Optimized day 6 1/2 solutions

src/day6/1.rs

@@ -21,23 +21,16 @@ fn main() {
 }
 
 fn solution(input: &[u8]) -> usize {
-    let mut buffer = [0; 26];
+    let mut buffer = 0u32;
 
-    for &c in input.iter().take(4) {
-        buffer[(c - b'a') as usize] += 1;
+    for i in 0..input.len() {
+        buffer ^= 1 << (input[i] - b'a');
+
+        if i >= 4 {
+            buffer ^= 1 << (input[i - 4] - b'a');
         }
 
-    if buffer.iter().all(|&c| c == 0 || c == 1) {
-        return 0;
-    }
-
-    for i in 4..input.len() {
-        let c = input[i];
-        buffer[(c - b'a') as usize] += 1;
-        let last = input[i - 4];
-        buffer[(last - b'a') as usize] -= 1;
-
-        if buffer.iter().all(|&c| c == 0 || c == 1) {
+        if buffer.count_ones() == 4 {
             return i + 1;
         }
     }

src/day6/2.rs

@@ -21,15 +21,18 @@ fn main() {
 }
 
 fn solution<const N: usize>(input: &[u8]) -> usize {
-    for idx in 0..input.len() - N {
     let mut buffer = 0u32;
+    for idx in 0..input.len() {
+        // Set the bit when we enter
+        buffer ^= 1 << (input[idx] - b'a');
 
-        for &c in &input[idx..(idx + N)] {
-            buffer |= 1 << (c - b'a') as usize;
+        // Reset it when we leave
+        if idx >= N {
+            buffer ^= 1 << (input[idx - N] - b'a');
         }
 
         if buffer.count_ones() as usize == N {
-            return idx + N;
+            return idx + 1;
         }
     }