Kotlin solution for day 12

kotlin/day12.kts

@@ -0,0 +1,79 @@
+import java.io.File
+
+fun findAllPaths(caves: MutableMap<String, MutableList<String>>,
+                 canVisitASmallCaveTwice: Boolean): MutableList<MutableList<String>> {
+
+    var visited = mutableListOf<String>()
+    var currentPath = mutableListOf<String>()
+    var paths = mutableListOf<MutableList<String>>()
+
+    visitCave(caves, visited, currentPath, paths,"start", canVisitASmallCaveTwice, false)
+
+    return paths
+}
+
+fun isBigCave(cave: String): Boolean {
+    for (c in cave) {
+        if (c.isLowerCase())
+            return false
+    }
+
+    return true
+}
+
+fun visitCave(caves: MutableMap<String, MutableList<String>>, visited: MutableList<String>,
+              currentPath: MutableList<String>, paths: MutableList<MutableList<String>>,
+              currentCave: String, canVisitASmallCaveTwice: Boolean, smallCaveVisitedTwice: Boolean) {
+
+    visited.add(currentCave)
+    currentPath.add(currentCave)
+
+    if (currentCave == "end") {
+        paths.add(currentPath.toMutableList())
+    } else {
+        val adjacentCaves = caves.getOrDefault(currentCave, mutableListOf<String>())
+
+        for (nextCave in adjacentCaves) {
+            if (nextCave == "start")
+                continue
+
+            var canVisit = (isBigCave(nextCave) || (nextCave !in visited))
+            var smallCaveVisitedTwiceNow = smallCaveVisitedTwice
+
+            // If we are in the second part of the puzzle, one small cave can be visited twice
+            // We check if we are in a position where we can still do this
+            if (!canVisit && canVisitASmallCaveTwice && !smallCaveVisitedTwice) {
+                canVisit = true
+                smallCaveVisitedTwiceNow = true
+            }
+
+            if (canVisit)
+                visitCave(caves, visited, currentPath, paths, nextCave, canVisitASmallCaveTwice, smallCaveVisitedTwiceNow)
+        }
+    }
+
+    currentPath.removeLast()
+    visited.remove(currentCave)
+}
+
+fun buildGraph(fileName: String): MutableMap<String, MutableList<String>> {
+    var caves = mutableMapOf<String, MutableList<String>>()
+
+    File(fileName).forEachLine {
+        var (start, end) = it.split("-")
+
+        // Bidirected graph, so we add both start->end and end->start into the list of edges
+        caves.put(start, (caves.getOrDefault(start, mutableListOf()) + end).toMutableList())
+        caves.put(end, (caves.getOrDefault(end, mutableListOf()) + start).toMutableList())
+    }
+
+    return caves
+}
+
+val fileName = if (args.size > 0) args[0] else "day12.txt"
+
+// List of edges of the bidirected graph
+var caves = buildGraph(fileName)
+
+println(findAllPaths(caves, false).size)
+println(findAllPaths(caves, true).size)