2021: day03 part 1

aoc2021/input/day03_provided.txt

@@ -0,0 +1,12 @@
+00100
+11110
+10110
+10111
+10101
+01111
+00111
+11100
+10000
+11001
+00010
+01010

aoc2021/src/day03.rs

@@ -0,0 +1,79 @@
+use std::fmt::Write;
+
+use anyhow::Result;
+
+const INPUT: &str = include_str!("../input/day03.txt");
+
+pub fn run() -> Result<String> {
+    let mut res = String::with_capacity(128);
+
+    writeln!(res, "part 1: {}", part1(INPUT)?)?;
+
+    Ok(res)
+}
+
+fn part1(input: &str) -> Result<u64> {
+    let binary_numbers: Vec<&str> = input.lines().collect();
+    // all binary numbers should have the same length
+    let size = binary_numbers[0].len();
+
+    let gamma = compute_gamma(&binary_numbers, size);
+    let epsilon = compute_epsilon(gamma, size);
+
+    Ok(gamma * epsilon)
+}
+
+fn compute_gamma(binary_numbers: &[&str], size: usize) -> u64 {
+    let mut gamma = 0;
+
+    for pos in 0..size {
+        let digit = if count_ones(binary_numbers, pos) > (binary_numbers.len() / 2) {
+            // majority of ones
+            1
+        } else {
+            // majority of zeroes
+            0
+        };
+        gamma = (gamma << 1) | digit;
+    }
+
+    gamma
+}
+
+fn count_ones(binary_numbers: &[&str], pos: usize) -> usize {
+    binary_numbers
+        .iter()
+        .filter(|&&num| num.chars().nth(pos).unwrap() == '1')
+        .count()
+}
+
+fn compute_epsilon(gamma: u64, size: usize) -> u64 {
+    // mask 0b000000000000000011111111 with `size` 1s.
+    let shift = u64::BITS - (size as u32);
+    let mask = (u64::MAX << shift) >> shift;
+
+    (!gamma) & mask
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const PROVIDED: &str = include_str!("../input/day03_provided.txt");
+    #[test]
+    fn part1_provided() {
+        let binary_numbers: Vec<&str> = PROVIDED.lines().collect();
+        let size = binary_numbers[0].len();
+
+        let gamma = compute_gamma(&binary_numbers, size);
+        let epsilon = compute_epsilon(gamma, size);
+
+        assert_eq!(gamma, 22);
+        assert_eq!(epsilon, 9);
+    }
+
+    #[test]
+    fn part1_real() {
+        assert_eq!(part1(INPUT).unwrap(), 3429254);
+    }
+}

aoc2021/src/lib.rs

@@ -2,3 +2,4 @@
 
 pub mod day01;
 pub mod day02;
+pub mod day03;

aoc2021/src/main.rs

@@ -4,9 +4,10 @@ use aoc::DayFunc;
 
 use aoc2021::day01;
 use aoc2021::day02;
+use aoc2021::day03;
 
 fn main() -> Result<()> {
-    let days: &[DayFunc] = &[day01::run, day02::run];
+    let days: &[DayFunc] = &[day01::run, day02::run, day03::run];
 
     aoc::run(days)
 }