105 lines
3 KiB
Rust
105 lines
3 KiB
Rust
use std::{cmp, fmt::Write};
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use anyhow::{Context, Result};
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const INPUT: &str = include_str!("../input/day03.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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writeln!(res, "part 2: {}", part2(INPUT)?)?;
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Ok(res)
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}
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fn find_max_digit(bank: &[u8]) -> Option<(usize, u8)> {
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bank.iter()
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.copied()
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.enumerate()
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// We want the max, but use min + Reverse because min selects the FIRST minimum, while max
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// selects the LAST maximum, and we want the first (to leave more choices available to us
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// for the units digit).
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.min_by_key(|&(_idx, val)| cmp::Reverse(val))
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}
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fn compute_max_joltage(mut bank: &[u8], batteries: usize) -> Result<u64> {
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let mut joltage = 0;
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for battery in 0..batteries {
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// We don't include the last [batteries - battery - 1] bytes since we need at least these other
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// digits for the final number
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let not_in_search = batteries - battery - 1;
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let (idx, digit) = find_max_digit(&bank[..(bank.len() - not_in_search)])
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.context("couldn't find max digit in bank")?;
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let digit = (digit - b'0') as u64;
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joltage = joltage * 10 + digit;
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bank = &bank[(idx + 1)..];
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}
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Ok(joltage)
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}
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fn part1(input: &str) -> Result<u64> {
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let mut sum = 0;
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for line in input.lines() {
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let joltage = compute_max_joltage(line.as_bytes(), 2)
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.with_context(|| format!("couldn't compute joltage for bank `{}'", line))?;
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sum += joltage;
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}
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Ok(sum)
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}
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fn part2(input: &str) -> Result<u64> {
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let mut sum = 0;
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for line in input.lines() {
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let joltage = compute_max_joltage(line.as_bytes(), 12)
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.with_context(|| format!("couldn't compute joltage for bank `{}'", line))?;
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sum += joltage;
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}
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Ok(sum)
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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/day03_provided.txt");
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#[test]
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fn part1_provided() {
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assert_eq!(
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compute_max_joltage("987654321111111".as_bytes(), 2).ok(),
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Some(98)
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);
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assert_eq!(
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compute_max_joltage("811111111111119".as_bytes(), 2).ok(),
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Some(89)
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);
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assert_eq!(
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compute_max_joltage("234234234234278".as_bytes(), 2).ok(),
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Some(78)
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);
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assert_eq!(
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compute_max_joltage("818181911112111".as_bytes(), 2).ok(),
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Some(92)
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);
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// added this test to check for min + Reverse usage
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assert_eq!(compute_max_joltage("9892".as_bytes(), 2).ok(), Some(99));
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assert_eq!(part1(PROVIDED).unwrap(), 357);
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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(), 17034);
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}
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#[test]
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fn part2_provided() {
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assert_eq!(part2(PROVIDED).unwrap(), 3121910778619);
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}
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#[test]
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fn part2_real() {
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assert_eq!(part2(INPUT).unwrap(), 168798209663590);
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}
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}
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