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input.lines().collect(); + // all binary numbers should have the same length + let size = binary_numbers[0].len(); + + let gamma = compute_gamma(&binary_numbers, size); + let epsilon = compute_epsilon(gamma, size); + + Ok(gamma * epsilon) +} + +fn compute_gamma(binary_numbers: &[&str], size: usize) -> u64 { + let mut gamma = 0; + + for pos in 0..size { + let digit = if count_ones(binary_numbers, pos) > (binary_numbers.len() / 2) { + // majority of ones + 1 + } else { + // majority of zeroes + 0 + }; + gamma = (gamma << 1) | digit; + } + + gamma +} + +fn count_ones(binary_numbers: &[&str], pos: usize) -> usize { + binary_numbers + .iter() + .filter(|&&num| num.chars().nth(pos).unwrap() == '1') + .count() +} + +fn compute_epsilon(gamma: u64, size: usize) -> u64 { + // mask 0b000000000000000011111111 with `size` 1s. + let shift = u64::BITS - (size as u32); + let mask = (u64::MAX << shift) >> shift; + + (!gamma) & mask +} + +#[cfg(test)] +mod tests { + use super::*; + + const PROVIDED: &str = include_str!("../input/day03_provided.txt"); + #[test] + fn part1_provided() { + let binary_numbers: Vec<&str> = PROVIDED.lines().collect(); + let size = binary_numbers[0].len(); + + let gamma = compute_gamma(&binary_numbers, size); + let epsilon = compute_epsilon(gamma, size); + + assert_eq!(gamma, 22); + assert_eq!(epsilon, 9); + } + + #[test] + fn part1_real() { + assert_eq!(part1(INPUT).unwrap(), 3429254); + } +} diff --git a/aoc2021/src/lib.rs b/aoc2021/src/lib.rs index c10d8ec..eef213c 100644 --- a/aoc2021/src/lib.rs +++ b/aoc2021/src/lib.rs @@ -2,3 +2,4 @@ pub mod day01; pub mod day02; +pub mod day03; diff --git a/aoc2021/src/main.rs b/aoc2021/src/main.rs index 316799b..2315917 100644 --- a/aoc2021/src/main.rs +++ b/aoc2021/src/main.rs @@ -4,9 +4,10 @@ use aoc::DayFunc; use aoc2021::day01; use aoc2021::day02; +use aoc2021::day03; fn main() -> Result<()> { - let days: &[DayFunc] = &[day01::run, day02::run]; + let days: &[DayFunc] = &[day01::run, day02::run, day03::run]; aoc::run(days) }