Day 9 part 1

2022/src/bin/day9.rs

@@ -0,0 +1,105 @@
+use std::collections::HashSet;
+use std::error::Error;
+
+type Pos = (i32, i32);
+
+fn main() -> Result<(), Box<dyn Error>> {
+    let input = std::fs::read_to_string("input/day_9.txt")?;
+
+    part1(&input)?;
+    part2(&input)?;
+
+    Ok(())
+}
+
+fn part1(input: &str) -> Result<(), Box<dyn Error>> {
+    let mut tail_visited = HashSet::new();
+    let mut head = (0, 0);
+    let mut tail = (0, 0);
+
+    for line in input.lines() {
+        execute(line, &mut head, &mut tail, &mut tail_visited)?;
+    }
+
+    println!("Part 1: tail visited {} positions", tail_visited.len());
+
+    Ok(())
+}
+
+fn part2(input: &str) -> Result<(), Box<dyn Error>> {
+    Ok(())
+}
+
+fn execute(
+    line: &str,
+    head: &mut Pos,
+    tail: &mut Pos,
+    visited: &mut HashSet<Pos>,
+) -> Result<(), Box<dyn Error>> {
+    // let h = *head;
+    // let t = *tail;
+    let instruction = line
+        .split_once(' ')
+        .and_then(|(x, count)| count.parse().ok().map(|count: u32| (x, count)));
+    let (delta, count) = match instruction {
+        Some(("U", count)) => ((0, 1), count),
+        Some(("D", count)) => ((0, -1), count),
+        Some(("L", count)) => ((-1, 0), count),
+        Some(("R", count)) => ((1, 0), count),
+        _ => Err(format!("invalid line: '{}'", line))?,
+    };
+    (0..count).for_each(|_| do_move(delta, head, tail, visited));
+    // println!("{}: H{:?} T{:?} -> H{:?} T{:?}", line, h, t, head, tail);
+    Ok(())
+}
+
+fn do_move(delta: Pos, head: &mut Pos, tail: &mut Pos, visited: &mut HashSet<Pos>) {
+    *head = add(*head, delta);
+    *tail = move_tail(*head, *tail);
+    visited.insert(*tail);
+}
+
+fn move_tail(head: Pos, tail: Pos) -> Pos {
+    match sub(head, tail) {
+        // No movement if tail is within 1 unit of head
+        (x, y) if x.abs() <= 1 && y.abs() <= 1 => tail,
+        // Left-right plane
+        (x, 0) if x.abs() == 2 => add(tail, (one(x), 0)),
+        // Up-down plane
+        (0, y) if y.abs() == 2 => add(tail, (0, one(y))),
+        // Diagonal
+        (x, y) if x.abs() <= 2 && y.abs() <= 2 => add(tail, (one(x), one(y))),
+        _ => unreachable!("head is more than 2 units away"),
+    }
+}
+
+fn add(a: Pos, b: Pos) -> Pos {
+    (a.0 + b.0, a.1 + b.1)
+}
+
+fn sub(a: Pos, b: Pos) -> Pos {
+    (a.0 - b.0, a.1 - b.1)
+}
+
+fn one(n: i32) -> i32 {
+    if n.is_negative() {
+        -1
+    } else {
+        1
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_move_tail() {
+        assert_eq!(move_tail((0, 0), (-1, -2)), (0, -1));
+        assert_eq!(move_tail((0, 0), (-2, -2)), (-1, -1));
+        assert_eq!(move_tail((0, 0), (1, 2)), (0, 1));
+        assert_eq!(move_tail((0, 0), (2, 2)), (1, 1));
+        assert_eq!(move_tail((0, 0), (0, 2)), (0, 1));
+        assert_eq!(move_tail((0, 0), (2, 0)), (1, 0));
+    }
+}