aoc2023/24/src/main.rs

use itertools::Itertools;
use std::fmt::Debug;
use std::fs::File;
use std::io::{BufRead, BufReader, Lines};
use std::ops::Index;
use std::time::Instant;

// BOILERPLATE
type InputIter = Lines<BufReader<File>>;

fn get_input() -> InputIter {
    let f = File::open("input").unwrap();
    let br = BufReader::new(f);
    br.lines()
}

fn main() {
    let start = Instant::now();
    let ans1 = problem1(get_input());
    let duration = start.elapsed();
    println!("Problem 1 solution: {} [{}s]", ans1, duration.as_secs_f64());

    let start = Instant::now();
    let ans2 = problem2(get_input());
    let duration = start.elapsed();
    println!("Problem 2 solution: {} [{}s]", ans2, duration.as_secs_f64());
}

// Parse

#[derive(Clone, Copy)]
struct Vec3 {
    x: f64,
    y: f64,
    z: f64,
}
impl Debug for Vec3 {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "{}, {}, {}", self.x, self.y, self.z)
    }
}

#[derive(Clone, Copy)]
struct Vec2 {
    x: f64,
    y: f64,
}
impl Debug for Vec2 {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "{}, {}", self.x, self.y)
    }
}


#[derive(Clone)]

struct Hailstone {
    pos: Vec3,
    vel: Vec3,
}

impl Hailstone {
    fn xy_vel_angle(&self) -> f64 {
        (self.vel.y / self.vel.x).atan()
    }
    fn xy_vel_abs(&self) -> f64 {
        (self.vel.x.powi(2) + self.vel.y.powi(2)).sqrt()
    }
    fn xy_slope(&self) -> f64 {
        self.vel.y / self.vel.x
    }
    fn y_crossing(&self) -> f64 {
        self.pos.y - self.xy_slope() * self.pos.x
    }
    fn xy_poi(&self, other: &Hailstone) -> Option<Vec2> {
        let our_slope = self.xy_slope();
        let other_slope = other.xy_slope();
        if our_slope == other_slope || our_slope * other_slope == -1. {
            None
        } else {
            let our_yint = self.y_crossing();
            let other_yint = other.y_crossing();
            let ratio = (other_yint - our_yint) / (our_slope - other_slope);
            Some(Vec2 {
                x: ratio,
                y: our_slope * ratio + our_yint,
            })
        }
    }
    fn xy_point_future(&self, point: &Vec2) -> bool {
        // a point will be in the past if the difference between its 'new' position and its 'start' position has a
        // different sign than the velocity for any component
        let diffs = [point.x - self.pos.x, point.y - self.pos.y];
        // for (diff, vel) in diffs.iter().zip([self.vel.x, self.vel.y].iter()) {
        //     println!(" diff: {:?} vel: {:?} mul: {:?}", diff, vel, diff * vel > 0.);
        // }
        diffs.iter().zip([self.vel.x, self.vel.y].iter()).any(|(diff, vel)| diff * vel > 0.)
    }
}

impl From<&str> for Hailstone {
    fn from(value: &str) -> Self {
        let (pos, vel) = value.split_once("@").unwrap();
        let [px, py, pz] = pos.split(",").map(|s| s.trim().parse::<f64>().unwrap()).collect_vec()[..] else {
            panic!()
        };
        let [vx, vy, vz] = vel.split(",").map(|s| s.trim().parse::<f64>().unwrap()).collect_vec()[..] else {
            panic!()
        };

        Self {
            pos: Vec3 { x: px, y: py, z: pz },
            vel: Vec3 { x: vx, y: vy, z: vz },
        }
    }
}

impl Debug for Hailstone {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "{:?} @ {:?}", self.pos, self.vel)
    }
}

#[derive(Debug)]
struct Hailstones {
    stones: Vec<Hailstone>,
}

impl Hailstones {
    fn count_xy_pois(&self, bounds: (f64, f64)) -> u64 {
        assert!(bounds.0 < bounds.1);
        let mut stones = self.stones.clone();

        let mut count = 0;
        while let Some(stone) = &stones.pop() {
            for other in &stones {
                let Some(poi) = stone.xy_poi(other) else { continue };
                // println!("intersection: {:?} / {:?} @ {:?}", stone, other, poi);
                if poi.x >= bounds.0 && poi.x <= bounds.1 && poi.y >= bounds.0 && poi.y <= bounds.1 && stone.xy_point_future(&poi) && other.xy_point_future(&poi) {
                    count += 1;
                }
            }
        }
        
        count
    }
}

impl Index<usize> for Hailstones {
    type Output = Hailstone;
    fn index(&self, index: usize) -> &Self::Output {
        &self.stones[index]
    }
}

impl<T: BufRead> From<Lines<T>> for Hailstones {
    fn from(value: Lines<T>) -> Self {
        let mut stones = Vec::new();
        for line in value {
            stones.push(Hailstone::from(line.unwrap().as_str()));
        }
        Hailstones { stones }
    }
}

// PROBLEM 1 solution

fn problem1<T: BufRead>(input: Lines<T>) -> u64 {
    problem1_impl(input, (200000000000000., 400000000000000.))
}

fn problem1_impl<T: BufRead>(input: Lines<T>, bounds: (f64, f64)) -> u64 {
    let stones = Hailstones::from(input);

    stones.count_xy_pois(bounds)
}

// PROBLEM 2 solution
fn problem2<T: BufRead>(input: Lines<T>) -> u64 {
    0
}

#[cfg(test)]
mod tests {
    use crate::*;
    use std::io::Cursor;

    const EXAMPLE: &str = &"19, 13, 30 @ -2,  1, -2
18, 19, 22 @ -1, -1, -2
20, 25, 34 @ -2, -2, -4
12, 31, 28 @ -1, -2, -1
20, 19, 15 @  1, -5, -3";

    #[test]
    fn problem1_example() {
        let c = Cursor::new(EXAMPLE);
        assert_eq!(problem1_impl(c.lines(), (7., 27.)), 2);
    }

    #[test]
    fn problem2_example() {
        let c = Cursor::new(EXAMPLE);
        assert_eq!(problem2(c.lines()), 0);
    }
}