2021: day10: part 2

aoc2021/src/day10.rs

@@ -8,6 +8,7 @@ pub fn run() -> Result<String> {
     let mut res = String::with_capacity(128);
 
     writeln!(res, "part 1: {}", part1(INPUT)?)?;
+    writeln!(res, "part 2: {}", part2(INPUT)?)?;
 
     Ok(res)
 }
@@ -18,7 +19,21 @@ fn part1(input: &str) -> Result<u64> {
         .map(str::parse::<Line>)
         .collect::<Result<Vec<_>>>()?;
 
-    Ok(lines.iter().map(Line::corrupt_score).sum())
+    Ok(lines.iter().filter_map(Line::syntax_checking_score).sum())
+}
+
+fn part2(input: &str) -> Result<u64> {
+    let lines = input
+        .lines()
+        .map(str::parse::<Line>)
+        .collect::<Result<Vec<_>>>()?;
+
+    let mut scores: Vec<u64> = lines.iter().filter_map(Line::completion_score).collect();
+    scores.sort_unstable();
+
+    debug_assert!(scores.len() % 2 == 1);
+
+    Ok(scores[scores.len() / 2])
 }
 
 enum SymbolState {
@@ -47,7 +62,7 @@ enum Symbol {
 }
 
 impl Symbol {
-    fn score(&self) -> u64 {
+    fn syntax_checking_score(&self) -> u64 {
         match self {
             Symbol::Parenthesis => 3,
             Symbol::Bracket => 57,
@@ -55,6 +70,15 @@ impl Symbol {
             Symbol::AngleBracket => 25137,
         }
     }
+
+    fn completion_score(&self) -> u64 {
+        match self {
+            Symbol::Parenthesis => 1,
+            Symbol::Bracket => 2,
+            Symbol::Brace => 3,
+            Symbol::AngleBracket => 4,
+        }
+    }
 }
 
 impl TryFrom<char> for Symbol {
@@ -76,7 +100,7 @@ struct Line {
 }
 
 impl Line {
-    fn corrupt_score(&self) -> u64 {
+    fn syntax_checking_score(&self) -> Option<u64> {
         let mut stack = Vec::new();
 
         for state in &self.symbols {
@@ -84,12 +108,34 @@ impl Line {
                 SymbolState::Open(symbol) => stack.push(symbol),
                 SymbolState::Close(symbol) => match stack.pop() {
                     Some(other_symbol) if symbol == other_symbol => continue,
-                    _ => return symbol.score(),
+                    _ => return Some(symbol.syntax_checking_score()),
                 },
             }
         }
 
-        0
+        None
+    }
+
+    fn completion_score(&self) -> Option<u64> {
+        let mut stack = Vec::new();
+
+        for state in &self.symbols {
+            match state {
+                SymbolState::Open(symbol) => stack.push(symbol),
+                SymbolState::Close(symbol) => match stack.pop() {
+                    Some(other_symbol) if symbol == other_symbol => continue,
+                    _ => return None, // ignore corrupt line
+                },
+            }
+        }
+
+        let mut score = 0;
+
+        for symbol in stack.into_iter().rev() {
+            score = score * 5 + symbol.completion_score();
+        }
+
+        Some(score)
     }
 }
 
@@ -122,4 +168,14 @@ mod tests {
     fn part1_real() {
         assert_eq!(part1(INPUT).unwrap(), 394647);
     }
+
+    #[test]
+    fn part2_provided() {
+        assert_eq!(part2(PROVIDED).unwrap(), 288957);
+    }
+
+    #[test]
+    fn part2_real() {
+        assert_eq!(part2(INPUT).unwrap(), 2380061249);
+    }
 }