2015: d19: ex2: add solution

2015/d19/ex2/ex2.py

@@ -0,0 +1,49 @@
+#!/usr/bin/env python
+
+import collections
+import sys
+
+
+def solve(input: str) -> int:
+    def parse(input: str) -> tuple[dict[str, list[str]], str]:
+        raw_replacements, molecule = input.strip().split("\n\n")
+        res: dict[str, list[str]] = collections.defaultdict(list)
+        for start, end in (
+            line.split(" => ") for line in raw_replacements.splitlines()
+        ):
+            res[start].append(end)
+        return res, molecule
+
+    def inverse(input: dict[str, list[str]]) -> dict[str, str]:
+        res: dict[str, str] = {}
+        for key, vals in input.items():
+            for val in vals:
+                res[val] = key
+        assert len(res) == sum(map(len, input.values()))  # Sanity check
+        return res
+
+    def build(replacements: dict[str, list[str]], molecule: str) -> int:
+        # We will be "building down" to the electron, so inverse the replacement mapping
+        inversed_replacements = inverse(replacements)
+        # Order the replacement needles to try the longest ones first
+        needles = sorted(inversed_replacements.keys(), key=len, reverse=True)
+        res = 0
+        while molecule != "e":
+            for needle in needles:
+                if needle not in molecule:
+                    continue
+                res += 1
+                molecule = molecule.replace(needle, inversed_replacements[needle], 1)
+        return res
+
+    replacements, molecule = parse(input)
+    return build(replacements, molecule)
+
+
+def main() -> None:
+    input = sys.stdin.read()
+    print(solve(input))
+
+
+if __name__ == "__main__":
+    main()