Tricky solution day 03, could possibly be improved

2021/src/day03.rs

@@ -1,9 +1,104 @@
 use std::io::Read;
 
-pub fn part1(_input: &mut dyn Read) -> String {
-    todo!()
+use crate::common::LineIter;
+
+fn parse_bit(bit: char) -> usize {
+    if bit == '1' {
+        1
+    } else {
+        0
+    }
 }
 
-pub fn part2(_input: &mut dyn Read) -> String {
-    todo!()
+pub fn part1(input: &mut dyn Read) -> String {
+    let mut reader = LineIter::new(input);
+
+    let mut totals: Vec<usize> = reader.next().unwrap().chars().map(parse_bit).collect();
+
+    let mut count = 1;
+
+    while let Some(line) = reader.next() {
+        line.chars()
+            .map(parse_bit)
+            .zip(totals.iter_mut())
+            .for_each(|(b, t)| *t += b);
+
+        count += 1;
+    }
+
+    let bitmask = (1 << totals.len()) - 1;
+
+    let gamma = totals.into_iter().fold(0, |mut current, total| {
+        current <<= 1;
+        if total > count / 2 {
+            current |= 1;
+        };
+
+        current
+    });
+
+    let epsilon = (!gamma) & bitmask;
+
+    (gamma * epsilon).to_string()
+}
+
+fn find_remaining(mut strings: Vec<Vec<u8>>, most: bool) -> u32 {
+    let comp: fn(usize, usize) -> bool = if most {
+        |occurrences, len| occurrences * 2 >= len
+    } else {
+        |occurrences, len| occurrences * 2 < len
+    };
+
+    for pos in 0..strings[0].len() {
+        if strings.len() == 1 {
+            break;
+        }
+
+        let occurrences = strings.iter().filter(|b| b[pos] == b'1').count();
+
+        let keep = if comp(occurrences, strings.len()) {
+            b'1'
+        } else {
+            b'0'
+        };
+
+        strings.retain(|s| s[pos] == keep);
+    }
+
+    strings[0]
+        .iter()
+        .fold(0, |n, &b| (n << 1) | (b - b'0') as u32)
+}
+
+pub fn part2(input: &mut dyn Read) -> String {
+    let mut strings = Vec::new();
+    let mut reader = LineIter::new(input);
+
+    while let Some(line) = reader.next() {
+        strings.push(line.as_bytes().to_owned());
+    }
+
+    let oxygen = find_remaining(strings.clone(), true);
+    let co2 = find_remaining(strings, false);
+
+    (oxygen * co2).to_string()
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::test_implementation;
+
+    use super::*;
+
+    const SAMPLE: &[u8] = include_bytes!("samples/03.txt");
+
+    #[test]
+    fn sample_part1() {
+        test_implementation(part1, SAMPLE, 198);
+    }
+
+    #[test]
+    fn sample_part2() {
+        test_implementation(part2, SAMPLE, 230)
+    }
 }

2021/src/samples/03.txt

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