From c4518496d3c5794d0125a26472946e8d0acd5120 Mon Sep 17 00:00:00 2001
From: Noratrieb <48135649+Noratrieb@users.noreply.github.com>
Date: Sat, 7 Dec 2024 12:19:30 +0100
Subject: [PATCH] d6p2

---
 2024/day05/src/lib.rs | 102 +++++++++++++++++++++++++++++++++++++++---
 1 file changed, 97 insertions(+), 5 deletions(-)

diff --git a/2024/day05/src/lib.rs b/2024/day05/src/lib.rs
index a6751d2..8c7fadc 100644
--- a/2024/day05/src/lib.rs
+++ b/2024/day05/src/lib.rs
@@ -1,3 +1,5 @@
+use std::collections::VecDeque;
+
 use helper::{parse_unwrap, Day, IteratorExt, Variants};
 
 pub fn main() {
@@ -12,7 +14,7 @@ helper::define_variants! {
         basic => crate::part1, sample_count=1000;
     }
     part2 {
-        basic => crate::part2;
+        basic => crate::part2, sample_count=1000;
     }
 }
 
@@ -124,8 +126,98 @@ fn part1(input: &str) -> u64 {
     result
 }
 
-fn part2(_input: &str) -> u64 {
-    0
+fn part2(input: &str) -> u64 {
+    struct Update {
+        values: Vec<u64>,
+        set: [bool; 100],
+    }
+
+    let mut rules = Vec::new();
+    let mut updates = Vec::new();
+    let mut lines = input.lines();
+    while let Some(line) = lines.next() {
+        if line.is_empty() {
+            break;
+        }
+        let values = line
+            .split('|')
+            .collect_array::<2>()
+            .unwrap()
+            .map(parse_unwrap);
+        rules.push((values[0], values[1]));
+    }
+    while let Some(line) = lines.next() {
+        let values = line.split(",").map(parse_unwrap).collect::<Vec<_>>();
+        let mut set = [false; 100];
+        for value in &values {
+            set[*value as usize] = true;
+        }
+        updates.push(Update { values, set });
+    }
+
+    fn build_nodes(edges: &mut Vec<[bool; 100]>, rules: impl Iterator<Item = (u64, u64)>) {
+        edges.clear();
+
+        edges.resize(100, [false; 100]);
+
+        for (first, then) in rules {
+            edges[first as usize][then as usize] = true;
+        }
+    }
+
+    let mut edges = Vec::new();
+
+    let mut result = 0;
+    for update in updates {
+        build_nodes(
+            &mut edges,
+            rules
+                .iter()
+                .filter(|(a, b)| update.set[*a as usize] && update.set[*b as usize])
+                .copied(),
+        );
+
+        let mut is_bad = false;
+        for ab in update.values.windows(2) {
+            let (a, b) = (ab[0], ab[1]);
+            if edges[b as usize][a as usize] {
+                is_bad = true;
+                break;
+            }
+        }
+        if is_bad {
+            // do Topological sort:
+            // https://en.wikipedia.org/wiki/Topological_sorting#Depth-first_search
+            let mut all_sorted = VecDeque::new();
+            let mut marked = [false; 100];
+
+            fn visit(
+                all_sorted: &mut VecDeque<u64>,
+                marked: &mut [bool; 100],
+                edges: &[[bool; 100]],
+                node: usize,
+            ) {
+                if marked[node] {
+                    return;
+                }
+
+                for (child, _) in edges[node].iter().enumerate().filter(|(_, edge)| **edge) {
+                    visit(all_sorted, marked, edges, child);
+                }
+
+                marked[node] = true;
+                all_sorted.push_front(node as u64);
+            }
+
+            while let Some(next) = update.values.iter().find(|value| !marked[**value as usize]) {
+                visit(&mut all_sorted, &mut marked, &edges, *next as usize);
+            }
+
+            result += all_sorted[all_sorted.len() / 2];
+        }
+    }
+
+    result
 }
 
 helper::tests! {
@@ -135,8 +227,8 @@ helper::tests! {
         default => 4959;
     }
     part2 {
-        small => 0;
-        default => 0;
+        small => 123;
+        default => 4655;
     }
 }
 helper::benchmarks! {}