From 1feda8609355a108997714fb0c6fa9110e564df9 Mon Sep 17 00:00:00 2001
From: Felix Van der Jeugt <felix.vanderjeugt@posteo.net>
Date: Sun, 12 Dec 2021 10:45:16 +0100
Subject: [PATCH] day 12

---
 day12/part1.lua | 34 ++++++++++++++++++++++++++++++++++
 day12/part2.lua | 36 ++++++++++++++++++++++++++++++++++++
 2 files changed, 70 insertions(+)
 create mode 100755 day12/part1.lua
 create mode 100755 day12/part2.lua

diff --git a/day12/part1.lua b/day12/part1.lua
new file mode 100755
index 0000000..8d67ce8
--- /dev/null
+++ b/day12/part1.lua
@@ -0,0 +1,34 @@
+#!/usr/bin/env luajit
+require("utils")
+require("set")
+
+local graph = {}
+local lower = set()
+for line in io.lines(arg[3]) do
+	local a, b = unpack(split(line, "-"))
+	local as, bs = graph[a] or {}, graph[b] or {}
+	table.insert(as, b)
+	table.insert(bs, a)
+	if string.lower(a) == a then lower:add(a) end
+	if string.lower(b) == b then lower:add(b) end
+	graph[a] = as
+	graph[b] = bs
+end
+
+local function backtrack(i, once)
+	if i == "end" then
+		return 1
+	else
+		local paths = 0
+		for k, v in pairs(graph[i]) do
+			if not lower:has(v) or not once:has(v) then
+				once:add(v)
+				paths = paths + backtrack(v, once)
+				once:remove(v)
+			end
+		end
+		return paths
+	end
+end
+
+print(backtrack("start", set({"start"})))
diff --git a/day12/part2.lua b/day12/part2.lua
new file mode 100755
index 0000000..869406b
--- /dev/null
+++ b/day12/part2.lua
@@ -0,0 +1,36 @@
+#!/usr/bin/env luajit
+require("utils")
+require("set")
+
+local graph = {}
+local lower = set()
+for line in io.lines(arg[3]) do
+	local a, b = unpack(split(line, "-"))
+	local as, bs = graph[a] or {}, graph[b] or {}
+	table.insert(as, b)
+	table.insert(bs, a)
+	if string.lower(a) == a then lower:add(a) end
+	if string.lower(b) == b then lower:add(b) end
+	graph[a] = as
+	graph[b] = bs
+end
+
+local function backtrack(i, once, smalls)
+	if i == "end" then
+		return 1
+	else
+		local paths = 0
+		for k, v in pairs(graph[i]) do
+			if not lower:has(v) or not once:has(v) then
+				once:add(v)
+				paths = paths + backtrack(v, once, smalls)
+				once:remove(v)
+			elseif smalls > 0 and v ~= "start" and v ~= "end" then
+				paths = paths + backtrack(v, once, smalls - 1)
+			end
+		end
+		return paths
+	end
+end
+
+print(backtrack("start", set({"start"}), 1))