day12: problem 2 solution
what a doozy. iterative solution from part 1 totally untenable. recursive solution a bit tricky, but still not fast enough for brute force. had to implement memoization cache for reasonable runtime.
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12/src/main.rs
126
12/src/main.rs
@ -1,6 +1,10 @@
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use itertools::Itertools;
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use rayon::prelude::*;
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use std::cell::RefCell;
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use std::collections::HashMap;
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use std::fs::File;
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use std::io::{BufRead, BufReader, Lines};
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use std::time::{Duration, Instant};
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// BOILERPLATE
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type InputIter = Lines<BufReader<File>>;
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@ -12,29 +16,38 @@ fn get_input() -> InputIter {
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}
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fn main() {
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println!("Problem 1 solution: {}", problem1(get_input()));
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println!("Problem 2 solution: {}", problem2(get_input()));
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let start = Instant::now();
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let ans1 = problem1(get_input());
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let duration = start.elapsed();
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println!("Problem 1 solution: {} [{}s]", ans1, duration.as_secs_f64());
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let start = Instant::now();
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let ans2 = problem2(get_input());
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let duration = start.elapsed();
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println!("Problem 2 solution: {} [{}s]", ans2, duration.as_secs_f64());
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}
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// PARSE
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#[derive(Debug)]
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struct SpringRow {
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struct SpringRow<'a> {
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springs: Vec<char>,
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spans: Vec<usize>,
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cache: RefCell<HashMap<(usize, &'a [usize], usize), u64>>,
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}
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impl From<&str> for SpringRow {
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impl<'a> From<&str> for SpringRow<'a> {
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fn from(s: &str) -> Self {
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let (springs_s, spans_s) = s.split_once(' ').unwrap();
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SpringRow {
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springs: springs_s.chars().collect(),
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spans: spans_s.split(',').map(|x| x.parse().unwrap()).collect(),
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cache: RefCell::new(HashMap::new()),
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}
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}
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}
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impl SpringRow {
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impl<'a> SpringRow<'a> {
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fn possible_arrangements(&self) -> u64 {
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let mut permutations = vec![self.springs.clone()];
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while let Some(cur) = permutations.iter().position(|perm| perm.contains(&'?')) {
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@ -66,6 +79,83 @@ impl SpringRow {
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})
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.count() as u64
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}
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fn memoize_arrangements_r(
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&self,
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idx: usize,
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spans_left: &'a [usize],
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cur_span_left: usize,
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) -> u64 {
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if let Some(value) = self.cache.borrow().get(&(idx, spans_left, cur_span_left)) {
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return *value;
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}
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let result = self.possible_arrangements_r(idx, spans_left, cur_span_left);
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self.cache
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.borrow_mut()
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.insert((idx, spans_left, cur_span_left), result);
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result
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}
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fn possible_arrangements_r(
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&self,
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idx: usize,
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spans_left: &'a [usize],
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cur_span_left: usize,
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) -> u64 {
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if cur_span_left == 0 && spans_left.len() == 1 {
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// everything is consumed. if we're at the end, we good
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// if we have stuff left, as long as it doesn't contain a fixed broken spring we good
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if idx == self.springs.len() || !self.springs[idx..].contains(&'#') {
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// println!("{:?} valid", self.springs);
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return 1;
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}
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// something invalid
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return 0;
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}
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if idx >= self.springs.len() {
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// reached the end of the string without consuming all our spans
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// println!(
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// "{:?} invalid idx: {} spans_left: {:?}",
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// self.springs, idx, spans_left
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// );
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return 0;
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}
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// println!(
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// "call idx: {} str: {:?} spans_left: {:?} cur_span_left: {}",
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// idx,
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// &self.springs[idx..],
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// spans_left,
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// cur_span_left
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// );
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return if cur_span_left == 0 {
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match self.springs[idx] {
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'#' => 0, // a broken spring after we have completed our span is invalid
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// a space or a ? (becomes a space) starts a new span
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'.' | '?' => self.memoize_arrangements_r(idx + 1, &spans_left[1..], spans_left[1]),
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_ => panic!("invalid spring"),
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}
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} else {
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// we are mid span
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match self.springs[idx] {
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// cur_span_left != 0, so we still have work to do with broken springs
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'#' => self.memoize_arrangements_r(idx + 1, spans_left, cur_span_left - 1),
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'.' => {
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if spans_left[0] == cur_span_left {
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self.memoize_arrangements_r(idx + 1, spans_left, cur_span_left)
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} else {
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0
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}
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}
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'?' => {
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if spans_left[0] == cur_span_left {
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self.memoize_arrangements_r(idx + 1, spans_left, cur_span_left - 1)
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+ self.memoize_arrangements_r(idx + 1, spans_left, cur_span_left)
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} else {
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self.memoize_arrangements_r(idx + 1, spans_left, cur_span_left - 1)
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}
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}
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_ => panic!("invalid spring"),
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}
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};
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}
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}
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// PROBLEM 1 solution
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@ -75,12 +165,32 @@ fn problem1<T: BufRead>(input: Lines<T>) -> u64 {
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.map(|row| SpringRow::from(row.unwrap().as_str()))
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.collect();
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rows.iter().map(|row| row.possible_arrangements()).sum()
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rows.iter()
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.map(|row| row.possible_arrangements_r(0, &row.spans, row.spans[0]))
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.sum()
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}
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// PROBLEM 2 solution
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fn problem2<T: BufRead>(input: Lines<T>) -> u64 {
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0
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let mut rows: Vec<SpringRow> = input
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.map(|row| SpringRow::from(row.unwrap().as_str()))
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.collect();
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for row in &mut rows {
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let orig_slice = &row.springs.clone();
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for _i in 0..4 {
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row.springs.push('?');
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row.springs.extend_from_slice(orig_slice);
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}
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let orig_span_slice = &row.spans.clone();
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for _i in 0..4 {
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row.spans.extend_from_slice(orig_span_slice);
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}
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}
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rows.iter()
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.map(|row| row.possible_arrangements_r(0, &row.spans, row.spans[0]))
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.sum()
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}
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#[cfg(test)]
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@ -104,6 +214,6 @@ mod tests {
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#[test]
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fn problem2_example() {
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let c = Cursor::new(EXAMPLE);
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assert_eq!(problem2(c.lines()), 0);
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assert_eq!(problem2(c.lines()), 525152);
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}
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}
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