2021: d10: ex2: add solution

2021/d10/ex2/ex2.py

@@ -0,0 +1,81 @@
+#!/usr/bin/env python
+
+import sys
+from typing import Iterator, List, Optional
+
+
+def solve(input: List[str]) -> int:
+    def find_illegal_char(input: str) -> Optional[int]:
+        chunks: List[str] = []
+        ends_to_start = {
+            ")": "(",
+            "]": "[",
+            "}": "{",
+            ">": "<",
+        }
+
+        for i, c in enumerate(input):
+            # Is it a chunk beginning
+            if c not in ends_to_start:
+                chunks.append(c)
+                continue
+            # Is it matching the last element on the chunk stack
+            current = chunks.pop()
+            # If not, corruption has been found
+            if ends_to_start[c] != current:
+                return i
+
+        return None
+
+    def complete_line(input: str) -> str:
+        chunks: List[str] = []
+        start_to_end = {
+            "(": ")",
+            "[": "]",
+            "{": "}",
+            "<": ">",
+        }
+
+        for c in input:
+            # Is it a chunk beginning
+            if c in start_to_end:
+                chunks.append(c)
+                continue
+            # Otherwise we must match the last open chunk
+            assert start_to_end[chunks.pop()] == c  # Sanity check
+
+        return "".join(reversed(list(start_to_end[c] for c in chunks)))
+
+    def score_completion(completion: str) -> int:
+        char_score = {
+            ")": 1,
+            "]": 2,
+            "}": 3,
+            ">": 4,
+        }
+
+        score = 0
+
+        for c in completion:
+            score *= 5
+            score += char_score[c]
+
+        return score
+
+    def score_completions(completions: Iterator[str]) -> int:
+        scores = sorted(map(score_completion, completions))
+        return scores[len(scores) // 2]
+
+    incomplete_lines = filter(lambda line: find_illegal_char(line) is None, input)
+    completions = map(complete_line, incomplete_lines)
+
+    return score_completions(completions)
+
+
+def main() -> None:
+    input = [line.strip() for line in sys.stdin.readlines()]
+    print(solve(input))
+
+
+if __name__ == "__main__":
+    main()