Add day 21

Author: LiquidFun

.aoc_tiles/tiles/2020/21.png

@@ -0,0 +1,33 @@
+use std::{collections::{HashMap, HashSet}, io::stdin};
+use itertools::Itertools;
+
+fn main() {
+    let lines = stdin().lines().filter_map(Result::ok).collect_vec();
+
+    let mut all: Vec<&str> = Vec::new();
+    let mut possible = HashMap::new();
+
+    for line in &lines {
+        let (left, right) = line.split_once(" (contains ").unwrap();
+        let ingredients: HashSet<&str> = left.split(" ").collect();
+        let allergens = right.trim_matches(')').split(", ").collect_vec();
+        all.extend(&ingredients);
+
+        for allergen in allergens {
+            possible.entry(allergen)
+                .or_insert(ingredients.clone())
+                .retain(|k| ingredients.contains(k))
+        }
+    }
+    println!("{}", all.iter().filter(|k| !possible.values().flatten().contains(k)).count());
+
+    let mut pairings: HashMap<&str, &str> = HashMap::new();
+    while possible.len() != pairings.len() {
+        possible.iter()
+            .filter(|(_, v)| v.len() == 1)
+            .for_each(|(k, v)| { pairings.insert(k, v.iter().next().unwrap()); });
+        possible.values_mut()
+            .for_each(|v| v.retain(|a| !pairings.values().contains(a)))
+    }
+    println!("{}", pairings.iter().sorted().map(|(_, b)| b).join(","));
+}

2020/21/21.rs

@@ -1,6 +1,6 @@
 <!-- AOC TILES BEGIN -->
 <h1 align="center">
-  Advent of Code - 190/450 ⭐
+  Advent of Code - 192/450 ⭐
 </h1>
 <h1 align="center">
   2023 - 50 ⭐
@@ -237,7 +237,7 @@
   <img src=".aoc_tiles/tiles/2021/25.png" width="161px">
 </a>
 <h1 align="center">
-  2020 - 40 ⭐
+  2020 - 42 ⭐
 </h1>
 <a href="2020/01/01.rs">
   <img src=".aoc_tiles/tiles/2020/01.png" width="161px">
@@ -299,6 +299,9 @@
 <a href="2020/20/20.rs">
   <img src=".aoc_tiles/tiles/2020/20.png" width="161px">
 </a>
+<a href="2020/21/21.rs">
+  <img src=".aoc_tiles/tiles/2020/21.png" width="161px">
+</a>
 <!-- AOC TILES END -->
 
 *The above tiles are clickable, leading to the solution of the corresponding day.*