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day05.rs
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day05.rs
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//! # How About a Nice Game of Chess?
//!
//! Essentially a repeat of [`Year 2015 Day 4`]. We brute force MD5 hashes as quickly as
//! possible in parallel in blocks of 1000 at a time.
//!
//! [`Year 2015 Day 4`]: crate::year2015::day04
use crate::util::md5::*;
use std::sync::atomic::{AtomicBool, AtomicU32, Ordering};
use std::sync::Mutex;
use std::thread;
struct Shared {
prefix: String,
done: AtomicBool,
counter: AtomicU32,
}
struct Exclusive {
found: Vec<(u32, u32)>,
mask: u16,
}
pub fn parse(input: &str) -> Vec<u32> {
let shared = Shared {
prefix: input.trim().to_owned(),
done: AtomicBool::new(false),
counter: AtomicU32::new(1000),
};
let mutex = Mutex::new(Exclusive { found: vec![], mask: 0 });
// Handle the first 999 numbers specially as the number of digits varies.
for n in 1..1000 {
let (mut buffer, size) = format_string(&shared.prefix, n);
check_hash(&mut buffer, size, n, &shared, &mutex);
}
// Use as many cores as possible to parallelize the remaining search.
thread::scope(|scope| {
for _ in 0..thread::available_parallelism().unwrap().get() {
scope.spawn(|| worker(&shared, &mutex));
}
});
let mut found = mutex.into_inner().unwrap().found;
found.sort_unstable();
found.iter().map(|&(_, n)| n).collect()
}
pub fn part1(input: &[u32]) -> String {
let password = input.iter().take(8).fold(0, |acc, n| (acc << 4) | (n >> 8));
format!("{password:08x}")
}
pub fn part2(input: &[u32]) -> String {
let mut password = 0;
let mut mask = 0xffffffff;
for n in input {
let sixth = n >> 8;
if sixth < 8 {
let shift = 4 * (7 - sixth);
let seventh = (n >> 4) & 0xf;
password |= (seventh << shift) & mask;
mask &= !(0xf << shift);
}
}
format!("{password:08x}")
}
fn format_string(prefix: &str, n: u32) -> ([u8; 64], usize) {
let string = format!("{prefix}{n}");
let size = string.len();
let mut buffer = [0; 64];
buffer[0..size].copy_from_slice(string.as_bytes());
(buffer, size)
}
fn check_hash(buffer: &mut [u8], size: usize, n: u32, shared: &Shared, mutex: &Mutex<Exclusive>) {
let (result, ..) = hash(buffer, size);
if result & 0xfffff000 == 0 {
let mut exclusive = mutex.lock().unwrap();
exclusive.found.push((n, result));
exclusive.mask |= 1 << (result >> 8);
if exclusive.mask & 0xff == 0xff {
shared.done.store(true, Ordering::Relaxed);
}
}
}
fn worker(shared: &Shared, mutex: &Mutex<Exclusive>) {
while !shared.done.load(Ordering::Relaxed) {
let offset = shared.counter.fetch_add(1000, Ordering::Relaxed);
let (mut buffer, size) = format_string(&shared.prefix, offset);
for n in 0..1000 {
// Format macro is very slow, so update digits directly
buffer[size - 3] = b'0' + (n / 100) as u8;
buffer[size - 2] = b'0' + ((n / 10) % 10) as u8;
buffer[size - 1] = b'0' + (n % 10) as u8;
check_hash(&mut buffer, size, offset + n, shared, mutex);
}
}
}