85 lines
1.9 KiB
Rust
85 lines
1.9 KiB
Rust
use std::fmt::Write;
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use anyhow::{Context, Result};
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const INPUT: &str = include_str!("../input/day25.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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const PUB_KEY_SUBJECT: u64 = 7;
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// - Set the value to itself multiplied by the subject number.
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// - Set the value to the remainder after dividing the value by 20201227.
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fn transform(mut value: u64, subject: u64) -> u64 {
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value *= subject;
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value % 20201227
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}
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fn find_loop_size(pub_key: u64) -> usize {
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let mut value = 1;
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for i in 1.. {
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value = transform(value, PUB_KEY_SUBJECT);
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if value == pub_key {
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return i;
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}
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}
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unreachable!()
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}
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fn part1(input: &str) -> Result<u64> {
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let mut lines = input.lines();
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let card_pub_key: u64 = lines
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.next()
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.context("no public key found for card")?
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.parse()?;
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let door_pub_key: u64 = lines
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.next()
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.context("no public key found for door")?
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.parse()?;
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let card_loop_size = find_loop_size(card_pub_key);
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let door_loop_size = find_loop_size(door_pub_key);
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let (loop_size, pub_key) = if card_loop_size < door_loop_size {
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(card_loop_size, door_pub_key)
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} else {
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(door_loop_size, card_pub_key)
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};
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let mut encryption_key = 1;
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for _ in 0..loop_size {
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encryption_key = transform(encryption_key, pub_key);
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}
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Ok(encryption_key)
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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/day25_provided.txt");
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#[test]
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fn part1_provided() {
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assert_eq!(find_loop_size(5764801), 8);
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assert_eq!(find_loop_size(17807724), 11);
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assert_eq!(part1(PROVIDED).unwrap(), 14897079);
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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(), 3015200);
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}
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}
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