2017: d09: ex2: add solution

2017/d09/ex2/ex2.py

@@ -0,0 +1,59 @@
+#!/usr/bin/env python
+
+import sys
+from typing import NamedTuple
+
+
+class Group(NamedTuple):
+    children: list["Group"]
+
+
+def solve(input: str) -> int:
+    def parse_group(input: str) -> tuple[Group, str, int]:
+        garbage = False
+        cancel = False
+        stack: list[Group] = []
+        total_garbage = 0
+        for i, c in enumerate(input):
+            if cancel:
+                assert garbage  # Sanity check
+                cancel = False
+                continue
+            if garbage:
+                if c == "!":
+                    cancel = True
+                elif c == ">":
+                    garbage = False
+                else:
+                    total_garbage += 1
+                continue
+            if c == "<":
+                garbage = True
+                continue
+            if c == "{":
+                stack.append(Group([]))
+                continue
+            if c == "}":
+                top = stack.pop()
+                if stack:
+                    stack[-1].children.append(top)
+                    continue
+                return top, input[i + 1 :], total_garbage
+        assert False  # Sanity check
+
+    def parse(input: str) -> tuple[Group, int]:
+        groups, input, total_garbage = parse_group(input)
+        assert not input  # Sanity check
+        return groups, total_garbage
+
+    _, total_garbage = parse(input.strip())
+    return total_garbage
+
+
+def main() -> None:
+    input = sys.stdin.read()
+    print(solve(input))
+
+
+if __name__ == "__main__":
+    main()