diff --git a/2021/src/main/kotlin/Day12.kt b/2021/src/main/kotlin/Day12.kt
new file mode 100644
index 0000000..c73f6c3
--- /dev/null
+++ b/2021/src/main/kotlin/Day12.kt
@@ -0,0 +1,57 @@
+package be.vandewalleh.aoc.days
+
+import be.vandewalleh.aoc.utils.BaseDay
+import be.vandewalleh.aoc.utils.input.Day
+
+@Day
+class Day12 : BaseDay() {
+
+    private val map by lazy {
+        HashMap<String, MutableSet<String>>().also { map ->
+            input.lines.value.map { it.split("-") }
+                .flatMap { (a, b) -> listOf(listOf(a, b), listOf(b, a)) }
+                .filter { (_, b) -> b != "start" }
+                .forEach { (a, b) -> map.computeIfAbsent(a) { HashSet() }.add(b) }
+        }
+    }
+
+    override fun part1(): Any {
+        val stack = map["start"]!!.map { listOf("start", it) }.toMutableList()
+
+        var i = 0
+        while (stack.isNotEmpty()) {
+            val path = stack.removeLast()
+            val node = path.last()
+            for (next in map[node]!!) {
+                when {
+                    next == "end" -> i++
+                    next[0].isUpperCase() -> stack.add(path + next)
+                    else -> if (next !in path) stack.add(path + next)
+                }
+            }
+        }
+        return i
+    }
+
+    override fun part2(): Any {
+        val stack = map["start"]!!.map { listOf("start", it) }.toMutableList()
+
+        var i = 0
+        while (stack.isNotEmpty()) {
+            val path = stack.removeLast()
+            val node = path.last()
+            for (next in map[node]!!) {
+                when {
+                    next == "end" -> i++
+                    next[0].isUpperCase() -> stack.add(path + next)
+                    else -> {
+                        val hasTwoSmallCaves = path.filter { it[0].isLowerCase() }
+                            .any { cur -> path.count { it == cur } == 2 }
+                        if (!hasTwoSmallCaves || next !in path) stack.add(path + next)
+                    }
+                }
+            }
+        }
+        return i
+    }
+}
diff --git a/2021/src/main/resources/day12.txt b/2021/src/main/resources/day12.txt
new file mode 100644
index 0000000..756681b
--- /dev/null
+++ b/2021/src/main/resources/day12.txt
@@ -0,0 +1,23 @@
+pn-TY
+rp-ka
+az-aw
+al-IV
+pn-co
+end-rp
+aw-TY
+rp-pn
+al-rp
+end-al
+IV-co
+end-TM
+co-TY
+TY-ka
+aw-pn
+aw-IV
+pn-IV
+IV-ka
+TM-rp
+aw-PD
+start-IV
+start-co
+start-pn
diff --git a/2021/src/test/kotlin/Day12Test.kt b/2021/src/test/kotlin/Day12Test.kt
new file mode 100644
index 0000000..bcbaffd
--- /dev/null
+++ b/2021/src/test/kotlin/Day12Test.kt
@@ -0,0 +1,19 @@
+package be.vandewalleh.aoc.days
+
+class Day12Test : BaseDayTest() {
+    override val example = """
+        start-A
+        start-b
+        A-c
+        A-b
+        b-d
+        A-end
+        b-end
+    """.trimIndent()
+
+    override val part1Example = 10
+    override val part1Answer = 4413
+
+    override val part2Example = 36
+    override val part2Answer = 118803
+}