Don’t use log2 to get around bit count
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@ -1,9 +1,8 @@
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#![feature(int_log)]
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#![feature(test)]
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extern crate test;
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use aoc2021::common::*;
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const DAY: usize = 03;
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const DAY: usize = 3;
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type Parsed = Vec<usize>;
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fn parse_input(raw: &str) -> Parsed {
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@ -22,13 +21,13 @@ fn most_common_at(parsed: &Parsed, n: usize) -> bool {
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parsed.iter().filter(|&&x| bit_at(x, n)).count() * 2 >= parsed.len()
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}
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fn invert(n: usize) -> usize {
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!n & ((1 << n.log2()) - 1)
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fn invert(n: usize, bits: usize) -> usize {
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!n & ((1 << bits) - 1)
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}
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fn part1(parsed: &Parsed, bits: usize) -> usize {
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let gamma = most_common(parsed, bits);
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let epsilon = invert(gamma);
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let epsilon = invert(gamma, bits);
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gamma * epsilon
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}
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@ -39,7 +38,9 @@ fn part2(parsed: &Parsed, bits: usize) -> usize {
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for i in (0..bits).rev() {
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let gamma = most_common_at(&matching_gamma, i);
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let epsilon = !most_common_at(&matching_epsilon, i);
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// TODO: Find out why retain is significantly slower than filter().collect()
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matching_gamma.retain(|&n| bit_at(n, i) == gamma);
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// matching_gamma = matching_gamma.into_iter().filter(|&n| bit_at(n, i) == gamma).collect();
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if matching_epsilon.len() > 1 {
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matching_epsilon.retain(|&n| bit_at(n, i) == epsilon);
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}
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@ -50,9 +51,11 @@ fn part2(parsed: &Parsed, bits: usize) -> usize {
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}
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fn main() {
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let input = parse_input(&read_file(DAY));
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println!("Part 1: {}", part1(&input, 12));
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println!("Part 2: {}", part2(&input, 12));
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let raw = read_file(DAY);
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let input = parse_input(&raw);
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let line_length = raw.find('\n').unwrap();
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println!("Part 1: {}", part1(&input, line_length));
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println!("Part 2: {}", part2(&input, line_length));
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}
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#[cfg(test)]
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@ -82,7 +85,7 @@ mod tests {
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#[test]
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fn invert_test() {
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let gamma = 0b10110;
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assert_eq!(invert(gamma), 0b01001);
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assert_eq!(invert(gamma, 4), 0b01001);
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}
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#[test]
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