounds(true, true);
        let y_bounds_en_route = self.get_bounds(false, true);
        let x_bounds_target = self.get_bounds(true, false);
        let y_bounds_target = self.get_bounds(false, false);
        match (
            x_bounds_en_route.contains(&position.0),
            y_bounds_en_route.contains(&position.1),
            x_bounds_target.contains(&position.0),
            y_bounds_target.contains(&position.1),
        ) {
            (_, _, true, true) => State::InTargetArea,
            (true, true, _, _) => State::EnRoute,
            _ => State::Overshot,
        }
    }

    pub fn get_bounds(&self, x: bool, en_route: bool) -> impl RangeBounds<isize> {
        let mut values = if x {
            vec![self.0 .0, self.0 .1]
        } else {
            vec![self.1 .0, self.1 .1]
        };
        if en_route && x {
            values.push(0);
        } else if en_route && !x {
            values.push(isize::MAX);
        }
        *values.iter().min().unwrap()..=*values.iter().max().unwrap()
    }

    pub fn get_lowest_possible_x_velocity(&self) -> Option<isize> {
        let bounds = self.get_bounds(true, false);
        for i in 1..=Ord::max(self.0 .0, self.0 .1) {
            if XLoc(0, i).any(|loc| bounds.contains(&loc)) {
                return Some(i);
            }
        }
        None
    }

    pub fn get_highest_possible_y_velocity(&self) -> Option<isize> {
        let bounds = self.get_bounds(false, false);
        let y_min = Ord::min(self.1 .0, self.1 .1);
        (y_min..500)
            .rev()
            .find(|i| YLoc(0, *i, y_min).any(|loc| bounds.contains(&loc)))
    }

    pub fn get_all_possible_velocities(&self) -> Vec<(isize, isize)> {
        let x_bounds = self.get_bounds(true, false);
        let x_velocities = (1..=Ord::max(self.0 .0, self.0 .1))
            .filter(|i| XLoc(0, *i).any(|loc| x_bounds.contains(&loc)));
        let y_bounds = self.get_bounds(false, false);
        let y_min = Ord::min(self.1 .0, self.1 .1);
        let y_velocities = (y_min..500)
            .rev()
            .filter(|i| YLoc(0, *i, y_min).any(|loc| y_bounds.contains(&loc)));

        let mut ret_val = Vec::new();

        x_velocities
            .cartesian_product(y_velocities)
            .filter(|velocity| Probe::new(*velocity).will_reach_target_area(self))
            .for_each(|velocity| ret_val.push(velocity));

        ret_val
    }
}

pub struct XLoc(isize, isize);

impl Iterator for XLoc {
    type Item = isize;

    fn next(&mut self) -> Option<Self::Item> {
        let velocity = self.1;
        self.0 += velocity;
        let item = self.0;
        match velocity {
            n if n > 0 => self.1 -= 1,
            n if n < 0 => self.1 += 1,
            _ => return None,
        };
        Some(item)
    }
}

pub struct YLoc(isize, isize, isize);

impl Iterator for YLoc {
    type Item = isize;

    fn next(&mut self) -> Option<Self::Item> {
        let velocity = self.1;
        self.0 += velocity;
        self.1 -= 1;
        let item = self.0;
        if item < self.2 {
            None
        } else {
            Some(item)
        }
    }
}

#[derive(Clone, Debug)]
pub struct Probe {
    pub position: (isize, isize),
    pub velocity: (isize, isize),
}

impl Probe {
    pub fn new(velocity: (isize, isize)) -> Self {
        Self {
            position: (0, 0),
            velocity,
        }
    }
    pub fn step(&mut self) {
        self.position.0 += self.velocity.0;
        self.position.1 += self.velocity.1;
        self.velocity.0 = match self.velocity.0 {
            n if n > 0 => self.velocity.0 - 1,
            n if n < 0 => self.velocity.0 + 1,
            _ => 0,
        };
        self.velocity.1 -= 1;
    }
    pub fn will_reach_target_area(&self, target_area: &TargetArea) -> bool {
        let mut probe = self.clone();
        while target_area.get_position_state(probe.position) == State::EnRoute {
            probe.step();
        }
        matches!(
            target_area.get_position_state(probe.position),
            State::InTargetArea
        )
    }
}

#[cfg(test)]
mod test {
    use crate::day_17::model::Probe;

    use super::TargetArea;

    fn example() -> TargetArea {
        TargetArea((20, 30), (-10, -5))
    }

    #[test]
    fn examples() {
        for velocity in [(7, 2), (6, 3), (9, 0), (6, 9)] {
            assert!(Probe::new(velocity).will_reach_target_area(&example()));
        }
        for velocity in [(17, -4)] {
            assert!(!Probe::new(velocity).will_reach_target_area(&example()));
        }

        for y_vel in 10..1000 {
            assert!(!Probe::new((6, y_vel)).will_reach_target_area(&example()));
        }
    }

    #[test]
    fn lowest_possible_x_velocity() {
        assert_eq!(example().get_lowest_possible_x_velocity(), Some(6));
    }

    #[test]
    fn highest_possible_y_velocity() {
        assert_eq!(example().get_highest_possible_y_velocity(), Some(9));
    }
}