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.

12/src/main.rs

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