part 2 should not have taken this long

2021/src/day3.rs

@@ -1,41 +1,71 @@
 use color_eyre::eyre;
 
 
-fn most_common_bits(numbers: &Vec<u16>) -> u16 {
-    let mut bit_diffs: [isize; 12] = [0; 12];
-    for num in numbers {
-        for i in 0..11 {
-            if num & (1 << i) > 0 {
-                bit_diffs[i] += 1
-            }
-            else {
-                bit_diffs[i] -= 1
-            }
-        }
-    }
+fn most_common_bit(numbers: &Vec<u16>, pos: u8) -> u16 {
+    // number of times the bit in question is 1,
+    // minus the number of times it's 0
+    let mask: u16 = 1 << pos;
+    let total: isize = numbers
+        .iter()
+        .map(|n| if n & mask > 0 {1} else {-1})
+        .sum();
 
-    let mut result = 0u16;
-    for (i, sum) in bit_diffs.iter().enumerate() {
-        let bit = if *sum >= 0 {1} else {0};
-        result |= bit << i;
-    }
-
-    result
-}
-
-
-fn invert_12bit(n: u16) -> u16 {
-    !n & (65535 >> 4) // since these are really only 12-bit numbers we have to clear the 4 spurious 1's that come from inverting the original
+    // if the total is positive, then 1 is more frequent than 0
+    if total >= 0 {1} else {0}
 }
 
 
 fn part1(numbers: &Vec<u16>) -> u64 {
-    let gamma = most_common_bits(numbers);
-    let epsilon = invert_12bit(gamma);
+    let mut gamma = 0u16;
+    for i in 0..12 {
+        let mask = most_common_bit(numbers, i) << i;
+        gamma |= mask;
+    }
+
+    // alternate solution: it feels super cool, but it's not any fewer lines than the above, and way less clear :(
+    // let gamma = (0..12)
+    //     .fold(
+    //         0u16,
+    //         |g, i| g | (most_common_bit(numbers, i) << i)
+    //     );
+
+    // epsilon is just the bit inverse of gamma, but since these are really 12-bit numbers
+    // we have to clear the 4 spurious 1's that come from inverting the original
+    let epsilon = !gamma & (65535 >> 4);
     epsilon as u64 * gamma as u64
 }
 
 
+fn filter_bit_criteria(numbers: Vec<u16>, bit_pos: u8, bit_value: u16) -> Vec<u16> {
+    numbers
+        .into_iter()
+        .filter(|n| (n >> bit_pos) & 1 == bit_value)
+        .collect()
+}
+
+
+fn part2(numbers: &Vec<u16>) -> u64 {
+    let mut oxy = numbers.clone();
+    let mut co2 = numbers.clone();
+    for pos in (0..12).rev() {
+        if oxy.len() > 1 {
+            let oxy_bit = most_common_bit(&oxy, pos);
+            oxy = filter_bit_criteria(oxy, pos, oxy_bit);
+        }
+
+        if co2.len() > 1 {
+            let co2_bit = most_common_bit(&co2, pos) ^ 1; // flip the rightmost bit, giving us the least common instead
+            co2 = filter_bit_criteria(co2, pos, co2_bit);
+        }
+    }
+
+    // assume both vecs have len == 1 now, since otherwise it means
+    // AoC gave us an invalid problem input
+    oxy[0] as u64 * co2[0] as u64
+}
+
+
+
 pub fn run(data: &str) -> eyre::Result<()> {
     let mut nums = Vec::new();
     for line in data.lines() {
@@ -43,5 +73,6 @@ pub fn run(data: &str) -> eyre::Result<()> {
     }
 
     println!("One: {}", part1(&nums));
+    println!("Two: {}", part2(&nums));
     Ok(())
 }