2 files changed, 108 insertions, 0 deletions
diff --git a/src/day_3.rs b/src/day_3.rs
new file mode 100644
index 0000000..89951ea
--- /dev/null
+++ b/src/day_3.rs
@@ -0,0 +1,106 @@
+use std::ops::AddAssign;
+
+#[derive(Clone)]
+struct Oneness(isize);
+
+fn part1(input: &[String])
+{
+    let scale = calculate_scale(input);
+
+    let mut gamma = 0;
+    let mut epsilon = 0;
+    for (i, oneness) in scale.iter().rev().enumerate() {
+        gamma += (oneness.is_one() || oneness.is_balanced()) as usize * 2_usize.pow(i as u32);
+        epsilon += (oneness.is_zero() || oneness.is_balanced()) as usize * 2_usize.pow(i as u32);
+    }
+
+    println!("Solution to part 1: {}", gamma * epsilon);
+}
+
+fn part2(input: Vec<String>)
+{
+    let mut oxygen = input.clone();
+    let mut pos: usize = 0;
+    while oxygen.len() != 1 {
+        let scale = calculate_scale(&oxygen);
+        let looking_for_one = scale[pos].is_one() || scale[pos].is_balanced();
+        oxygen = oxygen
+            .into_iter()
+            .filter(|s| {
+                let char_is_one = s.as_bytes()[pos] == '1' as u8;
+                looking_for_one == char_is_one
+            })
+            .collect();
+
+        pos += 1;
+    }
+
+    let mut co2 = input;
+    pos = 0;
+    while co2.len() != 1 {
+        let scale = calculate_scale(&co2);
+        let looking_for_one = scale[pos].is_zero();
+        co2 = co2
+            .into_iter()
+            .filter(|s| {
+                let char_is_one = s.as_bytes()[pos] == '1' as u8;
+                looking_for_one == char_is_one
+            })
+            .collect();
+
+        pos += 1;
+    }
+
+    let oxygen = usize::from_str_radix(&oxygen[0], 2).unwrap();
+    let co2 = usize::from_str_radix(&co2[0], 2).unwrap();
+
+    println!("Solution to part 2: {}", oxygen * co2);
+}
+
+fn calculate_scale(input: &[String]) -> Vec<Oneness>
+{
+    let mut one_scale = vec![Oneness::balanced(); input[0].len()];
+    for s in input.iter() {
+        for (i, c) in s.char_indices() {
+            one_scale[i] += match c {
+                '1' => true,
+                '0' => false,
+                o => panic!("Invalid input: {}", o),
+            };
+        }
+    }
+
+    one_scale
+}
+
+pub fn run(input: Vec<String>)
+{
+    // Determine the number of bits by taking a look at the first string.
+    let num_bits = input[0].len();
+    let mut one_scale = vec![Oneness::balanced(); num_bits];
+
+    part1(&input);
+    part2(input);
+}
+
+impl Oneness
+{
+    pub const fn balanced() -> Self { Self(0) }
+
+    pub fn is_one(&self) -> bool { self.0 > 0 }
+
+    pub fn is_zero(&self) -> bool { self.0 < 0 }
+
+    pub fn is_balanced(&self) -> bool { self.0 == 0 }
+}
+
+impl AddAssign<bool> for Oneness
+{
+    fn add_assign(&mut self, rhs: bool)
+    {
+        match rhs {
+            true => self.0 += 1,
+            false => self.0 -= 1,
+        }
+    }
+}
diff --git a/src/main.rs b/src/main.rs
index 0c3f0a7..094d48b 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,5 +1,6 @@
 mod day_1;
 mod day_2;
+mod day_3;
 
 use std::fs::File;
 use std::io::{BufRead, BufReader};
@@ -28,6 +29,7 @@ fn run(day: u8, input: Vec<String>)
     match day {
         1 => day_1::run(input),
         2 => day_2::run(input),
+        3 => day_3::run(input),
         o => panic!("Day {} is not implemented (yet)", o),
     }
 }