diff --git a/src/year2021/day23/Day23.kt b/src/year2021/day23/Day23.kt
new file mode 100644
index 0000000..6259775
--- /dev/null
+++ b/src/year2021/day23/Day23.kt
@@ -0,0 +1,220 @@
+package year2021.day23
+
+import product
+import readInput
+import java.util.PriorityQueue
+import kotlin.math.absoluteValue
+
+// region Position
+data class Position(val x: Int, val y: Int) {
+    fun distance(other: Position): Int =
+        if (y != 1 && other.y != 1) {
+            val mid = Position(other.x, 1)
+            this.distance(mid) + mid.distance(other)
+        } else {
+            (x - other.x).absoluteValue + (y - other.y).absoluteValue
+        }
+}
+// endregion Position
+
+// region Ampod
+fun energyPerStep(type: Char): Int = when (type) {
+    'A' -> 1
+    'B' -> 10
+    'C' -> 100
+    'D' -> 1000
+    else -> error("Invalid amphipod type")
+}
+
+fun targetRoom(type: Char): Int = when (type) {
+    'A' -> 3
+    'B' -> 5
+    'C' -> 7
+    'D' -> 9
+    else -> error("Invalid amphipod type")
+}
+
+data class Ampod(val type: Char, val position: Position) {
+    fun costTo(position: Position): Int = this.position.distance(position) * energyPerStep(type)
+
+    val isInHallway: Boolean
+        get() = position.y == 1
+
+    val isInRoom: Boolean
+        get() = position.y in 2..5 && position.x in (3..9 step 2)
+
+    val isInCorrectRoom: Boolean
+        get() = position.y in 2..5 && position.x == targetRoom(type)
+}
+// endregion Ampod
+
+// region Diagram
+data class Diagram(val ampods: Set<Ampod>, val maxY: Int = 3) {
+    private val hallwaySpots =
+        (1 until 12)
+            .filter { (it < 3 || it > 9 || it in (4..8 step 2)) }
+            .map { x -> Position(x, 1) }
+
+    private val freeHallwaySpots: Sequence<Position>
+        get() = hallwaySpots.asSequence().filter { !ampods.any { amphipod -> amphipod.position == it } }
+
+    private val ampodsInHallway: Sequence<Ampod>
+        get() = ampods.asSequence().filter(Ampod::isInHallway)
+
+    private val ampodsInRooms: Sequence<Ampod>
+        get() = ampods.asSequence().filter(Ampod::isInRoom)
+
+    // TODO: Refactor
+    private fun blocked(src: Position, dst: Position): Boolean {
+        if (src.y == 1 && dst.y in 2..maxY) {
+            return ampodsInHallway.any {
+                (it.position.x > src.x && it.position.x <= dst.x) || (it.position.x < src.x && it.position.x >= dst.x)
+            } || ampods.any {
+                it.position.x == dst.x && it.position.y <= dst.y
+            }
+        } else if (src.y in 2..maxY && dst.y == 1) {
+            return ampods.any {
+                it.position.x == src.x && it.position.y < src.y
+            } || ampodsInHallway.any {
+                (it.position.x in src.x..dst.x) || (it.position.x in dst.x..src.x)
+            }
+        }
+
+        return blocked(src, Position(dst.x, 1)) || blocked(Position(dst.x, 1), dst)
+    }
+
+    private fun moveAmpod(ampod: Ampod, position: Position): Diagram =
+        this.copy(ampods = ampods - setOf(ampod) + setOf(ampod.copy(position = position)))
+
+    private fun tryMoveFromHallway(ampod: Ampod): Pair<Diagram, Int>? {
+        val targetX = targetRoom(ampod.type)
+
+        if (ampodsInRooms.any { it.type != ampod.type && it.position.x == targetX }) {
+            // there is still an ampod of different type in the room
+            return null
+        }
+
+        val destination =
+            Position(
+                targetX,
+                (2..maxY).reversed().first { y ->
+                    !ampodsInRooms.any {
+                        it.type == ampod.type && it.position == Position(
+                            targetX, y
+                        )
+                    }
+                }
+            )
+        if (blocked(ampod.position, destination)) {
+            return null
+        }
+
+        return (moveAmpod(ampod, destination) to ampod.costTo(destination))
+    }
+
+    private fun tryMoveFromRoom(ampod: Ampod, destination: Position): Pair<Diagram, Int>? {
+        if (blocked(ampod.position, destination)) {
+            return null
+        }
+        return moveAmpod(ampod, destination) to ampod.costTo(destination)
+    }
+
+    fun moves(): Sequence<Pair<Diagram, Int>> = sequence {
+        yieldAll(product(ampodsInRooms, freeHallwaySpots).mapNotNull { (amphipod, freeSpot) ->
+            tryMoveFromRoom(amphipod, freeSpot)
+        })
+        yieldAll(ampodsInHallway.mapNotNull { tryMoveFromHallway(it) })
+    }
+
+    val goal: Diagram
+        get() = this.copy(
+            ampods = product('A'..'D', 2..maxY).map { (type, y) ->
+                Ampod(
+                    type,
+                    Position(targetRoom(type), y)
+                )
+            }.toSet()
+        )
+
+    override fun toString(): String {
+        fun charAt(x: Int, y: Int): Char =
+            if (y > 2 && (x < 2 || x > 10))
+                ' '
+            else if (y == 0 || y == 4 || x == 0 || x == 12)
+                '#'
+            else if (y == 1 || x == 3 || x == 5 || x == 7 || x == 9)
+                ampods.firstOrNull { it.position == Position(x, y) }?.type ?: '.'
+            else
+                '#'
+
+        return (0 until 5).joinToString("\n") { y ->
+            (0 until 13).map { x -> charAt(x, y) }.joinToString("")
+        }
+    }
+}
+
+fun List<String>.toDiagram(): Diagram = Diagram(
+    this@toDiagram.flatMapIndexed { y, row ->
+        row.mapIndexedNotNull { x, c ->
+            if (c !in 'A'..'D') {
+                null
+            } else {
+                Ampod(c, Position(x, y))
+            }
+        }
+    }.toSet()
+)
+// endregion Diagram
+
+// region solve
+fun solve(state: Diagram, goal: Diagram): Int {
+    val cost = mutableMapOf(state to 0)
+    val visited = mutableSetOf<Diagram>()
+    val queue = PriorityQueue<Pair<Diagram, Int>> { a, b ->
+        a.second - b.second
+    }
+
+    queue.add(state to 0)
+
+    while (queue.isNotEmpty()) {
+        val (currentDiagram, currentCost) = queue.remove()
+        if (visited.contains(currentDiagram)) {
+            continue
+        }
+
+        if (currentDiagram == goal) {
+            break
+        }
+
+        visited.add(currentDiagram)
+        currentDiagram.moves().forEach { (next, nextCost) ->
+            val alternativeCost = currentCost + nextCost
+            if (alternativeCost < cost.getOrDefault(next, Int.MAX_VALUE)) {
+                cost[next] = alternativeCost
+                queue.add(next to alternativeCost)
+            }
+        }
+    }
+
+    return cost[goal]!!
+}
+// endregion solve
+
+fun part1(input: Diagram): Int = solve(input, input.goal)
+fun part2(input: Diagram): Int = input.copy(maxY = 5).let { newInput -> solve(newInput, newInput.goal) }
+
+fun main() {
+    val sample = readInput(23, "sample").toDiagram()
+    val input = readInput(23, "input").toDiagram()
+
+    check(part1(sample) == 12521)
+    println("[PASS] Part #1 check")
+    println(part1(input))
+
+    val sample2 = readInput(23, "sample2").toDiagram()
+    val input2 = readInput(23, "input2").toDiagram()
+
+    check(part2(sample2) == 44169)
+    println("[PASS] Part #2 check")
+    println(part2(input2))
+}
diff --git a/src/year2021/day23/sample.txt b/src/year2021/day23/sample.txt
new file mode 100644
index 0000000..56a84a1
--- /dev/null
+++ b/src/year2021/day23/sample.txt
@@ -0,0 +1,5 @@
+#############
+#...........#
+###B#C#B#D###
+  #A#D#C#A#
+  #########
\ No newline at end of file
diff --git a/src/year2021/day23/sample2.txt b/src/year2021/day23/sample2.txt
new file mode 100644
index 0000000..b96553c
--- /dev/null
+++ b/src/year2021/day23/sample2.txt
@@ -0,0 +1,7 @@
+#############
+#...........#
+###B#C#B#D###
+  #D#C#B#A#
+  #D#B#A#C#
+  #A#D#C#A#
+  #########
\ No newline at end of file