80 lines
1.8 KiB
Rust
80 lines
1.8 KiB
Rust
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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/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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Ok(res)
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}
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fn part1(input: &str) -> Result<u64> {
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let binary_numbers: Vec<&str> = input.lines().collect();
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// all binary numbers should have the same length
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let size = binary_numbers[0].len();
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let gamma = compute_gamma(&binary_numbers, size);
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let epsilon = compute_epsilon(gamma, size);
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Ok(gamma * epsilon)
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}
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fn compute_gamma(binary_numbers: &[&str], size: usize) -> u64 {
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let mut gamma = 0;
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for pos in 0..size {
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let digit = if count_ones(binary_numbers, pos) > (binary_numbers.len() / 2) {
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// majority of ones
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1
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} else {
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// majority of zeroes
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0
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};
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gamma = (gamma << 1) | digit;
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}
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gamma
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}
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fn count_ones(binary_numbers: &[&str], pos: usize) -> usize {
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binary_numbers
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.iter()
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.filter(|&&num| num.chars().nth(pos).unwrap() == '1')
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.count()
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}
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fn compute_epsilon(gamma: u64, size: usize) -> u64 {
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// mask 0b000000000000000011111111 with `size` 1s.
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let shift = u64::BITS - (size as u32);
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let mask = (u64::MAX << shift) >> shift;
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(!gamma) & mask
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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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let binary_numbers: Vec<&str> = PROVIDED.lines().collect();
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let size = binary_numbers[0].len();
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let gamma = compute_gamma(&binary_numbers, size);
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let epsilon = compute_epsilon(gamma, size);
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assert_eq!(gamma, 22);
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assert_eq!(epsilon, 9);
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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(), 3429254);
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}
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}
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