aoc/year2021/
day03.rs

1//! # Binary Diagnostic
2//!
3//! Part 1 uses bit manipulation to build up the binary numbers directly one digit at a time.
4//!
5//! Part 2 clones the input `vec` then uses [`swap_remove`] to efficiently discard numbers that
6//! don't meet the criteria without having to move all subsequent elements.
7//!
8//! [`swap_remove`]: Vec::swap_remove
9
10pub struct Input<'a> {
11    width: usize,
12    numbers: Vec<&'a [u8]>,
13}
14
15pub fn parse(input: &str) -> Input<'_> {
16    let numbers: Vec<_> = input.lines().map(str::as_bytes).collect();
17    Input { width: numbers[0].len(), numbers }
18}
19
20pub fn part1(input: &Input<'_>) -> u32 {
21    let mut gamma = 0;
22    let mut epsilon = 0;
23
24    for i in 0..input.width {
25        let sum = sum(&input.numbers, i);
26        if sum > input.numbers.len() - sum {
27            gamma = (gamma << 1) | 1;
28            epsilon <<= 1;
29        } else {
30            gamma <<= 1;
31            epsilon = (epsilon << 1) | 1;
32        }
33    }
34
35    gamma * epsilon
36}
37
38pub fn part2(input: &Input<'_>) -> u32 {
39    let gamma = rating(input, |a, b| a >= b);
40    let epsilon = rating(input, |a, b| a < b);
41    gamma * epsilon
42}
43
44fn sum(numbers: &[&[u8]], i: usize) -> usize {
45    let total: usize = numbers.iter().map(|b| b[i] as usize).sum();
46    total - 48 * numbers.len()
47}
48
49fn fold(numbers: &[u8], width: usize) -> u32 {
50    numbers.iter().take(width).fold(0, |acc, &n| (acc << 1) | (n & 1) as u32)
51}
52
53fn rating(input: &Input<'_>, cmp: impl Fn(usize, usize) -> bool) -> u32 {
54    let mut numbers = input.numbers.clone();
55
56    for i in 0..input.width {
57        let sum = sum(&numbers, i);
58        let keep = if cmp(sum, numbers.len() - sum) { b'1' } else { b'0' };
59        filter(&mut numbers, i, keep);
60        if numbers.len() == 1 {
61            return fold(numbers[0], input.width);
62        }
63    }
64
65    unreachable!()
66}
67
68fn filter(numbers: &mut Vec<&[u8]>, i: usize, keep: u8) {
69    let mut j = 0;
70
71    while j < numbers.len() {
72        if numbers[j][i] == keep {
73            j += 1;
74        } else {
75            numbers.swap_remove(j);
76        }
77    }
78}
aoc/year2021/day03.rs

aoc/year2021/
day03.rs
