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
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204

//! # [Day 24: Electromagnetic Moat](http://adventofcode.com/2017/day/24)
//!
//! The CPU itself is a large, black building surrounded by a bottomless
//! pit. Enormous metal tubes extend outward from the side of the building
//! at regular intervals and descend down into the void. There's no way to
//! cross, but you need to get inside.
//!
//! No way, of course, other than building a *bridge* out of the magnetic
//! components strewn about nearby.

use itertools::Itertools;
use ::parse::unsigned_number;

/// Each component has two *ports*, one on each end. The ports come in all
/// different types, and only matching types can be connected. You take an
/// inventory of the components by their port types (your puzzle input).
/// Each port is identified by the number of *pins* it uses; more pins mean
/// a stronger connection for your bridge. A `3/7` component, for example,
/// has a type-`3` port on one side, and a type-`7` port on the other.
#[derive(Copy, Clone, PartialEq, Eq, Debug, Hash)]
pub struct Component(usize, usize);

impl Component {
    named!{ from_bytes (&[u8]) -> Component,
        do_parse!(
            a: unsigned_number >>
            char!('/') >>
            b: unsigned_number >>

            (Component(a, b))
        )
    }

    named!{ list_from_bytes (&[u8]) -> Vec<Component>,
        lines!(Component::from_bytes)
    }

    fn strength(self) -> usize {
        self.0 + self.1
    }
}

fn dfs_step(
    path: &mut Vec<usize>,
    last_index: usize,
    graph: &Vec<Component>,
    cmp_f: &Fn((usize, usize), (usize, usize)) -> bool,
) -> (usize, usize)
{
    let edges = graph.iter()
        .positions(|&Component(a, b)| last_index == a || last_index == b)
        .filter(|e| !path.contains(e))
        .collect::<Vec<_>>();

    if edges.len() == 0 {
        let strength = path.iter()
            .map(|&ei| graph[ei].strength())
            .sum::<usize>();

        return (path.len(), strength);
    }

    let mut max_len = 0;
    let mut max_strength = 0;

    for edge in edges {
        path.push(edge);

        let new_index = if graph[edge].0 == last_index {
            graph[edge].1
        } else {
            graph[edge].0
        };

        let (new_len, new_strength) = dfs_step(path, new_index, graph, cmp_f);

        if cmp_f((max_len, max_strength), (new_len, new_strength)) {
            max_len = new_len;
            max_strength = new_strength;
        }

        path.pop();
    }

    (max_len, max_strength)
}

/// Your side of the pit is metallic; a perfect surface to connect a
/// magnetic, *zero-pin port*. Because of this, the first port you use must
/// be of type `0`. It doesn't matter what type of port you end with; your
/// goal is just to make the bridge as strong as possible.
fn dfs(
    graph: &Vec<Component>,
    cmp_f: &Fn((usize, usize), (usize, usize)) -> bool,
) -> usize
{
    dfs_step(&mut vec![], 0, graph, cmp_f).1
}


/// The *strength* of a bridge is the sum of the port types in each
/// component. For example, if your bridge is made of components `0/3`,
/// `3/7`, and `7/4`, your bridge has a strength of `0+3 + 3+7 + 7+4 = 24`.
///
/// For example, suppose you had the following components:
///
/// 0/2
/// 2/2
/// 2/3
/// 3/4
/// 3/5
/// 0/1
/// 10/1
/// 9/10
///
/// With them, you could make the following valid bridges:
///
/// -   `0/1`
/// -   `0/1`--`10/1`
/// -   `0/1`--`10/1`--`9/10`
/// -   `0/2`
/// -   `0/2`--`2/3`
/// -   `0/2`--`2/3`--`3/4`
/// -   `0/2`--`2/3`--`3/5`
/// -   `0/2`--`2/2`
/// -   `0/2`--`2/2`--`2/3`
/// -   `0/2`--`2/2`--`2/3`--`3/4`
/// -   `0/2`--`2/2`--`2/3`--`3/5`
///
/// (Note how, as shown by `10/1`, order of ports within a component doesn't
/// matter. However, you may only use each port on a component once.)
///
/// Of these bridges, the *strongest* one is `0/1`--`10/1`--`9/10`; it has a
/// strength of `0+1 + 1+10 + 10+9 = 31`.
///
/// ```
/// # use advent_solutions::advent2017::day24::{ parse_input, part1 };
/// let input = parse_input("\
/// 0/2
/// 2/2
/// 2/3
/// 3/4
/// 3/5
/// 0/1
/// 10/1
/// 9/10
/// ");
///
/// assert_eq!(part1(&input), 31);
/// ```
///
/// *What is the strength of the strongest bridge you can make* with the
/// components you have available?
pub fn part1(graph: &Vec<Component>) -> usize {
    dfs(
        graph,
        &|(_, max_strength), (_, new_strength)| new_strength > max_strength
    )
}

/// The bridge you've built isn't long enough; you can't <span
/// title="Who do you think you are, Mario?">jump the rest of the way</span>.
///
/// In the example above, there are two longest bridges:
///
/// -   `0/2`--`2/2`--`2/3`--`3/4`
/// -   `0/2`--`2/2`--`2/3`--`3/5`
///
/// Of them, the one which uses the `3/5` component is stronger; its
/// strength is `0+2 + 2+2 + 2+3 + 3+5 = 19`.
///
/// ```
/// # use advent_solutions::advent2017::day24::{ parse_input, part2 };
/// let input = parse_input("\
/// 0/2
/// 2/2
/// 2/3
/// 3/4
/// 3/5
/// 0/1
/// 10/1
/// 9/10
/// ");
///
/// assert_eq!(part2(&input), 19);
/// ```
///
/// *What is the strength of the longest bridge you can make?* If you can
/// make multiple bridges of the longest length, pick the *strongest* one.
pub fn part2(graph: &Vec<Component>) -> usize {
    dfs(
        graph,
        &|(max_len, max_strength), (new_len, new_strength)|
            new_len > max_len || (new_len == max_len && new_strength > max_strength)
    )
}

pub fn parse_input(input: &str) -> Vec<Component> {
    Component::list_from_bytes(input.as_bytes())
        .to_full_result()
        .expect("Error parsing components")
}

test_day!("24", 1868, 1841);