day09: Chage stuff for part 2

Part 1 was missing the "y_diff.abs() == x_diff.abs()" condition in
Pos.move_after(), which made the program show the wrong output at first.

day09/Cargo.lock

@@ -0,0 +1,7 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 3
+
+[[package]]
+name = "day09"
+version = "0.1.0"

day09/Cargo.toml

@@ -0,0 +1,8 @@
+[package]
+name = "day09"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

day09/src/main.rs

@@ -0,0 +1,91 @@
+use std::collections::HashSet;
+use std::error;
+use std::fs::File;
+use std::io::BufRead;
+use std::io::BufReader;
+
+
+const DEBUG: bool = false;
+
+
+struct Pos {
+	x: i32,
+	y: i32
+}
+
+impl Pos {
+	// Move the position x into x direction and y into y direction
+	fn move_(&mut self, x: i32, y: i32) {
+		self.x += x;
+		self.y += y;
+	}
+
+	fn move_into_direction(&mut self, direction: &str) -> Result<(), Box<dyn error::Error>> {
+		match direction {
+			"R" => self.move_(1, 0),
+			"L" => self.move_(-1, 0),
+			"U" => self.move_(0, 1),
+			"D" => self.move_(0, -1),
+			_ => Err(format!("Unknown direction {direction}."))?
+		};
+		Ok(())
+	}
+
+	fn move_after(&mut self, other: &Pos) {
+		let x_diff = other.x - self.x;
+		let y_diff = other.y - self.y;
+
+		// no movement if we're close enough
+		if x_diff.abs() <= 1 && y_diff.abs() <= 1 {
+			return
+		}
+
+		if x_diff.abs() == y_diff.abs() {
+			self.move_((x_diff.abs() - 1) * x_diff.signum(), (y_diff.abs() - 1) * y_diff.signum());
+		} else if x_diff.abs() > y_diff.abs() {
+			self.move_((x_diff.abs() - 1) * x_diff.signum(), y_diff);
+		} else {
+			self.move_(x_diff, (y_diff.abs() - 1) * y_diff.signum());
+		}
+	}
+
+	fn show(&self) {
+		println!("x: {} y: {}", self.x, self.y)
+	}
+}
+
+fn main() -> Result<(), Box<dyn error::Error>> {
+	let f = File::open("input.txt")?;
+	let reader = BufReader::new(f);
+
+	let mut knot_positions = vec![];
+	for _ in 0..10 {
+		knot_positions.push(Pos { x: 0, y: 0 });
+	}
+
+	let mut visited_locations = HashSet::new();
+	for line in reader.lines() {
+		let line = line?;
+		let mut s_iter = line.split_whitespace();
+		let direction = s_iter.next().ok_or(format!("Could not split {line} on whitespace"))?;
+		let amount: i32 = s_iter.next().ok_or(format!("Could not split {line} into 2 parts."))?.parse()?;
+		for _ in 0..amount {
+			knot_positions[0].move_into_direction(direction)?;
+			for i in 1..knot_positions.len() {
+				let (head, tail) = knot_positions.split_at_mut(i);
+				tail[0].move_after(head.last().ok_or("head doesn't have an element")?);
+			}
+			if DEBUG {
+				knot_positions[0].show();
+				knot_positions[9].show();
+			}
+			visited_locations.insert((knot_positions[9].x.to_owned(), knot_positions[9].y.to_owned()));
+		}
+		if DEBUG {
+			println!("");
+		}
+	}
+
+    println!("Visited {} locations", visited_locations.len());
+    Ok(())
+}