1 files changed, 65 insertions, 0 deletions

diff --git a/2021/day03/src/main.rs b/2021/day03/src/main.rs
new file mode 100644
index 0000000..d943de0
--- /dev/null
+++ b/2021/day03/src/main.rs
@@ -0,0 +1,65 @@
+use std::fs;
+use std::path::Path;
+
+const ENTRY_LENGTH: usize = 12;
+
+/* AOC21 Day 3: https://adventofcode.com/2021/day/3 */
+fn main() {
+    let input = Path::new("resources").join("input.txt");
+    let content = fs::read_to_string(input).expect("Unable to read input file");
+    let report: Vec<u32> = content.lines().map(|l| u32::from_str_radix(l,2).expect("Malformed input")).collect();
+    ex1(&report);
+    ex2(report);
+}
+
+fn ex1(report: &[u32]) {
+    let gamma = compute_gamma(&report);
+    let epsilon = compute_epsilon(gamma);
+    println!("Ex1: the result is {} ({} * {})", gamma*epsilon, gamma, epsilon);
+}
+
+fn compute_gamma(report: &[u32]) -> u32 {
+    let length = report.len() as u32;
+    let mut counter: Vec<u32> = vec![0; ENTRY_LENGTH];
+    report.iter().for_each(|entry| counter.iter_mut().enumerate().for_each(|(i,v)| *v += (entry >> i) & 1));
+    counter.iter().enumerate().map(|(i,v)| if v*2 > length { 1 << i } else { 0 }).sum()
+}
+
+fn compute_epsilon(gamma: u32) -> u32 {
+    gamma ^ ((1 << ENTRY_LENGTH)-1)
+}
+
+fn ex2(report: Vec<u32>) {
+    let o2 = compute_rating(report.clone(), ENTRY_LENGTH, 1);
+    let co2 = compute_rating(report, ENTRY_LENGTH, 0);
+    println!("Ex2: the result is {} ({} * {})", o2*co2, o2, co2);
+}
+
+fn compute_rating(mut ratings: Vec<u32>, bit: usize, criterion: u32) -> u32 {
+    if ratings.len() == 1 || bit == 0 {
+        ratings[0]
+    } else {
+        let ones = ratings.iter().filter(|&r| ((r >> bit-1) & 1) == 1).count();
+        let sel = if 2*ones < ratings.len() { 1^criterion } else { criterion };
+        ratings.retain(|&r| ((r >> bit-1) & 1) == sel);
+        compute_rating(ratings, bit-1, criterion)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const TEST_INPUT: [u32;12] = [0b00100, 0b11110, 0b10110, 0b10111, 0b10101, 0b01111, 0b00111, 0b11100, 0b10000, 0b11001, 0b00010, 0b01010];
+    const ENTRY_LENGTH: usize = 5;
+
+    #[test]
+    fn oxygen_generator_rating() {
+        assert_eq!(23,compute_rating(Vec::from(TEST_INPUT), ENTRY_LENGTH, 1))
+    }
+
+    #[test]
+    fn co2_scrubber_rating() {
+        assert_eq!(10,compute_rating(Vec::from(TEST_INPUT), ENTRY_LENGTH, 0))
+    }
+}