Day 03: Perfectly Spherical Houses in a Vacuum

Give me a point class, and I shall plot the world. This is the first appearance of my Point2d class (closely related to my Space2d class). It is just a generic point class that I have built up after doing a few Advent-of-Code’s.

For part 1 I simply kept track of everywhere I had been in a HashSet. Rust’s scan method of iterators is pretty nifty here! It allows you to transform an iterator into another iterator, like map, but contains an internal state, like fold. Combined these allow you to transform an iterator of instructions into an iterator of states.

For part 2, unfortunately array_chunks is currently nightly-only. I would either have to use Itertools to chunk the array or do it myself. Once chunked into flesh santa and robot santa, we just combine the positions with the same method from part 1.

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day_03.rs

use crate::space::point::Point2d;
use std::collections::HashSet;

fn parse_input (input: &str) -> impl Iterator<Item = Point2d> + '_ {
	return input.chars().map(|x| match x {
		'^' => Point2d::cardinal_north(),
		'>' => Point2d::cardinal_east(),
		'v' => Point2d::cardinal_south(),
		'<' => Point2d::cardinal_west(),
		_ => panic!()
	});
}

fn houses_visited (directions: impl Iterator<Item = Point2d>) -> HashSet<Point2d> {
	return std::iter::once(Point2d::origin()) // Direction of not moving
		.chain(directions)
		.scan(Point2d::origin(), |santas_position, direction| {
			*santas_position = *santas_position + direction;
			Some(santas_position.clone())
		})
		.collect::<HashSet<Point2d>>();
}

pub fn day_03_1 (input: &str) -> String {
	return houses_visited(parse_input(input)).len().to_string();
}

pub fn day_03_2 (input: &str) -> String {
	let input = parse_input(input).collect::<Vec<_>>();
	let mut flesh_santa_route = vec![];
	let mut robot_santa_route = vec![];

	for i in (0..input.len()).step_by(2) {
		flesh_santa_route.push(input[i + 0]);
		robot_santa_route.push(input[i + 1]);
	}

	let mut all_houses = HashSet::new();
	all_houses.extend(houses_visited(flesh_santa_route.into_iter()));
	all_houses.extend(houses_visited(robot_santa_route.into_iter()));

	return all_houses.len().to_string();
}