2020: day23: part 2

This commit is contained in:
Antoine Martin 2020-12-24 01:14:15 +01:00
parent 9888140072
commit e341008746

View file

@ -4,10 +4,14 @@ use anyhow::{Context, Result};
const INPUT: &str = include_str!("../input/day23.txt");
const CUP_NUMBER: usize = 1_000_000;
const TURNS_NUMBER: usize = 10_000_000;
pub fn run() -> Result<String> {
let mut res = String::with_capacity(128);
writeln!(res, "part 1: {}", part1(INPUT)?)?;
writeln!(res, "part 2: {}", part2(INPUT)?)?;
Ok(res)
}
@ -22,40 +26,28 @@ fn part1(input: &str) -> Result<String> {
Ok(format!("{}", cup_circle))
}
struct CupCircle(VecDeque<u64>);
fn part2(input: &str) -> Result<usize> {
let mut cup_circle: FastCupCircle = input.parse()?;
for _ in 0..TURNS_NUMBER {
cup_circle.step();
}
let first = cup_circle.get_next_cup(1);
let second = cup_circle.get_next_cup(first);
Ok(first * second)
}
/// CupCircle provides an abstraction over a VecDeque to emulate the steps of the game
///
struct CupCircle(VecDeque<u64>);
/// The "current" cup should always be the first one in the VecDeque at the end of a turn, for
/// convenience
///
/// The Circle should never become empty if you only use its public interface, so all calls to
/// unwrap() in its implementation should never panic.
/// `unwrap()` in its implementation should never panic.
impl CupCircle {
pub fn front(&self) -> u64 {
*self.0.front().unwrap()
}
fn pop_front(&mut self) -> u64 {
self.0.pop_front().unwrap()
}
fn push_back(&mut self, value: u64) {
self.0.push_back(value);
}
pub fn max(&self) -> u64 {
*self.0.iter().max().unwrap()
}
fn remove_next_3(&mut self) -> [u64; 3] {
let first = self.pop_front();
let second = self.pop_front();
let third = self.pop_front();
[first, second, third]
}
/// Shifts the cup circle, putting the first cup at the end of the deque
///
/// This doesn't change anything to the cup circle layout, except that the new first cup is
@ -110,6 +102,30 @@ impl CupCircle {
}
self.shift();
}
fn remove_next_3(&mut self) -> [u64; 3] {
let first = self.pop_front();
let second = self.pop_front();
let third = self.pop_front();
[first, second, third]
}
pub fn front(&self) -> u64 {
*self.0.front().unwrap()
}
fn pop_front(&mut self) -> u64 {
self.0.pop_front().unwrap()
}
fn push_back(&mut self, value: u64) {
self.0.push_back(value);
}
pub fn max(&self) -> u64 {
*self.0.iter().max().unwrap()
}
}
impl std::str::FromStr for CupCircle {
@ -138,6 +154,108 @@ impl std::fmt::Display for CupCircle {
}
}
/// CupCircle provides a fast abstraction to emulate the steps of the game.
///
/// It is considerably faster than the above naive implementation, but its representation isn't as
/// intuitive. It uses a [`std::vec::Vec`] of indices, where `vec[cup]` returns the next cup in the
/// circle, for a given cup.
struct FastCupCircle {
cups: Vec<usize>,
current: usize,
}
impl FastCupCircle {
fn get_next_cup(&self, cup: usize) -> usize {
self.cups[cup - 1]
}
fn set_next_cup(&mut self, cup: usize, next: usize) {
self.cups[cup - 1] = next;
}
fn remove_next_3(&mut self, cup: usize) -> [usize; 3] {
let first = self.get_next_cup(cup);
let second = self.get_next_cup(first);
let third = self.get_next_cup(second);
// shortcut the links to remove them from the loop temporarily
self.set_next_cup(cup, self.get_next_cup(third));
[first, second, third]
}
fn step(&mut self) {
// The crab picks up the three cups that are immediately clockwise of the current cup. They
// are removed from the circle; cup spacing is adjusted as necessary to maintain the circle.
let removed_cups = self.remove_next_3(self.current);
// The crab selects a destination cup: the cup with a label equal to the current cup's label
// minus one. If this would select one of the cups that was just picked up, the crab will
// keep subtracting one until it finds a cup that wasn't just picked up. If at any point in
// this process the value goes below the lowest value on any cup's label, it wraps around to
// the highest value on any cup's label instead.
//
// TODO: use std::cmp::Ord::clamp when stabilized (Rust 1.50)
let mut destination = if self.current > 1 {
self.current - 1
} else {
self.cups.len()
};
while removed_cups.contains(&destination) {
destination = if destination > 1 {
destination - 1
} else {
self.cups.len()
};
}
// The crab places the cups it just picked up so that they are immediately clockwise of the
// destination cup. They keep the same order as when they were picked up.
//
// The links from first to second and from second to third haven't changed, no need to
// update them
let [first, _, third] = removed_cups;
self.set_next_cup(third, self.get_next_cup(destination));
self.set_next_cup(destination, first);
// The crab selects a new current cup: the cup which is immediately clockwise of the current
// cup.
self.current = self.get_next_cup(self.current);
}
}
impl std::str::FromStr for FastCupCircle {
type Err = anyhow::Error;
fn from_str(s: &str) -> Result<Self> {
let input_cups: Vec<usize> = s
.trim_end()
.chars()
.map(|c| Ok(c.to_digit(10).context("character was not a digit")? as usize))
.collect::<Result<_>>()?;
let max = *input_cups.iter().max().context("input contained 0 cups")?;
let mut cups = Vec::new();
cups.resize_with(CUP_NUMBER, Default::default);
let cup_iter = input_cups.clone().into_iter().chain((max + 1)..=CUP_NUMBER);
let next_cup_iter = input_cups
.into_iter()
.chain((max + 1)..=CUP_NUMBER)
.cycle()
.skip(1);
for (cup, next) in cup_iter.zip(next_cup_iter) {
cups[cup - 1] = next;
}
let current = cups[CUP_NUMBER - 1];
Ok(Self { cups, current })
}
}
#[cfg(test)]
mod tests {
use super::*;
@ -153,4 +271,16 @@ mod tests {
fn part1_real() {
assert_eq!(part1(INPUT).unwrap(), "72496583");
}
#[test]
#[ignore]
fn part2_provided() {
assert_eq!(part2(PROVIDED).unwrap(), 149245887792);
}
#[test]
#[ignore]
fn part2_real() {
assert_eq!(part2(INPUT).unwrap(), 41785843847);
}
}