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day16.rs
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day16.rs
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//! Packet Decoder
//!
//! [`BitStream`] is the key to making this problem tractable. It works like an iterator, allowing
//! us to consume an arbitrary number of bits from the input and convert this to a number.
//!
//! It works by maintaining an internal `u64` buffer. If the requested number of bits is larger than
//! the buffer's current size then additional bits are added to the buffer 4 at a time from each
//! hexadecimal digit of the input data.
//!
//! Additionally it keeps track of the total number of bits consumed so far. This is needed when
//! parsing packets that use the total length in bits to determine sub-packets.
//!
//! The decoded packet data is stored as a tree-like struct allowing recursive solutions to part 1
//! and part 2 to reuse the same decoded input.
use std::str::Bytes;
struct BitStream<'a> {
available: u64,
bits: u64,
read: u64,
iter: Bytes<'a>,
}
impl BitStream<'_> {
fn from(s: &str) -> BitStream<'_> {
BitStream { available: 0, bits: 0, read: 0, iter: s.bytes() }
}
fn next(&mut self, amount: u64) -> u64 {
while self.available < amount {
self.available += 4;
self.bits = (self.bits << 4) | self.hex_to_binary();
}
self.available -= amount;
self.read += amount;
let mask = (1 << amount) - 1;
(self.bits >> self.available) & mask
}
fn hex_to_binary(&mut self) -> u64 {
let hex_digit = self.iter.next().unwrap();
if hex_digit.is_ascii_digit() {
(hex_digit - 48) as u64
} else {
(hex_digit - 55) as u64
}
}
}
pub enum Packet {
Literal { version: u64, type_id: u64, value: u64 },
Operator { version: u64, type_id: u64, packets: Vec<Packet> },
}
impl Packet {
fn from(bit_stream: &mut BitStream<'_>) -> Packet {
let version = bit_stream.next(3);
let type_id = bit_stream.next(3);
if type_id == 4 {
let mut todo = true;
let mut value = 0;
while todo {
todo = bit_stream.next(1) == 1;
value = (value << 4) | bit_stream.next(4);
}
Packet::Literal { version, type_id, value }
} else {
let mut packets = Vec::new();
if bit_stream.next(1) == 0 {
let target = bit_stream.next(15) + bit_stream.read;
while bit_stream.read < target {
packets.push(Self::from(bit_stream));
}
} else {
let sub_packets = bit_stream.next(11);
for _ in 0..sub_packets {
packets.push(Self::from(bit_stream));
}
}
Packet::Operator { version, type_id, packets }
}
}
}
pub fn parse(input: &str) -> Packet {
let mut bit_stream = BitStream::from(input);
Packet::from(&mut bit_stream)
}
pub fn part1(packet: &Packet) -> u64 {
fn helper(packet: &Packet) -> u64 {
match packet {
Packet::Literal { version, .. } => *version,
Packet::Operator { version, packets, .. } => {
*version + packets.iter().map(helper).sum::<u64>()
}
}
}
helper(packet)
}
pub fn part2(packet: &Packet) -> u64 {
fn helper(packet: &Packet) -> u64 {
match packet {
Packet::Literal { value, .. } => *value,
Packet::Operator { type_id, packets, .. } => {
let mut iter = packets.iter().map(helper);
match type_id {
0 => iter.sum(),
1 => iter.product(),
2 => iter.min().unwrap(),
3 => iter.max().unwrap(),
5 => (iter.next().unwrap() > iter.next().unwrap()) as u64,
6 => (iter.next().unwrap() < iter.next().unwrap()) as u64,
7 => (iter.next().unwrap() == iter.next().unwrap()) as u64,
_ => unreachable!(),
}
}
}
}
helper(packet)
}