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
162
163
164
165
166
167
168
169
170
171
172
173

//! # Day 3: Toboggan Trajectory
//!
//! With the toboggan login problems resolved, you set off toward the airport. While travel by
//! toboggan might be easy, it's certainly not safe: there's very minimal steering and the area is
//! covered in trees. You'll need to see which angles will take you near the fewest trees.
//!
//! Due to the local geology, trees in this area only grow on exact integer coordinates in a grid.
//! You make a map (your puzzle input) of the open squares (`.`) and trees (`#`) you can see. For
//! example:
//!
//! ```txt
//! ..##.......
//! #...#...#..
//! .#....#..#.
//! ..#.#...#.#
//! .#...##..#.
//! ..#.##.....
//! .#.#.#....#
//! .#........#
//! #.##...#...
//! #...##....#
//! .#..#...#.#
//! ```
//!
//! These aren't the only trees, though; due to something you read about once involving arboreal
//! genetics and biome stability, the same pattern repeats to the right many times:
//!
//! ```txt
//! ..##.........##.........##.........##.........##.........##.......  --->
//! #...#...#..#...#...#..#...#...#..#...#...#..#...#...#..#...#...#..
//! .#....#..#..#....#..#..#....#..#..#....#..#..#....#..#..#....#..#.
//! ..#.#...#.#..#.#...#.#..#.#...#.#..#.#...#.#..#.#...#.#..#.#...#.#
//! .#...##..#..#...##..#..#...##..#..#...##..#..#...##..#..#...##..#.
//! ..#.##.......#.##.......#.##.......#.##.......#.##.......#.##.....  --->
//! .#.#.#....#.#.#.#....#.#.#.#....#.#.#.#....#.#.#.#....#.#.#.#....#
//! .#........#.#........#.#........#.#........#.#........#.#........#
//! #.##...#...#.##...#...#.##...#...#.##...#...#.##...#...#.##...#...
//! #...##....##...##....##...##....##...##....##...##....##...##....#
//! .#..#...#.#.#..#...#.#.#..#...#.#.#..#...#.#.#..#...#.#.#..#...#.#  --->
//! ```
//!
//! You start on the open square (`.`) in the top-left corner and need to reach the bottom (below
//! the bottom-most row on your map).
//!
//! The toboggan can only follow a few specific slopes (you opted for a cheaper model that prefers
//! rational numbers); start by **counting all the trees** you would encounter for the slope **right
//! 3, down 1**:
//!
//! From your starting position at the top-left, check the position that is right 3 and down 1.
//! Then, check the position that is right 3 and down 1 from there, and so on until you go past the
//! bottom of the map.
//!
//! The locations you'd check in the above example are marked here with **`O`** where there was an
//! open square and **`X`** where there was a tree:
//!
//! ```txt
//! ..##.........##.........##.........##.........##.........##.......  --->
//! #..O#...#..#...#...#..#...#...#..#...#...#..#...#...#..#...#...#..
//! .#....X..#..#....#..#..#....#..#..#....#..#..#....#..#..#....#..#.
//! ..#.#...#O#..#.#...#.#..#.#...#.#..#.#...#.#..#.#...#.#..#.#...#.#
//! .#...##..#..X...##..#..#...##..#..#...##..#..#...##..#..#...##..#.
//! ..#.##.......#.X#.......#.##.......#.##.......#.##.......#.##.....  --->
//! .#.#.#....#.#.#.#.O..#.#.#.#....#.#.#.#....#.#.#.#....#.#.#.#....#
//! .#........#.#........X.#........#.#........#.#........#.#........#
//! #.##...#...#.##...#...#.X#...#...#.##...#...#.##...#...#.##...#...
//! #...##....##...##....##...#X....##...##....##...##....##...##....#
//! .#..#...#.#.#..#...#.#.#..#...X.#.#..#...#.#.#..#...#.#.#..#...#.#  --->
//! ```
//!
//! In this example, traversing the map using this slope would cause you to encounter **`7`** trees.
//!
//! Starting at the top-left corner of your map and following a slope of right 3 and down 1, **how
//! many trees would you encounter?**
//!
//! ## Part Two
//!
//! Time to check the rest of the slopes - you need to minimize the probability of a sudden arboreal
//! stop, after all.
//!
//! Determine the number of trees you would encounter if, for each of the following slopes, you
//! start at the top-left corner and traverse the map all the way to the bottom:
//!
//! - Right 1, down 1.
//! - Right 3, down 1. (This is the slope you already checked.)
//! - Right 5, down 1.
//! - Right 7, down 1.
//! - Right 1, down 2.
//!
//! In the above example, these slopes would find `2`, `7`, `3`, `4`, and `2` tree(s) respectively;
//! multiplied together, these produce the answer **`336`**.
//!
//! **What do you get if you multiply together the number of trees encountered on each of the listed
//! slopes?**

use anyhow::{bail, Result};

pub const INPUT: &str = include_str!("d03.txt");

pub fn solve_part_one(input: &str) -> Result<u32> {
    let forest = parse_input(input);
    let mut pos = (0, 0);
    let mut count = 0;

    loop {
        pos.0 += 3;
        pos.1 += 1;

        if pos.1 >= forest.len() {
            return Ok(count);
        }

        if forest[pos.1][pos.0 % forest[pos.1].len()] {
            count += 1;
        }
    }
}

pub fn solve_part_two(input: &str) -> Result<u64> {
    let forest = parse_input(input);

    Ok([(1, 1), (3, 1), (5, 1), (7, 1), (1, 2)].iter().fold(1, |total, (right, down)| {
        let mut pos = (0, 0);
        let mut count = 0;

        loop {
            pos.0 += right;
            pos.1 += down;

            if pos.1 >= forest.len() {
                return total * count;
            }

            if forest[pos.1][pos.0 % forest[pos.1].len()] {
                count += 1;
            }
        }
    }))
}

fn parse_input(input: &str) -> Vec<Vec<bool>> {
    input.lines().map(|l| l.chars().map(|c| c == '#').collect()).collect()
}

#[cfg(test)]
mod tests {
    use indoc::indoc;

    use super::*;

    const INPUT: &str = indoc! {"
        ..##.......
        #...#...#..
        .#....#..#.
        ..#.#...#.#
        .#...##..#.
        ..#.##.....
        .#.#.#....#
        .#........#
        #.##...#...
        #...##....#
        .#..#...#.#
    "};

    #[test]
    fn part_one() {
        assert_eq!(7, solve_part_one(INPUT).unwrap());
    }

    #[test]
    fn part_two() {
        assert_eq!(336, solve_part_two(INPUT).unwrap());
    }
}