2022 day3/rust: use bitwise arithmetic

2022/day3/rust/src/main.rs

@@ -1,6 +1,6 @@
 use std::{
-    collections::HashSet,
     io::{stdin, Read},
+    ops::{BitAnd, BitOr},
 };
 
 fn item_priority(item: char) -> u32 {
@@ -11,9 +11,48 @@ fn item_priority(item: char) -> u32 {
     }
 }
 
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+struct ItemSet(u64);
+
+impl ItemSet {
+    pub fn priority_of_single_item(self) -> Option<u32> {
+        if self.0.is_power_of_two() {
+            Some(self.0.trailing_zeros())
+        } else {
+            None
+        }
+    }
+}
+
+impl FromIterator<char> for ItemSet {
+    fn from_iter<T: IntoIterator<Item = char>>(iter: T) -> Self {
+        Self(
+            iter.into_iter()
+                .map(item_priority)
+                .fold(0, |acc, i| acc | (1 << i)),
+        )
+    }
+}
+
+impl BitOr for ItemSet {
+    type Output = Self;
+
+    fn bitor(self, rhs: Self) -> Self::Output {
+        Self(BitOr::bitor(self.0, rhs.0))
+    }
+}
+
+impl BitAnd for ItemSet {
+    type Output = Self;
+
+    fn bitand(self, rhs: Self) -> Self::Output {
+        Self(BitAnd::bitand(self.0, rhs.0))
+    }
+}
+
 struct Rucksack {
-    left: HashSet<char>,
-    right: HashSet<char>,
+    left: ItemSet,
+    right: ItemSet,
 }
 
 fn main() {
@@ -26,8 +65,8 @@ fn main() {
             let (left, right) = line.split_at(line.len() / 2);
             assert_eq!(left.len(), right.len());
 
-            let left: HashSet<_> = left.chars().collect();
-            let right: HashSet<_> = right.chars().collect();
+            let left = left.chars().collect();
+            let right = right.chars().collect();
 
             Rucksack { left, right }
         })
@@ -35,21 +74,20 @@ fn main() {
 
     let sum1: u32 = sacks
         .iter()
-        .map(|sack| item_priority(*sack.left.intersection(&sack.right).next().unwrap()))
+        .map(|sack| (sack.left & sack.right).priority_of_single_item().unwrap())
         .sum();
 
     let sum2: u32 = sacks
         .chunks(3)
         .into_iter()
         .map(|sacks| {
-            let common = sacks
+            sacks
                 .iter()
-                .map(|sack| &sack.left | &sack.right)
-                .reduce(|a, b| &a & &b)
-                .unwrap();
-            assert_eq!(common.len(), 1);
-
-            item_priority(common.into_iter().next().unwrap())
+                .map(|sack| sack.left | sack.right)
+                .reduce(BitAnd::bitand)
+                .unwrap()
+                .priority_of_single_item()
+                .unwrap()
         })
         .sum();