2020: day23: part 1
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@ -21,6 +21,7 @@ use aoc2020::day18;
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use aoc2020::day19;
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use aoc2020::day21;
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use aoc2020::day22;
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use aoc2020::day23;
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fn aoc2020_all(c: &mut Criterion) {
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c.bench_function("day01", |b| b.iter(|| day01::run().unwrap()));
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@ -44,6 +45,7 @@ fn aoc2020_all(c: &mut Criterion) {
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c.bench_function("day19", |b| b.iter(|| day19::run().unwrap()));
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c.bench_function("day21", |b| b.iter(|| day21::run().unwrap()));
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c.bench_function("day22", |b| b.iter(|| day22::run().unwrap()));
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c.bench_function("day23", |b| b.iter(|| day23::run().unwrap()));
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}
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criterion_group! {
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1
aoc2020/input/day23.txt
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1
aoc2020/input/day23.txt
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@ -0,0 +1 @@
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315679824
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1
aoc2020/input/day23_provided.txt
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1
aoc2020/input/day23_provided.txt
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@ -0,0 +1 @@
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389125467
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156
aoc2020/src/day23.rs
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156
aoc2020/src/day23.rs
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@ -0,0 +1,156 @@
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use std::{collections::VecDeque, fmt::Write};
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use anyhow::{Context, Result};
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const INPUT: &str = include_str!("../input/day23.txt");
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pub fn run() -> Result<String> {
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let mut res = String::with_capacity(128);
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writeln!(res, "part 1: {}", part1(INPUT)?)?;
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Ok(res)
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}
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fn part1(input: &str) -> Result<String> {
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let mut cup_circle: CupCircle = input.parse()?;
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for _ in 0..100 {
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cup_circle.step();
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}
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Ok(format!("{}", cup_circle))
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}
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struct CupCircle(VecDeque<u64>);
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/// CupCircle provides an abstraction over a VecDeque to emulate the steps of the game
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///
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/// The "current" cup should always be the first one in the VecDeque at the end of a turn, for
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/// convenience
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///
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/// The Circle should never become empty if you only use its public interface, so all calls to
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/// unwrap() in its implementation should never panic.
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impl CupCircle {
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pub fn front(&self) -> u64 {
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*self.0.front().unwrap()
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}
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fn pop_front(&mut self) -> u64 {
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self.0.pop_front().unwrap()
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}
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fn push_back(&mut self, value: u64) {
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self.0.push_back(value);
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}
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pub fn max(&self) -> u64 {
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*self.0.iter().max().unwrap()
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}
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fn remove_next_3(&mut self) -> [u64; 3] {
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let first = self.pop_front();
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let second = self.pop_front();
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let third = self.pop_front();
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[first, second, third]
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}
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/// Shifts the cup circle, putting the first cup at the end of the deque
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///
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/// This doesn't change anything to the cup circle layout, except that the new first cup is
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/// considered the "current" cup at the end of a step in our implementation
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fn shift(&mut self) {
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let front = self.0.pop_front().unwrap();
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self.0.push_back(front);
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}
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/// Executes one step of the game
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pub fn step(&mut self) {
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let current = self.front();
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// skip current
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self.shift();
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// The crab picks up the three cups that are immediately clockwise of the current cup. They
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// are removed from the circle; cup spacing is adjusted as necessary to maintain the circle.
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let removed_cups = self.remove_next_3();
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// The crab selects a destination cup: the cup with a label equal to the current cup's label
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// minus one. If this would select one of the cups that was just picked up, the crab will
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// keep subtracting one until it finds a cup that wasn't just picked up. If at any point in
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// this process the value goes below the lowest value on any cup's label, it wraps around to
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// the highest value on any cup's label instead.
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//
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// TODO: use std::cmp::Ord::clamp when stabilized (Rust 1.50)
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let mut destination = if current > 1 { current - 1 } else { self.max() };
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while removed_cups.contains(&destination) {
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destination = if destination > 1 {
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destination - 1
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} else {
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self.max()
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};
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}
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// place destination in front
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while self.front() != destination {
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self.shift();
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}
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// The crab places the cups it just picked up so that they are immediately clockwise of the
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// destination cup. They keep the same order as when they were picked up.
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//
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// For this, let's put the destination at the end of the queue by shifting one last time
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self.shift();
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removed_cups.iter().for_each(|cup| self.push_back(*cup));
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// The crab selects a new current cup: the cup which is immediately clockwise of the current
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// cup.
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while self.front() != current {
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self.shift();
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}
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self.shift();
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}
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}
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impl std::str::FromStr for CupCircle {
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type Err = anyhow::Error;
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fn from_str(s: &str) -> Result<Self, Self::Err> {
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let circle = s
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.trim_end()
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.chars()
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.map(|c| Ok(c.to_digit(10).context("character was not a digit")? as u64))
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.collect::<Result<_>>()?;
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Ok(CupCircle(circle))
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}
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}
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impl std::fmt::Display for CupCircle {
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fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
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self.0
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.iter()
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.cycle()
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.skip_while(|cup| **cup != 1)
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.skip(1)
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.take(8)
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.try_for_each(|cup| write!(f, "{}", cup))
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}
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}
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#[cfg(test)]
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mod tests {
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use super::*;
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const PROVIDED: &str = include_str!("../input/day23_provided.txt");
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#[test]
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fn part1_provided() {
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assert_eq!(part1(PROVIDED).unwrap(), "67384529");
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}
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#[test]
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fn part1_real() {
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assert_eq!(part1(INPUT).unwrap(), "72496583");
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}
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}
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@ -21,3 +21,4 @@ pub mod day18;
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pub mod day19;
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pub mod day21;
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pub mod day22;
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pub mod day23;
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@ -23,6 +23,7 @@ use aoc2020::day18;
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use aoc2020::day19;
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use aoc2020::day21;
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use aoc2020::day22;
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use aoc2020::day23;
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fn main() -> Result<()> {
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let days: &[DayFunc] = &[
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@ -47,6 +48,7 @@ fn main() -> Result<()> {
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day19::run,
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day21::run,
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day22::run,
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day23::run,
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];
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aoc::run(days)
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