84 lines
2.1 KiB
Rust
84 lines
2.1 KiB
Rust
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use std::collections::HashMap;
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use std::fmt::Write;
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use anyhow::Result;
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const INPUT: &str = include_str!("../input/day15.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<u64> {
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let starting_numbers = input.trim_end()
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.split(',')
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.map(|num| num.parse().map_err(anyhow::Error::new))
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.collect::<Result<Vec<u64>>>()?;
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let mut spoken_numbers = HashMap::new();
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let mut next_number: u64 = 0;
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let mut turn: usize = 0;
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for num in starting_numbers {
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let last_turn = speak_number(num, turn, &mut spoken_numbers);
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match last_turn {
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Some(prev_turn) => next_number = (turn - prev_turn) as u64,
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None => next_number = 0,
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}
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turn += 1;
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}
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let mut last_number: u64 = 0;
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while turn < 2020 {
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// store the number we're about to speak for the solution
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last_number = next_number;
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// get the previous time this number was spoken, if any
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let last_turn = speak_number(next_number, turn, &mut spoken_numbers);
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match last_turn {
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// if it was spoken before, the number we'll say next turn is the difference between the
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// two turns where it was last spoken
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Some(prev_turn) => next_number = (turn - prev_turn) as u64,
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// otherwise we'll say 0 next turn
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None => next_number = 0,
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}
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turn += 1
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}
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Ok(last_number)
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}
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/// Inserts the turn a number was spoken in the map. Returns the previous turn the number was
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/// spoken, if any
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fn speak_number(n: u64, turn: usize, spoken_numbers: &mut HashMap<u64, usize>) -> Option<usize> {
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let res = spoken_numbers.get(&n).copied();
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spoken_numbers.insert(n, turn);
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res
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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/day15_provided.txt");
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#[test]
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fn part1_provided() {
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assert_eq!(part1(PROVIDED).unwrap(), 436);
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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(), 447);
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}
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}
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