Solution for 2022/day09-part2

2022/day09-part2/Cargo.toml

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+[package]
+name = "day09-part2"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

2022/day09-part2/src/main.rs

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+use std::collections::HashSet;
+
+#[derive(Copy, Clone, Debug)]
+enum Direction {
+    Up,
+    Down,
+    Left,
+    Right,
+}
+
+use Direction::*;
+
+impl Direction {
+    fn from_char(dir: char) -> Direction {
+        match dir {
+            'R' => Right,
+            'L' => Left,
+            'U' => Up,
+            'D' => Down,
+            _ => {
+                panic!("Invalid character to construct Direction form");
+            }
+        }
+    }
+}
+
+#[derive(Eq, PartialEq, Hash, Copy, Clone, Debug)]
+struct Coord {
+    x: i32,
+    y: i32,
+}
+
+impl Coord {
+    fn new() -> Coord {
+        Coord {
+            x: 0,
+            y: 0,
+        }
+    }
+
+    fn move_towards(&mut self, dir: Direction) {
+        match dir {
+            Up => {
+                self.y += 1;
+            }
+            Down => {
+                self.y -= 1;
+            }
+            Left => {
+                self.x -= 1;
+            }
+            Right => {
+                self.x += 1;
+            }
+        }
+    }
+
+    fn correct_tail(&mut self, head: Coord) {
+        let dx = head.x - self.x;
+        let dy = head.y - self.y;
+
+        // Do nothing if there isn't any actual distance between
+        // the tail and head, i.e. at least one of the dimensions
+        // has distance value 2 or greater.
+        if dx.abs() > 1 || dy.abs() > 1 {
+            // Use the sign of the delta to move the tail towards the head.
+            // This automatically takes care of the diagonal vs. horizontal vs. vertical
+            // behvaior outlined in the task description.
+            self.x += dx.signum();
+            self.y += dy.signum();
+        }
+    }
+}
+
+// Move the whole rope towards a direction.
+fn move_rope(rope: &mut Vec<Coord>, dir: Direction) {
+    // Move the head in the given direction.
+    rope.first_mut().unwrap().move_towards(dir);
+    // Then iterate over all remaining "links" of the rope, starting from the head.
+    for i in 1..rope.len() {
+        // Copy the destination coordinate.
+        let prev = rope[i-1];
+        // And then correct the current link towards that previous one.
+        rope[i].correct_tail(prev);
+    }
+}
+
+fn main() {
+    // Use command line arguments to specify the input filename.
+    let args: Vec<String> = std::env::args().collect();
+    if args.len() < 2 {
+        panic!("Usage: ./main <input-file>\nNo input file provided. Exiting.");
+    }
+
+    // Next, read the contents of the input file into a string for easier processing.
+    let input = std::fs::read_to_string(&args[1]).expect("Error opening file");
+    // Line-by-line processing is easiest.
+    let input = input.lines();
+
+    // --- TASK BEGIN ---
+
+    // Keep track of the rope as a series of coordinates.
+    let mut rope: Vec<Coord> = vec![Coord::new(); 10];
+
+    // Keep track of all the coordnates visited by the tail.
+    let mut visited_coordinates: HashSet<Coord> = HashSet::new();
+    visited_coordinates.insert(*rope.last().unwrap());
+
+    // Go through the instructions line-by-line.
+    for line in input {
+        // Split and parse each line into direction (char) and distance (i32).
+        let s: Vec<_> = line.split(' ').collect();
+        let direction = Direction::from_char(s[0].chars().next().unwrap());
+        let distance = s[1].parse::<i32>().unwrap();
+
+        // Then move the head `distance` times and correct the tail afterwards.
+        for _ in 0..distance {
+            // Move the entire rope along the given direction.
+            move_rope(&mut rope, direction);
+            // Finally, store the new tail coordinate in the set.
+            visited_coordinates.insert(*rope.last().unwrap());
+            // dbg!(&tail);
+        }
+    }
+
+    println!("Number of visited coordinates: {}", visited_coordinates.len());
+}
+