Day 3

input/03-test.txt

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

src/bin/01.rs

@@ -26,4 +26,4 @@ fn input() -> impl Iterator<Item = i32> {
     include_str!("../../input/01a.txt")
         .lines()
         .map(|l| l.parse().unwrap())
-}
+}

src/bin/03.rs

@@ -0,0 +1,74 @@
+fn main() {
+    let gamma_bits = gamma(&input());
+    let gamma = to_dec(&gamma_bits);
+    let epsilon = to_dec(&invert(&gamma_bits));
+    println!(
+        "First solution: gamma={}, epsilon={}, product={}",
+        gamma,
+        epsilon,
+        gamma * epsilon
+    );
+
+    let oxygen = to_dec(&life_support(input(), false));
+    let co2 = to_dec(&life_support(input(), true));
+    println!(
+        "Second solution: oxygen={}, co2={}, product={}",
+        oxygen,
+        co2,
+        oxygen * co2
+    );
+}
+
+type Binary = Vec<i32>;
+
+fn life_support(mut it: Vec<Binary>, co2: bool) -> Binary {
+    let mut i = 0;
+    while it.len() > 1 {
+        let gamma = gamma(&it);
+        it = it
+            .into_iter()
+            .filter(|b| b[i] == gamma[i] ^ co2 as i32)
+            .collect();
+        i += 1;
+    }
+    it.remove(0)
+}
+
+fn to_dec(b: &Binary) -> i32 {
+    let mut acc = 0;
+
+    for (i, n) in b.iter().rev().copied().enumerate() {
+        acc |= n << i;
+    }
+
+    acc
+}
+
+fn invert(b: &Binary) -> Binary {
+    b.iter().map(|&n| n ^ 1).collect()
+}
+
+/// gamma returns a pattern of the most common bits for each position.
+fn gamma(it: &Vec<Binary>) -> Binary {
+    let mut iter = it.clone().into_iter();
+    let mut acc = iter.next().unwrap();
+
+    for n in iter {
+        for i in 0..acc.len() {
+            acc[i] += 2 * n[i] - 1;
+        }
+    }
+
+    for i in 0..acc.len() {
+        acc[i] = (acc[i] > 0) as i32;
+    }
+    acc
+}
+
+/// several lines, each line has several digits
+fn input() -> Vec<Binary> {
+    include_str!("../../input/03.txt")
+        .lines()
+        .map(|l| l.chars().map(|c| c.to_digit(2).unwrap() as i32).collect())
+        .collect()
+}