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use regex::Regex;
use std::collections::{HashMap, HashSet};
use std::str::FromStr;
type Page = u32;
#[derive(Debug, Clone)]
struct Rule {
before: Page,
after: Page,
}
#[derive(Debug)]
enum ParseError {
ParseError,
}
impl FromStr for Rule {
type Err = ParseError;
fn from_str(s: &str) -> Result<Self, Self::Err> {
let Some(cs) = Regex::new("^([0-9]+)\\|([0-9]+)$").unwrap().captures(s) else {
return Err(ParseError::ParseError);
};
let (_full, [bf, af]) = cs.extract();
Ok(Rule {
before: bf.parse().unwrap(),
after: af.parse().unwrap(),
})
}
}
#[derive(Debug)]
struct RuleStatus {
previous: HashMap<Page, HashSet<Page>>,
invalid: HashSet<Page>,
}
impl RuleStatus {
fn new(rules: &[Rule]) -> Self {
let mut previous = HashMap::new();
for &Rule { before, after } in rules.iter() {
match previous.get_mut(&after) {
None => {
let mut set = HashSet::new();
set.insert(before);
previous.insert(after, set);
}
Some(set) => {
set.insert(before);
}
}
}
RuleStatus {
previous,
invalid: HashSet::new(),
}
}
fn step(&mut self, page: &Page) -> bool {
let RuleStatus { previous, invalid } = self;
if invalid.contains(page) {
return false;
}
if let Some(prev) = previous.get(page) {
for p in prev.iter() {
invalid.insert(*p);
}
}
true
}
}
type Update = Vec<Page>;
fn update_from_str(s: &str) -> Update {
Regex::new("([0-9]+)")
.unwrap()
.find_iter(s)
.map(|s| s.as_str().parse().unwrap())
.collect()
}
fn is_update_valid(update: &Update, rules: &[Rule]) -> bool {
let mut status = RuleStatus::new(rules);
for page in update.iter() {
if !status.step(page) {
return false;
}
}
true
}
fn middle_page(update: &Update) -> Page {
update[update.len() / 2]
}
#[derive(Debug)]
struct Input {
rules: Vec<Rule>,
updates: Vec<Update>,
}
impl FromStr for Input {
type Err = ParseError;
fn from_str(s: &str) -> Result<Self, Self::Err> {
let mut lines = s.lines();
let mut input = Input {
rules: vec![],
updates: vec![],
};
// Read rules
for l in lines.by_ref() {
if l.is_empty() {
break;
}
input.rules.push(Rule::from_str(l).unwrap());
}
// Read updates
for l in lines.by_ref() {
input.updates.push(update_from_str(l));
}
Ok(input)
}
}
fn sum_valid_updates(Input { rules, updates }: &Input) -> u32 {
updates
.iter()
.filter(|u| is_update_valid(u, rules))
.map(middle_page)
.sum()
}
fn p1(input: &str) -> String {
let input = Input::from_str(input).unwrap();
sum_valid_updates(&input).to_string()
}
// Remove useless rules for this particular update
fn filter_rules(rules: &[Rule], update: &Update) -> Vec<Rule> {
let updated: HashSet<&Page> = update.iter().collect();
rules
.iter()
.filter(|r| updated.contains(&r.before))
.cloned()
.collect()
}
fn sort_rules(rules: &[Rule]) -> Vec<Rule> {
let mut sorted: Vec<Rule> = vec![];
let mut afters: HashMap<Page, u8> = HashMap::new();
for a in rules.iter().map(|r| r.after) {
*afters.entry(a).or_default() += 1;
}
// Allocate new "droppable" rules by wrapping the originals into Options
let mut rules: Vec<Option<&Rule>> = rules.iter().map(Some).collect();
while rules.iter().any(|x| x.is_some()) {
for rule in rules.iter_mut() {
let Some(r) = *rule else { continue };
if !afters.contains_key(&r.before) {
sorted.push(r.clone());
match afters.get_mut(&r.after) {
None => {
panic!("This should not happen");
}
Some(1) => {
afters.remove(&r.after);
}
Some(x) => {
*x -= 1;
}
};
*rule = None;
}
}
}
sorted
}
fn update_reorder(update: &Update, rules: &[Rule]) -> Option<Update> {
let filtered_rules = filter_rules(rules, update);
let sorted_rules = sort_rules(&filtered_rules);
let mut result: Update = vec![];
let mut pages: HashSet<Page> = update.iter().cloned().collect();
for Rule { before, .. } in sorted_rules.iter() {
if pages.contains(before) {
result.push(*before);
pages.remove(before);
}
}
// Add unrestricted pages at the end
for &p in pages.iter() {
result.push(p);
}
if is_update_valid(&result, &sorted_rules) {
Some(result)
} else {
None
}
}
fn sum_reordered_updates(Input { rules, updates }: &Input) -> u32 {
updates
.iter()
.filter(|u| !is_update_valid(u, rules)) // drop valid updates
.filter_map(|u| update_reorder(u, rules)) // reorder updates
.map(|u| middle_page(&u))
.sum()
}
fn p2(input: &str) -> String {
let input = Input::from_str(input).unwrap();
sum_reordered_updates(&input).to_string()
}
fn main() {
aoc2024::run_day("5", p1, p2);
}
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