aoc/year2019/
day17.rs

1//! # Set and Forget
2//!
3//! The key insight is that this is not a path finding problem but a *compression*
4//! problem. We need to reduce the robot's path into repetitions of three patterns.
5//! This is essentially a very simple version of the well-known
6//! [LZW](https://en.wikipedia.org/wiki/Lempel%E2%80%93Ziv%E2%80%93Welch)
7//! algorithm used by the `GIF` and `ZIP` file formats.
8//!
9//! First we find the complete path with a simple heuristic:
10//! * Rotate left or right to face the current path segment (a horizontal or vertical line).
11//! * Go forward until we hit the end of the current path segment.
12//! * If it's a dead end then finish.
13//!
14//! Then we look for three patterns that can be repeated in any order to form the whole path.
15//! Without loss of any generality the first pattern anchored at the start is always `A`,
16//! the next `B` and the last `C`.
17use super::intcode::*;
18use crate::util::hash::*;
19use crate::util::parse::*;
20use crate::util::point::*;
21use std::fmt::Write as _;
22use std::ops::ControlFlow;
23
24pub struct Input {
25    code: Vec<i64>,
26    scaffold: FastSet<Point>,
27    position: Point,
28    direction: Point,
29}
30
31struct Movement<'a> {
32    routine: String,
33    functions: [Option<&'a str>; 3],
34}
35
36/// The camera output points from left to right, top to bottom.
37pub fn parse(input: &str) -> Input {
38    let code: Vec<_> = input.iter_signed().collect();
39    let mut computer = Computer::new(&code);
40
41    let mut x = 0;
42    let mut y = 0;
43    let mut scaffold = FastSet::new();
44    let mut position = ORIGIN;
45    let mut direction = ORIGIN;
46
47    while let State::Output(next) = computer.run() {
48        match next {
49            // '\n'
50            10 => {
51                y += 1;
52            }
53            // '#'
54            35 => {
55                scaffold.insert(Point::new(x, y));
56            }
57            // '<'
58            60 => {
59                position = Point::new(x, y);
60                direction = LEFT;
61            }
62            // '>'
63            62 => {
64                position = Point::new(x, y);
65                direction = RIGHT;
66            }
67            // '^'
68            94 => {
69                position = Point::new(x, y);
70                direction = UP;
71            }
72            // 'v'
73            118 => {
74                position = Point::new(x, y);
75                direction = DOWN;
76            }
77            // '.'
78            _ => (),
79        }
80        x = if next == 10 { 0 } else { x + 1 };
81    }
82
83    Input { code, scaffold, position, direction }
84}
85
86pub fn part1(input: &Input) -> i32 {
87    input
88        .scaffold
89        .iter()
90        .filter(|&point| ORTHOGONAL.iter().all(|&delta| input.scaffold.contains(&(*point + delta))))
91        .map(|point| point.x * point.y)
92        .sum()
93}
94
95pub fn part2(input: &Input) -> i64 {
96    let path = build_path(input);
97    let mut movement = Movement { routine: String::new(), functions: [None; 3] };
98
99    let _unused = compress(&path, &mut movement);
100
101    // Convert trailing comma ',' into a trailing newline '\n'
102    let mut rules = String::new();
103    let mut newline_ending = |s: &str| {
104        rules.push_str(s);
105        rules.pop();
106        rules.push('\n');
107    };
108
109    newline_ending(&movement.routine);
110    movement.functions.into_iter().flatten().for_each(newline_ending);
111
112    let mut modified = input.code.clone();
113    modified[0] = 2;
114
115    let mut computer = Computer::new(&modified);
116    computer.input_ascii(&rules);
117
118    visit(computer)
119}
120
121/// Use a simple heuristic to build a path that visits every part of the scaffold at least once.
122/// This string will be too long to use directly in the robot's movement functions, so we'll
123/// need to compress it first.
124fn build_path(input: &Input) -> String {
125    let scaffold = &input.scaffold;
126    let mut position = input.position;
127    let mut direction = input.direction;
128    let mut path = String::new();
129
130    loop {
131        let left = direction.counter_clockwise();
132        let right = direction.clockwise();
133
134        if scaffold.contains(&(position + left)) {
135            direction = left;
136        } else if scaffold.contains(&(position + right)) {
137            direction = right;
138        } else {
139            break path;
140        }
141
142        let mut next = position + direction;
143        let mut magnitude = 0;
144
145        while scaffold.contains(&next) {
146            position = next;
147            next += direction;
148            magnitude += 1;
149        }
150
151        let direction = if direction == left { 'L' } else { 'R' };
152        let _ = write!(path, "{direction},{magnitude},");
153    }
154}
155
156/// Find three patterns that can be repeated in any order to build the whole path.
157///
158/// Uses a greedy backtracking algorithm that attempts to match as much of the remaining string
159/// as possible with known patterns, before trying combinations of a new pattern (up to the maximum
160/// movement function length of 20 characters).
161fn compress<'a>(path: &'a str, movement: &mut Movement<'a>) -> ControlFlow<()> {
162    // Nothing left to match, we've finished successfully.
163    if path.is_empty() {
164        return ControlFlow::Break(());
165    }
166    // Safety check just in case very short sequences can match the entire input.
167    if movement.routine.len() > 21 {
168        return ControlFlow::Continue(());
169    }
170
171    for (i, &name) in ['A', 'B', 'C'].iter().enumerate() {
172        movement.routine.push(name);
173        movement.routine.push(',');
174
175        if let Some(needle) = movement.functions[i] {
176            // Try known patterns first
177            if let Some(remaining) = path.strip_prefix(needle) {
178                compress(remaining, movement)?;
179            }
180        } else {
181            // Then combinations up to length 20 characters
182            for (needle, remaining) in segments(path) {
183                movement.functions[i] = Some(needle);
184                compress(remaining, movement)?;
185                movement.functions[i] = None;
186            }
187        }
188
189        movement.routine.pop();
190        movement.routine.pop();
191    }
192
193    ControlFlow::Continue(())
194}
195
196/// Fun with iterators.
197fn segments(path: &str) -> impl Iterator<Item = (&str, &str)> {
198    path.bytes()
199        .enumerate()
200        // Index of every comma ',' in the string
201        .filter_map(|(i, b)| (b == b',').then_some(i))
202        // Maximum length for movement function is 20 characters
203        .take_while(|&i| i < 21)
204        // Include trailing comma in "needle" to make matching easier
205        .map(|i| path.split_at(i + 1))
206        // Movement is always pairs of (rotation, magnitude) so return every second comma
207        .skip(1)
208        .step_by(2)
209}
210
211#[cfg(not(feature = "frivolity"))]
212fn visit(mut computer: Computer) -> i64 {
213    // Disable continuous video feed
214    computer.input_ascii("n\n");
215
216    let mut result = 0;
217    while let State::Output(next) = computer.run() {
218        result = next;
219    }
220    result
221}
222
223/// Non essential but fun. Animates the robot traversing the scaffold.
224#[cfg(feature = "frivolity")]
225fn visit(mut computer: Computer) -> i64 {
226    use crate::util::ansi::*;
227    use std::thread::sleep;
228    use std::time::Duration;
229
230    let mut result = 0;
231    let mut previous = ' ';
232    let mut buffer = String::new();
233
234    // Enable continuous video feed
235    computer.input_ascii("y\n");
236
237    while let State::Output(next) = computer.run() {
238        result = next;
239        let ascii = (next as u8) as char;
240
241        // Highlight the robot's position
242        match ascii {
243            '^' | 'v' | '<' | '>' => {
244                let _ = write!(&mut buffer, "{BOLD}{YELLOW}{ascii}{RESET}");
245            }
246            _ => buffer.push(ascii),
247        }
248
249        // Each frame is separated by a blank line
250        if ascii == '\n' && previous == '\n' {
251            print!("{HOME}{CLEAR}{buffer}");
252            sleep(Duration::from_millis(25));
253            buffer.clear();
254        }
255
256        previous = ascii;
257    }
258
259    result
260}
aoc/year2019/day17.rs

aoc/year2019/
day17.rs
