feat: solve day 10 part 2

src/day10.rs

@@ -15,17 +15,22 @@ fn is_opening(c: char) -> bool {
     c == '(' || c == '[' || c == '{' || c == '<'
 }
 
+// I know that it is not ideal to throw a panic here. But I'm too lazy to do
+// error handling. And the input is guaranteed to be correct to this is fine.
+fn get_chunk_variant(c: char) -> ChunkVariant {
+    match c {
+        '('|')' => ChunkVariant::Parenthesis,
+        '['|']' => ChunkVariant::Bracket,
+        '{'|'}' => ChunkVariant::Curly,
+        '<'|'>' => ChunkVariant::Pointy,
+        _ => panic!("Invalid chunk character")
+    }
+}
+
 fn find_corrupted_chunk_symbol(line: &str) -> Option<ChunkVariant> {
     let mut active_chunks: Vec<ChunkVariant> = Vec::new();
     for c in line.chars() {
-        let variant = match c {
-            '('|')' => ChunkVariant::Parenthesis,
-            '['|']' => ChunkVariant::Bracket,
-            '{'|'}' => ChunkVariant::Curly,
-            '<'|'>' => ChunkVariant::Pointy,
-            _ => panic!("Invalid character found while finding corrupted chunks")
-        };
-
+        let variant = get_chunk_variant(c);
         if is_opening(c) {
             active_chunks.push(variant);
         } else {
@@ -53,6 +58,42 @@ pub fn part1(lines: &Vec<String>) -> u32 {
     return score;
 }
 
+fn find_unclosed_chunks(line: &str) -> Vec<ChunkVariant> {
+    let mut active_chunks: Vec<ChunkVariant> = Vec::new();
+    for c in line.chars() {
+        if is_opening(c) {
+            active_chunks.push(get_chunk_variant(c));
+        } else {
+            active_chunks.pop();
+        }
+    }
+    active_chunks
+}
+
+pub fn part2(lines: &Vec<String>) -> u64 {
+    let mut scores = Vec::new();
+    for line in lines {
+        let corrupted = find_corrupted_chunk_symbol(line);
+        if corrupted == None {
+            let mut score = 0;
+            let mut unclosed_chunks = find_unclosed_chunks(line);
+            unclosed_chunks.reverse();
+            for chunk in unclosed_chunks {
+                score *= 5;
+                score += match chunk {
+                    ChunkVariant::Parenthesis => 1,
+                    ChunkVariant::Bracket => 2,
+                    ChunkVariant::Curly => 3,
+                    ChunkVariant::Pointy => 4
+                }
+            }
+            scores.push(score);
+        }
+    }
+    scores.sort();
+    return scores[scores.len()/2];
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -74,5 +115,23 @@ mod tests {
         let result = part1(&input);
         assert_eq!(result, 26397);
     }
+
+    #[test]
+    fn part2_example() {
+        let input = vec![
+            "[({(<(())[]>[[{[]{<()<>>".into(),
+            "[(()[<>])]({[<{<<[]>>(".into(),
+            "{([(<{}[<>[]}>{[]{[(<()>".into(),
+            "(((({<>}<{<{<>}{[]{[]{}".into(),
+            "[[<[([]))<([[{}[[()]]]".into(),
+            "[{[{({}]{}}([{[{{{}}([]".into(),
+            "{<[[]]>}<{[{[{[]{()[[[]".into(),
+            "[<(<(<(<{}))><([]([]()".into(),
+            "<{([([[(<>()){}]>(<<{{".into(),
+            "<{([{{}}[<[[[<>{}]]]>[]]".into(),
+        ];
+        let result = part2(&input);
+        assert_eq!(result, 288957);
+    }
 }
 

src/main.rs

@@ -43,6 +43,7 @@ fn run(day: i32, part: i32, input_filename: &str) {
         "9.1" => println!("{}", day9::part1(day9::parse_input(&contents))),
         "9.2" => println!("{}", day9::part2(day9::parse_input(&contents))),
         "10.1" => println!("{}", day10::part1(&day10::parse_input(&contents))),
+        "10.2" => println!("{}", day10::part2(&day10::parse_input(&contents))),
         _ => println!("Day {} part {} not found", day, part)
     }
 }