diff --git a/src/year2016/day18.rs b/src/year2016/day18.rs
index 9a655d9..955a2e7 100644
--- a/src/year2016/day18.rs
+++ b/src/year2016/day18.rs
@@ -41,7 +41,7 @@ fn count(input: &str, rows: u32) -> u32 {
         // Count the traps in each row.
         total += row.count_ones();
         // Only consider the left and right values for the next row.
-        row = (row << 1) ^ (row >> 1) & mask;
+        row = ((row << 1) ^ (row >> 1)) & mask;
     }
 
     // We want the number of safe tiles so convert from the number of traps.
diff --git a/src/year2020/day21.rs b/src/year2020/day21.rs
index 25ce4cc..49f09f2 100644
--- a/src/year2020/day21.rs
+++ b/src/year2020/day21.rs
@@ -45,6 +45,7 @@
 //! [`Day 16`]: crate::year2020::day16
 //! [`count_ones`]: u32::count_ones
 use crate::util::hash::*;
+use std::collections::BTreeMap;
 
 pub struct Input<'a> {
     ingredients: FastMap<&'a str, Ingredient>,
@@ -102,34 +103,31 @@ pub fn part1(input: &Input<'_>) -> u32 {
 }
 
 pub fn part2(input: &Input<'_>) -> String {
-    let mut ingredients = input.ingredients.clone();
-    ingredients.retain(|_, v| v.candidates != 0);
-
     let inverse_allergens: FastMap<_, _> =
         input.allergens.iter().map(|(k, v)| (1 << v, k)).collect();
 
-    // There must be at least one ingredient with only one allergen.
-    let mut todo: Vec<_> = ingredients
-        .iter()
-        .filter(|(_, v)| v.candidates.count_ones() == 1)
-        .map(|(k, v)| (*k, v.candidates))
-        .collect();
-    let mut done = Vec::new();
+    let mut ingredients = input.ingredients.clone();
+    ingredients.retain(|_, v| v.candidates != 0);
+
+    let mut done = BTreeMap::new();
 
     // Eliminate known allergens from other ingredients.
-    while let Some(pair @ (next, allergen)) = todo.pop() {
-        ingredients.remove(next);
-        done.push(pair);
+    while done.len() < ingredients.len() {
+        let mut mask = 0;
 
-        for (name, ingredient) in &mut ingredients {
-            ingredient.candidates &= !allergen;
+        // There must be at least one ingredient with only one allergen.
+        for (name, ingredient) in &ingredients {
             if ingredient.candidates.count_ones() == 1 {
-                todo.push((name, ingredient.candidates));
+                let allergen = inverse_allergens[&ingredient.candidates];
+                done.insert(*allergen, *name);
+
+                mask |= ingredient.candidates;
             }
         }
+
+        ingredients.values_mut().for_each(|ingredient| ingredient.candidates &= !mask);
     }
 
     // Sort by alphabetical order of the allergens.
-    done.sort_by_cached_key(|(_, v)| inverse_allergens[v]);
-    done.iter().map(|(k, _)| *k).collect::<Vec<_>>().join(",")
+    done.into_values().collect::<Vec<_>>().join(",")
 }