Day 12 Complete. Thanks Reddit. The cache was a godsend!

src/solutions/day12.rs

@@ -1,3 +1,5 @@
+use std::collections::HashMap;
+
 use super::Solution;
 
 pub struct Day12 {}
@@ -18,7 +20,7 @@ impl Solution for Day12 {
                 .map(|n| n.parse::<usize>().unwrap())
                 .collect::<Vec<_>>();
 
-            ans += self.solve(&springs.as_bytes(), None, &nums);
+            ans += self.solve(&springs.as_bytes(), None, &nums, &mut HashMap::new());
         }
         Ok(Box::new(ans))
     }
@@ -43,7 +45,7 @@ impl Solution for Day12 {
                 .map(|n| n.parse::<usize>().unwrap())
                 .collect::<Vec<_>>();
 
-            ans += self.solve(&springs.as_bytes(), None, &nums);
+            ans += self.solve(&springs.as_bytes(), None, &nums, &mut HashMap::new());
         }
         Ok(Box::new(ans))
     }
@@ -56,7 +58,13 @@ impl Solution for 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 {
+    fn solve<'a, 'b>(
+        &self,
+        s: &'a [u8],
+        in_group: Option<usize>,
+        cons: &'b [usize],
+        map: &mut HashMap<(&'a [u8], Option<usize>, &'b [usize]), usize>,
+    ) -> usize {
         if s.is_empty() {
             return match in_group {
                 Some(n) if cons == &[n] => 1,
@@ -64,15 +72,36 @@ impl Day12 {
                 _ => 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,
+
+        // Check for a cache hit
+        if s[0] == b'?' {
+            if let Some(result) = map.get(&(s, in_group, cons)) {
+                return *result;
+            }
         }
+
+        // Resursively match based on the whether we are in a block and/or we have spaces left to fill
+        let ans = match (s[0], in_group, cons) {
+            (b'.', None, _) | (b'?', None, []) => self.solve(&s[1..], None, cons, map),
+            (b'.' | b'?', Some(n), [e, ..]) if n == *e => {
+                self.solve(&s[1..], None, &cons[1..], map)
+            }
+            (b'#' | b'?', Some(n), [e, ..]) if n < *e => {
+                self.solve(&s[1..], Some(n + 1), cons, map)
+            }
+            (b'#', None, [_, ..]) => self.solve(&s[1..], Some(1), cons, map),
+            (b'?', None, _) => {
+                self.solve(&s[1..], None, cons, map) + self.solve(&s[1..], Some(1), cons, map)
+            }
+            _ => 0,
+        };
+
+        // Store in cache
+        if s[0] == b'?' {
+            map.insert((s, in_group, cons), ans);
+        }
+
+        ans
     }
 }