1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161

//! This is my solution for [Advent of Code - Day 18 - _Title_](https://adventofcode.com/2022/day/18)
//!
//!

use std::collections::{HashSet, VecDeque};
use std::fs;
use itertools::Itertools;

type Coordinate = (isize, isize, isize);

/// The entry point for running the solutions with the 'real' puzzle input.
///
/// - The puzzle input is expected to be at `<project_root>/res/day-18-input`
/// - It is expected this will be called by [`super::main()`] when the user elects to run day 18.
pub fn run() {
    let contents = fs::read_to_string("res/day-18-input").expect("Failed to read file");
    let coords = parse_input(&contents);

    println!(
        "The surface area is {}.",
        get_surface_area(&coords)
    );

    println!(
        "The external surface area is {}.",
        get_external_surface_area(&coords)
    );
}

fn parse_input(input: &String) -> Vec<Coordinate> {
    input.trim().lines().map(parse_coordinate).collect()
}

fn parse_coordinate(line: &str) -> Coordinate {
    let mut values = line.split(',').flat_map(|s| s.parse::<isize>());

    (values.next().unwrap(), values.next().unwrap(), values.next().unwrap())
}

fn get_surface_area(coords: &Vec<Coordinate>) -> usize {
    let mut surface_area = 0;
    let mut coords_seen: HashSet<Coordinate> = HashSet::new();
    let deltas = vec![
        (1, 0, 0),
        (0, 1, 0),
        (0, 0, 1),
    ];

    for &(x, y, z) in coords {
        surface_area += 6;
        coords_seen.insert((x, y, z));
        for &(dx, dy, dz) in deltas.iter() {
            if coords_seen.contains(&(x + dx, y + dy, z + dz)) {
                surface_area -= 2;
            }

            if coords_seen.contains(&(x - dx, y - dy, z - dz)) {
                surface_area -= 2;
            }
        }
    }

    surface_area
}

fn get_external_surface_area(coords: &Vec<Coordinate>) -> usize {
    let (x_min, x_max) = coords.iter().map(|&c| c.0).minmax().into_option().unwrap();
    let (y_min, y_max) = coords.iter().map(|&c| c.1).minmax().into_option().unwrap();
    let (z_min, z_max) = coords.iter().map(|&c| c.2).minmax().into_option().unwrap();

    let coord_set: HashSet<Coordinate> = coords.into_iter().map(|&c| c).collect();
    let mut visited: HashSet<Coordinate> = HashSet::new();
    let mut surface_area = 0;

    let mut to_process: VecDeque<Coordinate> = VecDeque::new();
    to_process.push_back((x_min - 1, y_min - 1, z_min - 1));

    let deltas = vec![
        (1, 0, 0),
        (0, 1, 0),
        (0, 0, 1),
    ];

    while let Some((x, y, z)) = to_process.pop_front() {
        if visited.contains(&(x, y, z)) {
            continue;
        }

        if coord_set.contains(&(x, y, z)) {
            surface_area += 1;
            continue;
        }

        if x < x_min - 1 || x > x_max + 1
            || y < y_min - 1 || y > y_max + 1
            || z < z_min - 1|| z > z_max + 1 {
            continue;
        }

        visited.insert((x, y, z));

        for &(dx, dy, dz) in deltas.iter() {
            to_process.push_back((x + dx, y + dy, z + dz));
            to_process.push_back((x - dx, y - dy, z - dz));
        }
    }

    surface_area
}

#[cfg(test)]
mod tests {
    use crate::day_18::{Coordinate, get_external_surface_area, get_surface_area, parse_input};

    fn sample_coords() -> Vec<Coordinate> {
        vec![
            (2, 2, 2),
            (1, 2, 2),
            (3, 2, 2),
            (2, 1, 2),
            (2, 3, 2),
            (2, 2, 1),
            (2, 2, 3),
            (2, 2, 4),
            (2, 2, 6),
            (1, 2, 5),
            (3, 2, 5),
            (2, 1, 5),
            (2, 3, 5),
        ]
    }

    #[test]
    fn can_parse() {
        let input = "2,2,2
1,2,2
3,2,2
2,1,2
2,3,2
2,2,1
2,2,3
2,2,4
2,2,6
1,2,5
3,2,5
2,1,5
2,3,5".to_string();

        assert_eq!(parse_input(&input), sample_coords());
    }

    #[test]
    fn can_get_surface_area() {
        assert_eq!(get_surface_area(&sample_coords()), 64)
    }

    #[test]
    fn can_get_external_surface_area() {
        assert_eq!(get_external_surface_area(&sample_coords()), 58)
    }
}