[2023] Day 24: Part A

README.md

@@ -28,6 +28,7 @@ Thank you to [Eric Wastl](http://was.tl) for running this incredible yearly even
 - [Day 21: Step Counter](aoc_2023/src/day_21.rs)
 - [Day 22: Sand Slabs](aoc_2023/src/day_22.rs)
 - [Day 23: A Long Walk](aoc_2023/src/day_23.rs)
+- [Day 24: Never Tell Me The Odds](aoc_2023/src/day_24.rs)
 <!-- MARKER -->
 
 ## [2022](https://adventofcode.com/2022) [![aoc_2022](https://github.com/Basicprogrammer10/advent-of-code/actions/workflows/aoc_2022.yml/badge.svg)](https://github.com/Basicprogrammer10/advent-of-code/actions/workflows/aoc_2022.yml)

aoc_2023/src/day_24.rs

@@ -0,0 +1,105 @@
+use std::ops::RangeInclusive;
+
+use common::{Answer, Solution};
+use itertools::Itertools;
+use nd_vec::{vector, Vec2, Vec3};
+
+type Pos = Vec3<f64>;
+
+pub struct Day24;
+
+impl Solution for Day24 {
+    fn name(&self) -> &'static str {
+        "Never Tell Me The Odds"
+    }
+
+    fn part_a(&self, input: &str) -> Answer {
+        let stones = parse(input);
+
+        const RANGE: RangeInclusive<f64> = 200000000000000.0..=400000000000000.0;
+        stones
+            .iter()
+            .tuple_combinations()
+            .filter_map(|(a, b)| a.collision(b))
+            .filter(|&pos| RANGE.contains(&pos.x()) && RANGE.contains(&pos.y()))
+            .count()
+            .into()
+    }
+
+    fn part_b(&self, input: &str) -> Answer {
+        Answer::Unimplemented
+    }
+}
+
+#[derive(Debug)]
+struct HailStone {
+    pos: Pos,
+    vel: Pos,
+}
+
+impl HailStone {
+    fn collision(&self, other: &Self) -> Option<Vec2<f64>> {
+        let a_slope = self.vel.y() / self.vel.x();
+        let a_intercept = self.pos.y() - a_slope * self.pos.x();
+
+        let b_slope = other.vel.y() / other.vel.x();
+        let b_intercept = other.pos.y() - b_slope * other.pos.x();
+
+        let x_pos = (b_intercept - a_intercept) / (a_slope - b_slope);
+        let y_pos = a_slope * x_pos + a_intercept;
+
+        (x_pos.is_normal()
+            && y_pos.is_normal()
+            && !(self.vel.x() > 0.0 && x_pos < self.pos.x())
+            && !(self.vel.x() < 0.0 && x_pos > self.pos.x())
+            && !(other.vel.x() > 0.0 && x_pos < other.pos.x())
+            && !(other.vel.x() < 0.0 && x_pos > other.pos.x()))
+        .then(|| vector!(x_pos, y_pos))
+    }
+}
+
+fn parse(input: &str) -> Vec<HailStone> {
+    let mut out = Vec::new();
+
+    for line in input.lines() {
+        let parts = line.split_whitespace().collect::<Vec<_>>();
+        let parse = |i| {
+            parts[i as usize]
+                .trim_end_matches(',')
+                .parse::<f64>()
+                .unwrap()
+        };
+        out.push(HailStone {
+            pos: vector!(parse(0), parse(1), parse(2)),
+            vel: vector!(parse(4), parse(5), parse(6)),
+        });
+    }
+
+    out
+}
+
+#[cfg(test)]
+mod test {
+    use common::Solution;
+    use indoc::indoc;
+
+    use super::Day24;
+
+    const CASE: &str = indoc! {"
+        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 part_a() {
+        assert_eq!(Day24.part_a(CASE), 0.into());
+    }
+
+    #[test]
+    fn part_b() {
+        assert_eq!(Day24.part_b(CASE), ().into());
+    }
+}

aoc_2023/src/lib.rs

@@ -25,6 +25,7 @@ mod day_20;
 mod day_21;
 mod day_22;
 mod day_23;
+mod day_24;
 // [import_marker]
 
 #[rustfmt::skip]
@@ -52,5 +53,6 @@ pub const ALL: &[&dyn Solution] = &[
     &day_21::Day21,
     &day_22::Day22,
     &day_23::Day23,
+    &day_24::Day24,
     // [list_marker]
 ];