From 9d15c56597c8f14e380c07799eaa2b8bb7d1949c Mon Sep 17 00:00:00 2001
From: Bruno BELANYI <bruno@belanyi.fr>
Date: Thu, 22 May 2025 00:23:54 +0100
Subject: [PATCH] 2015: d19: ex1: add solution

---
 2015/d19/ex1/ex1.py | 45 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 45 insertions(+)
 create mode 100755 2015/d19/ex1/ex1.py

diff --git a/2015/d19/ex1/ex1.py b/2015/d19/ex1/ex1.py
new file mode 100755
index 0000000..4f038b2
--- /dev/null
+++ b/2015/d19/ex1/ex1.py
@@ -0,0 +1,45 @@
+#!/usr/bin/env python
+
+import collections
+import sys
+from collections.abc import Iterator
+
+
+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 replace_needle(input: str, needle: str, to: str) -> Iterator[str]:
+        assert needle in input  # Sanity check
+        i = input.find(needle)
+        while i != -1:
+            yield input[:i] + to + input[i + len(needle) :]
+            i = input.find(needle, i + 1)
+
+    def do_replacements(
+        replacements: dict[str, list[str]], molecule: str
+    ) -> Iterator[str]:
+        for needle, vals in replacements.items():
+            if needle not in molecule:
+                continue
+            for to in vals:
+                yield from replace_needle(molecule, needle, to)
+
+    replacements, molecule = parse(input)
+    new_molecules = set(do_replacements(replacements, molecule))
+    return len(new_molecules)
+
+
+def main() -> None:
+    input = sys.stdin.read()
+    print(solve(input))
+
+
+if __name__ == "__main__":
+    main()