day 9

Cargo.toml

@@ -9,4 +9,5 @@ members = [
     "day6",
     "day7",
     "day8",
+    "day9",
 ]

day9/Cargo.toml

@@ -0,0 +1,9 @@
+[package]
+name = "day9"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+nalgebra = "0.31.4"

day9/sample_input.txt

@@ -0,0 +1,8 @@
+R 5
+U 8
+L 8
+D 3
+R 17
+D 10
+L 25
+U 20

day9/src/main.rs

@@ -0,0 +1,64 @@
+use std::collections::HashSet;
+
+use nalgebra::base::Vector2;
+
+type IVec2 = Vector2<i32>;
+
+const RIGHT: IVec2 = IVec2::new(1, 0);
+const LEFT: IVec2 = IVec2::new(-1, 0);
+const UP: IVec2 = IVec2::new(0, 1);
+const DOWN: IVec2 = IVec2::new(0, -1);
+
+const ZERO: IVec2 = IVec2::new(0, 0);
+
+fn main() {
+    const INPUT: &str = include_str!("../input.txt");
+    
+    let mut visited: HashSet<IVec2> = HashSet::new();
+
+    let mut knots: [IVec2; 10] = [ZERO; 10];
+
+    for line in INPUT.lines() {
+        let (dir, steps) = line.split_once(' ').unwrap();
+        let dir: IVec2 = match dir.chars().next().unwrap() {
+            'R' => RIGHT,
+            'L' => LEFT,
+            'U' => UP,
+            'D' => DOWN,
+            _   => ZERO,
+        };
+        let steps: u32 = steps.parse().unwrap();
+
+        for _ in 0..steps {
+            *knots.get_mut(0).unwrap() += dir;
+
+            for i in 1..knots.len() {
+                let diff: IVec2;
+                {
+                    let knot = knots.get(i).unwrap();
+                    let prev = knots.get(i - 1).unwrap();
+                    diff = *prev - *knot;
+                }
+
+                let knot = knots.get_mut(i).unwrap();
+                if diff.x.abs() > 1 || diff.y.abs() > 1 {
+                    // need to move
+                    if diff.x == 0 {
+                        knot.y += diff.y.signum();
+                    } else if diff.y == 0 {
+                        knot.x += diff.x.signum(); 
+                    } else {
+                        knot.x += diff.x.signum();
+                        knot.y += diff.y.signum();
+                    }
+                }
+
+                if i == 9 {
+                    visited.insert(*knot);
+                }
+            }
+        }
+    }
+
+    println!("{}", visited.len());
+}