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day03.rs
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day03.rs
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//! # Binary Diagnostic
//!
//! Part 1 uses bit manipulation to build up the binary numbers directly one digit at a time.
//!
//! Part 2 clones the input `vec` then uses [`swap_remove`] to efficiently discard numbers that
//! don't meet the criteria without having to move all subsequent elements.
//!
//! [`swap_remove`]: Vec::swap_remove
pub struct Input<'a> {
width: usize,
numbers: Vec<&'a [u8]>,
}
pub fn parse(input: &str) -> Input<'_> {
let numbers: Vec<_> = input.lines().map(str::as_bytes).collect();
Input { width: numbers[0].len(), numbers }
}
pub fn part1(input: &Input<'_>) -> u32 {
let mut gamma = 0;
let mut epsilon = 0;
for i in 0..input.width {
let sum = sum(&input.numbers, i);
if sum > input.numbers.len() - sum {
gamma = (gamma << 1) | 1;
epsilon <<= 1;
} else {
gamma <<= 1;
epsilon = (epsilon << 1) | 1;
}
}
gamma * epsilon
}
pub fn part2(input: &Input<'_>) -> u32 {
let gamma = rating(input, |a, b| a >= b);
let epsilon = rating(input, |a, b| a < b);
gamma * epsilon
}
fn sum(numbers: &[&[u8]], i: usize) -> usize {
let total: usize = numbers.iter().map(|b| b[i] as usize).sum();
total - 48 * numbers.len()
}
fn fold(numbers: &[u8], width: usize) -> u32 {
numbers.iter().take(width).fold(0, |acc, &n| (acc << 1) | (n & 1) as u32)
}
fn rating(input: &Input<'_>, cmp: impl Fn(usize, usize) -> bool) -> u32 {
let mut numbers = input.numbers.clone();
for i in 0..input.width {
let sum = sum(&numbers, i);
let keep = if cmp(sum, numbers.len() - sum) { b'1' } else { b'0' };
filter(&mut numbers, i, keep);
if numbers.len() == 1 {
return fold(numbers[0], input.width);
}
}
unreachable!()
}
fn filter(numbers: &mut Vec<&[u8]>, i: usize, keep: u8) {
let mut j = 0;
while j < numbers.len() {
if numbers[j][i] == keep {
j += 1;
} else {
numbers.swap_remove(j);
}
}
}