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| 1 | +use utils::prelude::*; |
| 2 | + |
| 3 | +/// Simulating 1D cellular automata. |
| 4 | +#[derive(Clone, Debug)] |
| 5 | +pub struct Day12 { |
| 6 | + initial: State, |
| 7 | + rules: u32, |
| 8 | +} |
| 9 | + |
| 10 | +#[derive(Copy, Clone, Debug)] |
| 11 | +struct State { |
| 12 | + pots: [u64; WIDTH], |
| 13 | + start: i64, |
| 14 | + len: usize, |
| 15 | + sum: i64, |
| 16 | +} |
| 17 | + |
| 18 | +const WIDTH: usize = 4; |
| 19 | +const PART1_GENERATIONS: i64 = 20; |
| 20 | +const PART2_GENERATIONS: i64 = 50_000_000_000; |
| 21 | + |
| 22 | +impl Day12 { |
| 23 | + pub fn new(input: &str, _: InputType) -> Result<Self, InputError> { |
| 24 | + let cell = parser::byte_map!( |
| 25 | + b'#' => true, |
| 26 | + b'.' => false, |
| 27 | + ); |
| 28 | + |
| 29 | + let (initial, rules) = cell |
| 30 | + .repeat_arrayvec::<100, _>(parser::noop(), 1) |
| 31 | + .with_prefix("initial state: ") |
| 32 | + .with_suffix(parser::eol().then(parser::eol())) |
| 33 | + .then( |
| 34 | + cell.repeat_n::<5, _>(parser::noop()) |
| 35 | + .with_suffix(" => ") |
| 36 | + .then(cell) |
| 37 | + .repeat_arrayvec::<32, _>(parser::eol(), 1), |
| 38 | + ) |
| 39 | + .parse_complete(input)?; |
| 40 | + |
| 41 | + let mut pots = [0; WIDTH]; |
| 42 | + let mut sum = 0; |
| 43 | + for (i, &b) in initial.iter().enumerate() { |
| 44 | + pots[i / 64] |= (b as u64) << (i % 64); |
| 45 | + sum += i64::from(b) * i as i64; |
| 46 | + } |
| 47 | + |
| 48 | + let mut rules_mask = 0; |
| 49 | + for &(lhs, rhs) in rules.iter() { |
| 50 | + let index = lhs |
| 51 | + .iter() |
| 52 | + .enumerate() |
| 53 | + .fold(0, |acc, (i, &b)| acc | (b as u32) << i); |
| 54 | + rules_mask |= (rhs as u32) << index; |
| 55 | + } |
| 56 | + |
| 57 | + Ok(Self { |
| 58 | + initial: State { |
| 59 | + pots, |
| 60 | + start: 0, |
| 61 | + len: initial.len(), |
| 62 | + sum, |
| 63 | + }, |
| 64 | + rules: rules_mask, |
| 65 | + }) |
| 66 | + } |
| 67 | + |
| 68 | + #[must_use] |
| 69 | + pub fn part1(&self) -> i64 { |
| 70 | + let mut state = self.initial; |
| 71 | + for _ in 0..PART1_GENERATIONS { |
| 72 | + state = state.next(self.rules); |
| 73 | + } |
| 74 | + state.sum |
| 75 | + } |
| 76 | + |
| 77 | + #[must_use] |
| 78 | + pub fn part2(&self) -> i64 { |
| 79 | + let mut state = self.initial; |
| 80 | + for remaining in (PART2_GENERATIONS - 1000..PART2_GENERATIONS).rev() { |
| 81 | + let next = state.next(self.rules); |
| 82 | + |
| 83 | + // In the input the pattern eventually stabilizes and just moves to the right each |
| 84 | + // iteration |
| 85 | + if state.len == next.len && state.pots == next.pots { |
| 86 | + return next.sum + remaining * (next.sum - state.sum); |
| 87 | + } |
| 88 | + |
| 89 | + state = next; |
| 90 | + } |
| 91 | + panic!("no solution found: reached generation limit") |
| 92 | + } |
| 93 | +} |
| 94 | + |
| 95 | +impl State { |
| 96 | + fn next(&self, rules: u32) -> State { |
| 97 | + if self.len + 4 >= WIDTH * 64 { |
| 98 | + panic!("no solution found: reached width limit"); |
| 99 | + } |
| 100 | + |
| 101 | + let (mut pots, mut start, mut len, mut sum) = ([0; WIDTH], self.start - 2, 0, 0); |
| 102 | + let (mut index, mut rule) = (0, 0); |
| 103 | + for (old_index, pot_num) in (0..self.len + 4).zip(start..) { |
| 104 | + rule >>= 1; |
| 105 | + rule |= ((self.pots[old_index / 64] >> (old_index % 64)) & 1) << 4; |
| 106 | + |
| 107 | + let value = (rules & (1 << rule)) != 0; |
| 108 | + if index == 0 && !value { |
| 109 | + // Skip over empty plant pots at the start |
| 110 | + start += 1; |
| 111 | + continue; |
| 112 | + } |
| 113 | + pots[index / 64] |= (value as u64) << (index % 64); |
| 114 | + |
| 115 | + index += 1; |
| 116 | + if value { |
| 117 | + sum += pot_num; |
| 118 | + |
| 119 | + // Don't include empty plant pots at the end in the length |
| 120 | + len = index; |
| 121 | + } |
| 122 | + } |
| 123 | + |
| 124 | + State { |
| 125 | + pots, |
| 126 | + start, |
| 127 | + len, |
| 128 | + sum, |
| 129 | + } |
| 130 | + } |
| 131 | +} |
| 132 | + |
| 133 | +examples!(Day12 -> (i64, i64) [ |
| 134 | + {file: "day12_example0.txt", part1: 325}, |
| 135 | + // Custom examples |
| 136 | + { |
| 137 | + input: "initial state: .#\n\n..#.. => #", |
| 138 | + part1: 1, |
| 139 | + part2: 1 |
| 140 | + }, |
| 141 | + { |
| 142 | + input: "initial state: #\n\n.#... => #", |
| 143 | + part1: PART1_GENERATIONS, |
| 144 | + part2: PART2_GENERATIONS |
| 145 | + }, |
| 146 | + { |
| 147 | + input: "initial state: #\n\n....# => #", |
| 148 | + part1: -2 * PART1_GENERATIONS, |
| 149 | + part2: -2 * PART2_GENERATIONS |
| 150 | + }, |
| 151 | + { |
| 152 | + input: "initial state: ##\n\n.##.. => #\n##... => #", |
| 153 | + part1: 2 * PART1_GENERATIONS + 1, |
| 154 | + part2: 2 * PART2_GENERATIONS + 1, |
| 155 | + }, |
| 156 | + // 01234 |
| 157 | + // 0: ##.## |
| 158 | + // 1: ##.# |
| 159 | + // 2: ##...# |
| 160 | + // 3: ##....## |
| 161 | + // 4: ##....## |
| 162 | + // 5: ##....## |
| 163 | + { |
| 164 | + input: "initial state: ##.##\n\n##.## => #\n..#.. => #\n.#... => #\n..##. => #\n...## => #", |
| 165 | + part1: -4 * PART1_GENERATIONS + 14, |
| 166 | + part2: -4 * PART2_GENERATIONS + 14, |
| 167 | + }, |
| 168 | +]); |
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