Completed day 15

day15/Cargo.lock

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+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+[[package]]
+name = "anyhow"
+version = "1.0.35"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "2c0df63cb2955042487fad3aefd2c6e3ae7389ac5dc1beb28921de0b69f779d4"
+
+[[package]]
+name = "day15"
+version = "0.1.0"
+dependencies = [
+ "anyhow",
+]

day15/Cargo.toml

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+[package]
+name = "day15"
+version = "0.1.0"
+authors = ["Tyler Hallada <tyler@hallada.net>"]
+edition = "2018"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+anyhow = "1.0"

day15/input/input.txt

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+16,1,0,18,12,14,19

day15/input/test.txt

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+0,3,6

day15/src/main.rs

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+use anyhow::{anyhow, Result};
+use std::collections::HashMap;
+use std::fs::File;
+use std::io::prelude::*;
+use std::time::Instant;
+
+const INPUT: &str = "input/input.txt";
+
+fn take_turn(nums: &mut Vec<usize>) {
+    if let Some(position) = nums[..nums.len() - 1]
+        .iter()
+        .rposition(|&num| num == nums[nums.len() - 1])
+    {
+        nums.push(nums.len() - (position + 1));
+    } else {
+        nums.push(0);
+    }
+}
+
+#[derive(Debug)]
+struct Game {
+    turn: usize,
+    last_num: usize,
+    prev_nums: HashMap<usize, usize>,
+}
+
+impl Game {
+    fn new(input: &str) -> Result<Self> {
+        let mut nums = input
+            .split(",")
+            .enumerate()
+            .map(|(position, num)| Ok((num.trim().parse()?, position + 1)))
+            .collect::<Result<Vec<(usize, usize)>>>()?;
+        let (last_num, turn) = nums.pop().ok_or_else(|| anyhow!("input is empty"))?;
+        let prev_nums = nums.into_iter().collect();
+        Ok(Self {
+            turn,
+            last_num,
+            prev_nums,
+        })
+    }
+
+    fn take_turn(&mut self) {
+        self.turn += 1;
+        let new_num = if let Some(prev_turn) = self.prev_nums.get(&self.last_num) {
+            self.turn - prev_turn - 1
+        } else {
+            0
+        };
+        self.prev_nums.insert(self.last_num, self.turn - 1);
+        self.last_num = new_num;
+    }
+}
+
+fn solve_part1(input_path: &str) -> Result<usize> {
+    let mut file = File::open(input_path)?;
+    let mut nums = String::new();
+    file.read_to_string(&mut nums)?;
+    let mut nums: Vec<usize> = nums
+        .split(",")
+        .map(|num| Ok(num.trim().parse()?))
+        .collect::<Result<Vec<usize>>>()?;
+    let num_turns = 2020 - nums.len();
+    for _ in 0..num_turns {
+        take_turn(&mut nums);
+    }
+    Ok(nums.pop().expect("non-empty nums"))
+}
+
+fn solve_part2(input_path: &str) -> Result<usize> {
+    let mut file = File::open(input_path)?;
+    let mut nums = String::new();
+    file.read_to_string(&mut nums)?;
+    let mut game = Game::new(&nums)?;
+    let num_turns = 30000000 - game.turn;
+    for _ in 0..num_turns {
+        game.take_turn();
+    }
+    Ok(game.last_num)
+}
+
+fn main() {
+    let mut now = Instant::now();
+    println!("Part 1: {}", solve_part1(INPUT).unwrap());
+    println!("(elapsed: {:?})", now.elapsed());
+    now = Instant::now();
+    println!("");
+    println!("Part 2: {}", solve_part2(INPUT).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).unwrap(), 436);
+    }
+
+    #[test]
+    fn solves_part2() {
+        assert_eq!(solve_part2(TEST_INPUT).unwrap(), 175594);
+    }
+}