From d4524174ccd9d78233690936f5dde7a70c7c74a4 Mon Sep 17 00:00:00 2001
From: Maciej Jur <mj@kamoshi.org>
Date: Sun, 12 Dec 2021 12:57:30 +0100
Subject: [PATCH] [2021] day 12 python

---
 2021/.input/day12    | 23 +++++++++++++++++++++
 2021/Python/day12.py | 48 ++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 71 insertions(+)
 create mode 100644 2021/.input/day12
 create mode 100644 2021/Python/day12.py

diff --git a/2021/.input/day12 b/2021/.input/day12
new file mode 100644
index 0000000..1c0b034
--- /dev/null
+++ b/2021/.input/day12
@@ -0,0 +1,23 @@
+start-YA
+ps-yq
+zt-mu
+JS-yi
+yq-VJ
+QT-ps
+start-yq
+YA-yi
+start-nf
+nf-YA
+nf-JS
+JS-ez
+yq-JS
+ps-JS
+ps-yi
+yq-nf
+QT-yi
+end-QT
+nf-yi
+zt-QT
+end-ez
+yq-YA
+end-JS
\ No newline at end of file
diff --git a/2021/Python/day12.py b/2021/Python/day12.py
new file mode 100644
index 0000000..d3e5194
--- /dev/null
+++ b/2021/Python/day12.py
@@ -0,0 +1,48 @@
+import re
+
+
+input_pattern = re.compile(r'([a-zA-Z]+)-([a-zA-Z]+)')
+small_pattern = re.compile(r'([a-z]+)')
+
+
+def load():
+    with open('../.input/day12', 'r') as f:
+        return input_pattern.findall(f.read())
+
+
+def construct_paths(graph, start, end, path=None, single_reentry=False):
+    if path is None:
+        path = [start]
+
+    if start == end:
+        yield path
+    else:
+        for node in graph[start]:
+            is_small = small_pattern.match(node)
+            if not is_small or node not in path:
+                yield from construct_paths(graph, node, end, path + [node], single_reentry)
+            elif is_small and single_reentry and node not in ["start", "end"]:
+                yield from construct_paths(graph, node, end, path + [node])
+
+
+def solve1():
+    graph = {}
+    for node1, node2 in load():
+        graph.setdefault(node1, set()).add(node2)
+        graph.setdefault(node2, set()).add(node1)
+
+    return sum(1 for _ in construct_paths(graph, 'start', 'end'))
+
+
+def solve2():
+    graph = {}
+    for node1, node2 in load():
+        graph.setdefault(node1, set()).add(node2)
+        graph.setdefault(node2, set()).add(node1)
+
+    return sum(1 for _ in construct_paths(graph, 'start', 'end', single_reentry=True))
+
+
+if __name__ == '__main__':
+    print(solve1())  # 4304
+    print(solve2())  # 118242