day5 part2

src/day5.rs

···
-#[derive(Copy, Clone, Debug)]
+use std::collections::{HashMap, HashSet};
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
 struct OrderRule {
     before: u8,
     after: u8,
···
     }
 }
 
+fn sort_by_order(order_rules: &[OrderRule], update: &[u8]) -> Vec<u8> {
+    let pages: HashSet<u8> = update.iter().cloned().collect();
+    let relevant_order_rules = order_rules
+        .iter()
+        .filter(|rule| pages.contains(&rule.before) && pages.contains(&rule.after))
+        .collect::<Vec<_>>();
+
+    //directed adjacency list
+    let mut digraph = HashMap::new();
+    for rule in relevant_order_rules {
+        digraph
+            .entry(rule.before)
+            .or_insert(vec![rule.after])
+            .push(rule.after)
+    }
+
+    topological_sort(digraph)
+        .into_iter()
+        .filter(|page| update.contains(page))
+        .collect()
+}
+
+fn topological_sort(mut dag: HashMap<u8, Vec<u8>>) -> Vec<u8> {
+    let mut sort = vec![];
+
+    while !dag.is_empty() {
+        let mut start_nodes: HashSet<u8> = dag.keys().cloned().collect();
+
+        for destination in dag.values().flatten() {
+            start_nodes.remove(destination);
+        }
+        assert_eq!(start_nodes.len(), 1);
+
+        let start = start_nodes.into_iter().next().unwrap();
+        sort.push(start);
+        dag.remove(&start);
+    }
+
+    sort
+}
+
 pub fn day5_part1(input: &str) -> String {
     let (rules, updates) = parse(input);
     let sum: u32 = updates
         .iter()
         .filter(|update| rules.iter().all(|rule| rule.follows_rule(update)))
+        .map(|update| update[update.len() / 2] as u32)
+        .sum();
+    sum.to_string()
+}
+
+pub fn day5_part2(input: &str) -> String {
+    let (rules, updates) = parse(input);
+    let sum: u32 = updates
+        .iter()
+        .filter(|update| rules.iter().any(|rule| !rule.follows_rule(update)))
+        .map(|update| sort_by_order(&rules, update))
         .map(|update| update[update.len() / 2] as u32)
         .sum();
     sum.to_string()

src/lib.rs

···
     }
 
     #[test]
+    fn day5_part2_test() {
+        let test_result = day5::day5_part2(include_str!("../input/day5.test.txt"));
+        assert_eq!(test_result, "123");
+        let result = day5::day5_part2(include_str!("../input/day5.txt"));
+        assert_eq!(result, "4598");
+    }
+
+    #[test]
     fn day4_part1_test() {
         let simplest = "..X...\n.SAMX.\n.A..A.\nXMAS.S\n.X....\n......";
         let test_result = day4::day4_part1(simplest);