Pretty much there with Day 12... just need an eternity to let it run :)

src/input/day12_test1

@@ -0,0 +1,6 @@
+???.### 1,1,3
+.??..??...?##. 1,1,3
+?#?#?#?#?#?#?#? 1,3,1,6
+????.#...#... 4,1,1
+????.######..#####. 1,6,5
+?###???????? 3,2,1

src/input/day12_test2

@@ -0,0 +1,6 @@
+???.### 1,1,3
+.??..??...?##. 1,1,3
+?#?#?#?#?#?#?#? 1,3,1,6
+????.#...#... 4,1,1
+????.######..#####. 1,6,5
+?###???????? 3,2,1

src/main.rs

@@ -19,7 +19,7 @@ async fn main() -> Result<(), Box<dyn Error>> {
         Box::new(day09::Day09 {}),
         Box::new(day10::Day10 {}),
         Box::new(day11::Day11 {}),
-        // Box::new(day12::Day12 {}),
+        Box::new(day12::Day12 {}),
         // Box::new(day13::Day13 {}),
         // Box::new(day14::Day14 {}),
         // Box::new(day15::Day15 {}),

src/solutions/day12.rs

@@ -5,16 +5,47 @@ pub struct Day12 {}
 impl Solution for Day12 {
     fn part1(
         &self,
-        _input: &mut Vec<String>,
+        input: &mut Vec<String>,
     ) -> Result<Box<dyn std::fmt::Display + Sync>, Box<dyn std::error::Error>> {
-        Ok(Box::new("Ready"))
+        let mut ans = 0usize;
+        for s in input {
+            let springs = s.split_whitespace().nth(0).unwrap().to_owned();
+            let nums = s
+                .split_whitespace()
+                .nth(1)
+                .unwrap()
+                .split(',')
+                .map(|n| n.parse::<usize>().unwrap())
+                .collect::<Vec<_>>();
+
+            ans += self.solve(&springs.as_bytes(), None, &nums);
+        }
+        Ok(Box::new(ans))
     }
 
     fn part2(
         &self,
-        _input: &mut Vec<String>,
+        input: &mut Vec<String>,
     ) -> Result<Box<dyn std::fmt::Display + Sync>, Box<dyn std::error::Error>> {
-        Ok(Box::new("Ready"))
+        for l in input.iter_mut() {
+            let (s, n) = l.split_once(" ").unwrap();
+            *l = format!("{s}?{s}?{s}?{s}?{s} {n},{n},{n},{n},{n}\n");
+        }
+
+        let mut ans = 0usize;
+        for s in input {
+            let springs = s.split_whitespace().nth(0).unwrap().to_owned();
+            let nums = s
+                .split_whitespace()
+                .nth(1)
+                .unwrap()
+                .split(',')
+                .map(|n| n.parse::<usize>().unwrap())
+                .collect::<Vec<_>>();
+
+            ans += self.solve(&springs.as_bytes(), None, &nums);
+        }
+        Ok(Box::new(ans))
     }
 
     fn get_day(&self) -> u8 {
@@ -22,7 +53,28 @@ impl Solution for Day12 {
     }
 }
 
-impl Day12 {}
+impl Day12 {
+    /// recursively counts the number of permutations of spring we could get from
+    /// the splits specified in 'nums'
+    fn solve(&self, s: &[u8], in_group: Option<usize>, cons: &[usize]) -> usize {
+        if s.is_empty() {
+            return match in_group {
+                Some(n) if cons == &[n] => 1,
+                None if cons.is_empty() => 1,
+                _ => 0,
+            };
+        }
+        // Resursively match based on the whether we are in a block and/or we have spaces left to fill
+        match (s[0], in_group, cons) {
+            (b'.', None, _) | (b'?', None, []) => self.solve(&s[1..], None, cons),
+            (b'.' | b'?', Some(n), [e, ..]) if n == *e => self.solve(&s[1..], None, &cons[1..]),
+            (b'#' | b'?', Some(n), [e, ..]) if n < *e => self.solve(&s[1..], Some(n + 1), cons),
+            (b'#', None, [_, ..]) => self.solve(&s[1..], Some(1), cons),
+            (b'?', None, _) => self.solve(&s[1..], None, cons) + self.solve(&s[1..], Some(1), cons),
+            _ => 0,
+        }
+    }
+}
 
 /// Test from puzzle input
 #[cfg(test)]
@@ -44,7 +96,7 @@ mod test {
             .unwrap()
             .to_string();
 
-        assert_eq!(answer, "Ready");
+        assert_eq!(answer, "21");
     }
 
     #[test]
@@ -61,6 +113,6 @@ mod test {
             .unwrap()
             .to_string();
 
-        assert_eq!(answer, "Ready");
+        assert_eq!(answer, "525152");
     }
 }