Improve perf of day 10 (#10)

day_10/src/lib.rs

@@ -8,7 +8,7 @@ pub mod day_10 {
         parse(include_str!("../input.txt"))
     }
 
-    fn closing(c: char) -> Option<char> {
+    const fn closing(c: char) -> Option<char> {
         if c == '[' {
             Some(']')
         } else if c == '{' {
@@ -23,7 +23,11 @@ pub mod day_10 {
     }
 
     /// Return Ok(first illegal char), or Err(ending) for an incomplete string.
-    fn syntax_error(s: &str) -> Result<char, Vec<char>> {
+    /// The function takes the stack with the earliest element being the first one pushed.
+    fn syntax_error<F, T>(s: &str, incomplete_handle: F) -> Result<char, T>
+    where
+        F: FnOnce(Vec<char>) -> T,
+    {
         let mut stack = Vec::new();
         for c in s.chars() {
             match closing(c) {
@@ -42,13 +46,13 @@ pub mod day_10 {
                 },
             }
         }
-        stack.reverse();
-        Err(stack)
+        let answer = incomplete_handle(stack);
+        Err(answer)
     }
 
     pub fn part_1(data: &[&str]) -> u32 {
         data.iter()
-            .map(|line| match syntax_error(line) {
+            .map(|line| match syntax_error(line, |_| {}) {
                 Ok(')') => 3,
                 Ok(']') => 57,
                 Ok('}') => 1197,
@@ -62,10 +66,12 @@ pub mod day_10 {
     pub fn part_2(data: &[&str]) -> u64 {
         let mut scores: Vec<u64> = data
             .iter()
-            .filter_map(|line| match syntax_error(line) {
-                Err(completion) => Some(
+            .filter_map(|line| {
+                match syntax_error(line, |completion| {
+                    Some(
                         completion
                             .iter()
+                            .rev()
                             .map(|c| match c {
                                 ')' => 1,
                                 ']' => 2,
@@ -76,10 +82,14 @@ pub mod day_10 {
                                 }
                             })
                             .fold(0, |state, new_val| state * 5 + new_val),
-                ),
+                    )
+                }) {
                     Ok(_) => None,
+                    Err(v) => v,
+                }
             })
             .collect();
+        // This can be done in linear time using two heaps, but life is too short
         scores.sort_unstable();
         scores[scores.len() / 2]
     }