Completed day 23 part 1

day23/Cargo.lock

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+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 3
+
+[[package]]
+name = "anyhow"
+version = "1.0.48"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "62e1f47f7dc0422027a4e370dd4548d4d66b26782e513e98dca1e689e058a80e"
+
+[[package]]
+name = "day23"
+version = "0.1.0"
+dependencies = [
+ "anyhow",
+]

day23/Cargo.toml

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+[package]
+name = "day23"
+version = "0.1.0"
+edition = "2018"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+anyhow = "1.0"

day23/input/input.txt

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+538914762

day23/input/test.txt

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+389125467

day23/src/main.rs

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+use anyhow::{Error, Result};
+use std::fs::File;
+use std::io::{BufRead, BufReader};
+use std::time::Instant;
+use std::str::FromStr;
+use std::fmt;
+
+const INPUT: &str = "input/input.txt";
+
+#[derive(Debug, PartialEq, Eq)]
+struct Circle {
+    cups: Vec<u64>,
+    current_cup: usize,
+}
+
+impl FromStr for Circle {
+    type Err = Error;
+
+    fn from_str(s: &str) -> Result<Self> {
+        let cups = s
+            .chars()
+            .map(|c| c.to_digit(10).unwrap() as u64)
+            .collect::<Vec<u64>>();
+        Ok(Circle { cups, current_cup: 0 })
+    }
+}
+
+impl fmt::Display for Circle {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        for (index, cup) in self.cups.iter().enumerate() {
+            if self.current_cup == index {
+                write!(f, " ({}) ", cup)?;
+            } else {
+                write!(f, " {} ", cup)?;
+            }
+        }
+        writeln!(f, "")?;
+        Ok(())
+    }
+}
+
+impl Circle {
+    // need to rewrite this for part 2 to use a secondary "next" Vec that uses a cup's label as the
+    // index and the next (clockwise) cup's label as the value.
+    fn run_move(&mut self) {
+        let current_cup_label = self.cups[self.current_cup];
+        if self.current_cup + 4 > self.cups.len() {
+            self.cups.rotate_left(3);
+            self.current_cup = self.current_cup - 3;
+        }
+        let three_cups_start = (self.current_cup + 1) % self.cups.len();
+        let three_cups_end = (self.current_cup + 4) % (self.cups.len() + 1);
+        let three_cups: Vec<u64> = self.cups.drain(three_cups_start..three_cups_end).collect();
+        let mut destination_cup_label = (current_cup_label - 1) % 10;
+        loop {
+            let destination_cup = self.cups.iter().position(|&c| c == destination_cup_label);
+            if let Some(destination_cup) = destination_cup {
+                let destination_cup = destination_cup + 1 % self.cups.len();
+                self.cups.splice(destination_cup..destination_cup, three_cups);
+                break;
+            }
+            destination_cup_label = destination_cup_label.checked_sub(1).unwrap_or(9);
+        }
+        self.current_cup = self.cups.iter().position(|&c| c == current_cup_label).unwrap();
+        self.current_cup = (self.current_cup + 1) % self.cups.len();
+    }
+
+    fn expand_for_part2(&mut self) {
+        let mut max = *self.cups.iter().max().unwrap() as u64;
+        while max <= 1000000 {
+            self.cups.push(max);
+            max += 1;
+        }
+    }
+}
+
+fn solve_part1(input_path: &str, n: usize) -> Result<u64> {
+    let file = File::open(input_path)?;
+    let reader = BufReader::new(file);
+    let line = reader.lines().next().unwrap()?;
+    let mut circle = Circle::from_str(&line)?;
+    for _ in 0..n {
+        circle.run_move();
+    }
+    let one_index = circle.cups.iter().position(|&c| c == 1).unwrap();
+    circle.cups.rotate_left(one_index);
+    Ok(circle.cups.iter().skip(1).map(|cup| cup.to_string()).collect::<Vec<String>>().join("").parse()?)
+}
+
+fn solve_part2(input_path: &str, n: usize) -> Result<u64> {
+    let file = File::open(input_path)?;
+    let reader = BufReader::new(file);
+    let line = reader.lines().next().unwrap()?;
+    let mut circle = Circle::from_str(&line)?;
+    circle.expand_for_part2();
+    for _ in 0..n {
+        circle.run_move();
+    }
+    let one_index = circle.cups.iter().position(|&c| c == 1).unwrap();
+    Ok(
+        circle.cups[(one_index + 1) % circle.cups.len()] as u64 *
+        circle.cups[(one_index + 2) % circle.cups.len()] as u64
+    )
+}
+
+fn main() {
+    let mut now = Instant::now();
+    println!("Part 1: {}", solve_part1(INPUT, 100).unwrap());
+    println!("(elapsed: {:?})", now.elapsed());
+    now = Instant::now();
+    println!("");
+    println!("Part 2: {}", solve_part2(INPUT, 10000000).unwrap());
+    println!("(elapsed: {:?})", now.elapsed());
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const TEST_INPUT: &str = "input/test.txt";
+
+    #[test]
+    fn solves_part1() {
+        assert_eq!(solve_part1(TEST_INPUT, 10).unwrap(), 92658374);
+        assert_eq!(solve_part1(TEST_INPUT, 100).unwrap(), 67384529);
+    }
+
+    #[test]
+    fn solves_part2() {
+        assert_eq!(
+            solve_part2(TEST_INPUT, 10000000).unwrap(),
+            149245887792
+        );
+    }
+}