From f3304cff3fc3614a691539c3174980a02f72f19a Mon Sep 17 00:00:00 2001
From: Justin Wernick <justin@worthe-it.co.za>
Date: Sun, 24 Dec 2023 13:53:00 +0200
Subject: Day 24 part 1

---
 2023/src/bin/day_24.rs | 95 +++++++++++++++++++++++++++++++++++++++++++++-----
 1 file changed, 87 insertions(+), 8 deletions(-)

(limited to '2023/src/bin')

diff --git a/2023/src/bin/day_24.rs b/2023/src/bin/day_24.rs
index b3a610b..325b4cc 100644
--- a/2023/src/bin/day_24.rs
+++ b/2023/src/bin/day_24.rs
@@ -1,19 +1,98 @@
-use nom::IResult;
+use nalgebra::{Matrix2, Point2, Point3, Vector2, Vector3};
+use nom::{
+    bytes::complete::tag,
+    character::complete::{i64, line_ending},
+    combinator::map,
+    multi::separated_list1,
+    sequence::{separated_pair, tuple},
+    IResult,
+};
 use std::fs;
 
 fn main() -> Result<(), Box<dyn std::error::Error>> {
-    let input = fs::read_to_string("inputs/day_2.txt")?;
-    let parsed = Example::parser(&input).unwrap().1;
-    dbg!(&parsed);
+    let input = fs::read_to_string("inputs/day_24.txt")?;
+    let parsed = Hailstones::parser(&input).unwrap().1;
+    dbg!(&parsed.count_intersections_2d(200000000000000., 400000000000000.));
 
     Ok(())
 }
 
 #[derive(Debug)]
-struct Example;
+struct Hailstones(Vec<Hailstone>);
 
-impl Example {
-    fn parser(_input: &str) -> IResult<&str, Self> {
-        todo!()
+#[derive(Debug)]
+struct Hailstone {
+    initial_position: Point3<f64>,
+    velocity: Vector3<f64>,
+}
+
+impl Hailstones {
+    fn parser(input: &str) -> IResult<&str, Self> {
+        map(separated_list1(line_ending, Hailstone::parser), Hailstones)(input)
+    }
+
+    fn count_intersections_2d(&self, min: f64, max: f64) -> usize {
+        self.0
+            .iter()
+            .enumerate()
+            .map(|(i, hailstone)| {
+                self.0
+                    .iter()
+                    .skip(i + 1)
+                    .filter(|other_hailstone| hailstone.intersects_2d(other_hailstone, min, max))
+                    .count()
+            })
+            .sum()
+    }
+}
+
+impl Hailstone {
+    fn parser(input: &str) -> IResult<&str, Self> {
+        map(
+            separated_pair(
+                map(
+                    tuple((i64, tag(", "), i64, tag(", "), i64)),
+                    |(x, _, y, _, z)| Point3::new(x as f64, y as f64, z as f64),
+                ),
+                tag(" @ "),
+                map(
+                    tuple((i64, tag(", "), i64, tag(", "), i64)),
+                    |(x, _, y, _, z)| Vector3::new(x as f64, y as f64, z as f64),
+                ),
+            ),
+            |(initial_position, velocity)| Hailstone {
+                initial_position,
+                velocity,
+            },
+        )(input)
+    }
+
+    fn intersects_2d(&self, other: &Hailstone, min: f64, max: f64) -> bool {
+        let variables = Matrix2::new(
+            self.velocity.x,
+            -other.velocity.x,
+            self.velocity.y,
+            -other.velocity.y,
+        );
+        let constants = Vector2::new(
+            other.initial_position.x - self.initial_position.x,
+            other.initial_position.y - self.initial_position.y,
+        );
+
+        if let Some(variables_inverse) = variables.try_inverse() {
+            let intersection = variables_inverse * constants;
+            let self_t = intersection.x;
+            let other_t = intersection.y;
+
+            let intersection = self.initial_position.xy() + self.velocity.xy() * self_t;
+            self_t >= 0.
+                && other_t >= 0.
+                && intersection.x >= min
+                && intersection.x <= max
+                && intersection.y >= min
+                && intersection.y <= max
+        } else {
+            false
+        }
     }
 }
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