1 files changed, 72 insertions, 0 deletions
diff --git a/aoc-2021-kotlin/src/Day12.kt b/aoc-2021-kotlin/src/Day12.kt
new file mode 100644
index 0000000..d3368da
--- /dev/null
+++ b/aoc-2021-kotlin/src/Day12.kt
@@ -0,0 +1,72 @@
+object Day12 {
+    private class CaveGraph {
+        companion object {
+            fun fromLines(input: List<String>): CaveGraph =
+                CaveGraph().apply { input.forEach { addConnection(it) } }
+        }
+
+        private val adjacencyList = mutableMapOf<Node, Set<Node>>()
+
+        private fun addConnection(connectionString: String) {
+            val (from, to) = connectionString.split("-").map(Node::fromString)
+
+            adjacencyList.merge(from, setOf(to), Set<Node>::union)
+            adjacencyList.merge(to, setOf(from), Set<Node>::union)
+        }
+
+        fun countPaths(canUseDoubleTraversal: Boolean = false): Int =
+            traverse(Node.Start, setOf(), !canUseDoubleTraversal)
+
+        private fun traverse(from: Node, visitedBefore: Set<Node>, usedUpDoubleTraverse: Boolean): Int {
+            return adjacencyList[from]!!.sumOf {
+                when (it) {
+                    is Node.Start -> 0
+                    is Node.End -> 1
+                    is Node.Big -> traverse(it, visitedBefore + from, usedUpDoubleTraverse)
+                    is Node.Small -> {
+                        if (!visitedBefore.contains(it)) {
+                            traverse(it, visitedBefore + from, usedUpDoubleTraverse)
+                        } else if (!usedUpDoubleTraverse) {
+                            traverse(it, visitedBefore + from, true)
+                        } else {
+                            0
+                        }
+                    }
+                }
+            }
+        }
+
+        private sealed class Node {
+            companion object {
+                fun fromString(text: String): Node = when (text) {
+                    "start" -> Start
+                    "end" -> End
+                    else -> if (text == text.uppercase()) {
+                        Big(text)
+                    } else {
+                        Small(text)
+                    }
+                }
+            }
+
+            object Start : Node()
+            object End : Node()
+            data class Small(private val identifier: String) : Node()
+            data class Big(private val identifier: String) : Node()
+        }
+    }
+
+    fun bothParts(input: List<String>) = CaveGraph.fromLines(input).let { it.countPaths() to it.countPaths(true) }
+}
+
+fun main() {
+    val testInput = readInputAsLines("Day12_test")
+    val testOutput = Day12.bothParts(testInput)
+    check(testOutput.first == 226)
+    check(testOutput.second == 3509)
+
+    val input = readInputAsLines("Day12")
+    val output = Day12.bothParts(input)
+    println(output.first)
+    println(output.second)
+}