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| 1 | +//! [Day 16: Packet Decoder](https://adventofcode.com/2021/day/16) |
| 2 | +
|
| 3 | +fn int_value(bits: &str) -> u32 { |
| 4 | + bits.chars() |
| 5 | + .map_while(|c| c.to_digit(2)) |
| 6 | + .fold(0, |acc, d| (acc << 1) + d) |
| 7 | +} |
| 8 | + |
| 9 | +fn read_packet(packet: &str, mut offset: usize, sum_of_versions: &mut u32) -> (usize, u64) { |
| 10 | + assert!(offset < packet.len()); |
| 11 | + |
| 12 | + let version = int_value(&packet[offset..offset + 3]); |
| 13 | + let type_id = int_value(&packet[offset + 3..offset + 6]); |
| 14 | + offset += 6; |
| 15 | + |
| 16 | + // compute part 1 answer |
| 17 | + *sum_of_versions += version; |
| 18 | + |
| 19 | + // |
| 20 | + if type_id == 4 { |
| 21 | + let mut literal_value = 0; |
| 22 | + loop { |
| 23 | + let last = &packet[offset..=offset]; |
| 24 | + let nibble = int_value(&packet[offset + 1..offset + 5]); |
| 25 | + literal_value = (literal_value * 16) + u64::from(nibble); |
| 26 | + offset += 5; |
| 27 | + if offset > packet.len() || last == "0" { |
| 28 | + return (offset, literal_value); |
| 29 | + } |
| 30 | + } |
| 31 | + } |
| 32 | + |
| 33 | + // subpackets |
| 34 | + let mut values = Vec::new(); |
| 35 | + |
| 36 | + // length of subpackets |
| 37 | + let length_id = int_value(&packet[offset..=offset]); |
| 38 | + offset += 1; |
| 39 | + |
| 40 | + if length_id == 0 { |
| 41 | + // |
| 42 | + let length = int_value(&packet[offset..offset + 15]); |
| 43 | + offset += 15; |
| 44 | + |
| 45 | + let end_offset: usize = offset + usize::try_from(length).unwrap(); |
| 46 | + |
| 47 | + while length != 0 && offset < end_offset { |
| 48 | + let (new_offset, value) = read_packet(packet, offset, sum_of_versions); |
| 49 | + values.push(value); |
| 50 | + offset = new_offset; |
| 51 | + } |
| 52 | + } else { |
| 53 | + let mut count = int_value(&packet[offset..offset + 11]); |
| 54 | + offset += 11; |
| 55 | + |
| 56 | + while count > 0 && offset < packet.len() { |
| 57 | + let (new_offset, value) = read_packet(packet, offset, sum_of_versions); |
| 58 | + values.push(value); |
| 59 | + count -= 1; |
| 60 | + offset = new_offset; |
| 61 | + } |
| 62 | + }; |
| 63 | + |
| 64 | + let computed = match type_id { |
| 65 | + 0 => values.iter().sum(), |
| 66 | + 1 => values.iter().product(), |
| 67 | + 2 => *values.iter().min().unwrap(), |
| 68 | + 3 => *values.iter().max().unwrap(), |
| 69 | + 5 => u64::from(values[0] > values[1]), |
| 70 | + 6 => u64::from(values[0] < values[1]), |
| 71 | + 7 => u64::from(values[0] == values[1]), |
| 72 | + _ => panic!(), |
| 73 | + }; |
| 74 | + |
| 75 | + (offset, computed) |
| 76 | +} |
| 77 | + |
| 78 | +struct Puzzle { |
| 79 | + part1: u32, |
| 80 | + part2: u64, |
| 81 | +} |
| 82 | + |
| 83 | +impl Puzzle { |
| 84 | + /// Get the puzzle input. |
| 85 | + fn solve_from_file(path: &str) -> Self { |
| 86 | + let data = std::fs::read_to_string(path).unwrap_or_else(|_| { |
| 87 | + eprintln!("cannot read input file {path}"); |
| 88 | + std::process::exit(1); |
| 89 | + }); |
| 90 | + |
| 91 | + Self::solve(&data) |
| 92 | + } |
| 93 | + |
| 94 | + fn solve(data: &str) -> Self { |
| 95 | + let bin_data = data |
| 96 | + .chars() |
| 97 | + .map_while(|c| c.to_digit(16)) |
| 98 | + .map(|d| match d { |
| 99 | + 0 => "0000", |
| 100 | + 1 => "0001", |
| 101 | + 2 => "0010", |
| 102 | + 3 => "0011", |
| 103 | + 4 => "0100", |
| 104 | + 5 => "0101", |
| 105 | + 6 => "0110", |
| 106 | + 7 => "0111", |
| 107 | + 8 => "1000", |
| 108 | + 9 => "1001", |
| 109 | + 10 => "1010", |
| 110 | + 11 => "1011", |
| 111 | + 12 => "1100", |
| 112 | + 13 => "1101", |
| 113 | + 14 => "1110", |
| 114 | + 15 => "1111", |
| 115 | + _ => panic!(), |
| 116 | + }) |
| 117 | + .collect::<Vec<_>>() |
| 118 | + .join(""); |
| 119 | + |
| 120 | + let mut part1 = 0; |
| 121 | + let (_, part2) = read_packet(&bin_data, 0, &mut part1); |
| 122 | + |
| 123 | + Self { part1, part2 } |
| 124 | + } |
| 125 | +} |
| 126 | + |
| 127 | +fn main() { |
| 128 | + let args = aoc::parse_args(); |
| 129 | + let puzzle = Puzzle::solve_from_file(args.path.as_str()); |
| 130 | + println!("{}", puzzle.part1); |
| 131 | + println!("{}", puzzle.part2); |
| 132 | +} |
| 133 | + |
| 134 | +/// Test from puzzle input |
| 135 | +#[cfg(test)] |
| 136 | +mod test { |
| 137 | + use super::*; |
| 138 | + |
| 139 | + #[test] |
| 140 | + fn test_bin() { |
| 141 | + assert_eq!(int_value("a"), 0); |
| 142 | + assert_eq!(int_value("0"), 0); |
| 143 | + assert_eq!(int_value("01"), 1); |
| 144 | + assert_eq!(int_value("11"), 3); |
| 145 | + assert_eq!(int_value("10000"), 16); |
| 146 | + assert_eq!(int_value("10101010101010101010101010101010"), 0xAAAAAAAA); |
| 147 | + } |
| 148 | + |
| 149 | + #[test] |
| 150 | + fn test_part1() { |
| 151 | + let puzzle = Puzzle::solve("8A004A801A8002F478"); |
| 152 | + assert_eq!(puzzle.part1, 16); |
| 153 | + |
| 154 | + let puzzle = Puzzle::solve("620080001611562C8802118E34"); |
| 155 | + assert_eq!(puzzle.part1, 12); |
| 156 | + |
| 157 | + let puzzle = Puzzle::solve("C0015000016115A2E0802F182340"); |
| 158 | + assert_eq!(puzzle.part1, 23); |
| 159 | + |
| 160 | + let puzzle = Puzzle::solve("A0016C880162017C3686B18A3D4780"); |
| 161 | + assert_eq!(puzzle.part1, 31); |
| 162 | + } |
| 163 | + |
| 164 | + #[test] |
| 165 | + fn test_part2() { |
| 166 | + let puzzle = Puzzle::solve("C200B40A82"); |
| 167 | + assert_eq!(puzzle.part2, 3); |
| 168 | + |
| 169 | + let puzzle = Puzzle::solve("04005AC33890"); |
| 170 | + assert_eq!(puzzle.part2, 54); |
| 171 | + |
| 172 | + let puzzle = Puzzle::solve("880086C3E88112"); |
| 173 | + assert_eq!(puzzle.part2, 7); |
| 174 | + |
| 175 | + let puzzle = Puzzle::solve("CE00C43D881120"); |
| 176 | + assert_eq!(puzzle.part2, 9); |
| 177 | + |
| 178 | + let puzzle = Puzzle::solve("D8005AC2A8F0"); |
| 179 | + assert_eq!(puzzle.part2, 1); |
| 180 | + |
| 181 | + let puzzle = Puzzle::solve("F600BC2D8F"); |
| 182 | + assert_eq!(puzzle.part2, 0); |
| 183 | + |
| 184 | + let puzzle = Puzzle::solve("9C005AC2F8F0"); |
| 185 | + assert_eq!(puzzle.part2, 0); |
| 186 | + |
| 187 | + let puzzle = Puzzle::solve("9C0141080250320F1802104A08"); |
| 188 | + assert_eq!(puzzle.part2, 1); |
| 189 | + } |
| 190 | +} |
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