diff --git a/src/main/kotlin/com/basdado/adventofcode/Day12.kt b/src/main/kotlin/com/basdado/adventofcode/Day12.kt
new file mode 100644
index 0000000..0082c46
--- /dev/null
+++ b/src/main/kotlin/com/basdado/adventofcode/Day12.kt
@@ -0,0 +1,155 @@
+package com.basdado.adventofcode
+
+import java.util.*
+import java.util.stream.Collectors
+import kotlin.math.max
+import kotlin.math.min
+
+const val INPUT = "/day/12/input.txt"
+
+fun main() {
+    val day = Day12()
+    day.puzzle1()
+    day.puzzle2()
+}
+
+class Day12 {
+
+    fun puzzle1() {
+        val initialState = parseInitialState()
+        val plantPatterns = parsePlantPatterns()
+
+        var lastPlantState = getPlantStateAfterGeneration(initialState, plantPatterns, 20)
+
+        println(lastPlantState.calcPlantSum())
+    }
+
+    fun puzzle2() {
+
+        val initialState = parseInitialState()
+        val plantPatterns = parsePlantPatterns()
+
+        val lastPlantState = getPlantStateAfterGeneration(initialState, plantPatterns, 50000000000L)
+
+        println(lastPlantState.calcPlantSum())
+    }
+
+    private fun getPlantStateAfterGeneration(
+        initialState: BooleanArray,
+        plantPatterns: BooleanArray,
+        generationCount: Long
+    ): PlantState {
+        var lastPlantState = PlantState(0, initialState)
+//        println("0:    $lastPlantState")
+        for (i in 1L..generationCount) {
+            val newPlantState = PlantState(lastPlantState.firstPlantIndex - 2, BooleanArray(lastPlantState.state.size + 4))
+            for (plantIndex in (newPlantState.firstPlantIndex) until (newPlantState.firstPlantIndex + newPlantState.state.size)) {
+                val patternIndex = lastPlantState.getPatternIndex(plantIndex)
+                newPlantState.set(plantIndex, plantPatterns[patternIndex])
+            }
+
+            val trimmedNewState = newPlantState.trimmed()
+            if (lastPlantState.equalsShifted(trimmedNewState)) {
+                // So the only changes from now on is shifting the plants a little bit every generation
+                val shift = trimmedNewState.firstPlantIndex - lastPlantState.firstPlantIndex
+                val totalShift = shift * (generationCount - i)
+                return PlantState(trimmedNewState.firstPlantIndex + totalShift, trimmedNewState.state)
+            }
+            lastPlantState = trimmedNewState
+//            println("$i:  $newPlantState")
+        }
+        return lastPlantState
+    }
+
+    private fun parsePlantPatterns(): BooleanArray {
+        val plantPatternRegex = Regex("""([.#]{5}) => ([.#])""")
+        val plantPatternIndices = lines(INPUT)
+            .map { plantPatternRegex.matchEntire(it) }
+            .filter { it != null }
+            .filter { it!!.groupValues[2] == "#" }
+            .map { plantPatternToIndex(it!!.groupValues[1]) }
+            .collect(Collectors.toList())
+        return BooleanArray(32) { plantPatternIndices.contains(it) }
+    }
+
+    private fun parseInitialState(): BooleanArray {
+        val initialStateRegex = Regex("""initial state: ([.#]+)""")
+        return plantsToBooleanArray(lines(INPUT).filter { initialStateRegex.matches(it) }.findFirst()
+            .map { initialStateRegex.matchEntire(it) }.get().groupValues[1])
+    }
+
+    companion object {
+
+        @JvmStatic
+        fun plantsToBooleanArray(plants: String): BooleanArray {
+            return plants.chars().mapToObj { it == '#'.toInt() }.toArray { Array(it) { false } }.toBooleanArray()
+        }
+
+        @JvmStatic
+        fun plantPatternToIndex(plantPattern: String): Int {
+            assert(plantPattern.length == 5)
+
+            var res = 0
+            (0 until plantPattern.length).forEach { i ->
+                if (plantPattern[i] == '#') {
+                    res = res or (1 shl i)
+                }
+            }
+            return res
+        }
+    }
+
+
+
+    class PlantState(val firstPlantIndex: Long, val state: BooleanArray) {
+        val lastPlantIndex = firstPlantIndex + state.size
+
+        fun getPatternIndex(plantIndex: Long): Int {
+            var res = 0
+            for (i in 0 until 5) {
+                if (get(plantIndex + i - 2L)) {
+                    res = res or (1 shl i)
+                }
+            }
+            return res
+        }
+
+        private fun stateIndex(plantIndex: Long): Int = (plantIndex - firstPlantIndex).toInt()
+        private fun plantIndex(stateIndex: Int): Long = firstPlantIndex + stateIndex
+
+        fun set(plantIndex: Long, value: Boolean) {
+            state[stateIndex(plantIndex)] = value
+        }
+
+        fun get(plantIndex: Long): Boolean {
+            val stateIndex = stateIndex(plantIndex)
+            return stateIndex >= 0 && stateIndex < state.size && state[stateIndex]
+        }
+
+        override fun toString(): String {
+            return state.joinToString("") { if (it) "#" else "." }
+        }
+
+        fun calcPlantSum(): Long = (firstPlantIndex until (firstPlantIndex + state.size)).filter { get(it) }.sum()
+
+        fun trimmed(): PlantState {
+            val firstPlantStateIndex = state.indexOfFirst { it }
+            val lastPlantStateIndex = state.indexOfLast { it }
+            if (firstPlantStateIndex == 0 && lastPlantStateIndex == state.size - 1) {
+                return this
+            }
+            return PlantState(plantIndex(firstPlantStateIndex), state.copyOfRange(firstPlantStateIndex, lastPlantStateIndex + 1))
+        }
+
+        override fun equals(other: Any?): Boolean {
+            return other is PlantState &&
+                    (min(firstPlantIndex, other.firstPlantIndex)..(max(lastPlantIndex, other.lastPlantIndex))).all { get(it) == other.get(it) }
+        }
+
+        fun equalsShifted(other: PlantState): Boolean {
+            val thisTrimmed = this.trimmed()
+            val otherTrimmed = other.trimmed()
+            return Arrays.equals(thisTrimmed.state, otherTrimmed.state)
+        }
+    }
+}
\ No newline at end of file
diff --git a/src/main/resources/day/12/example.txt b/src/main/resources/day/12/example.txt
new file mode 100644
index 0000000..8335d88
--- /dev/null
+++ b/src/main/resources/day/12/example.txt
@@ -0,0 +1,16 @@
+initial state: #..#.#..##......###...###
+
+...## => #
+..#.. => #
+.#... => #
+.#.#. => #
+.#.## => #
+.##.. => #
+.#### => #
+#.#.# => #
+#.### => #
+##.#. => #
+##.## => #
+###.. => #
+###.# => #
+####. => #
\ No newline at end of file
diff --git a/src/main/resources/day/12/input.txt b/src/main/resources/day/12/input.txt
new file mode 100644
index 0000000..c9e4a95
--- /dev/null
+++ b/src/main/resources/day/12/input.txt
@@ -0,0 +1,34 @@
+initial state: ..#..####.##.####...#....#######..#.#..#..#.#.#####.######..#.#.#.#..##.###.#....####.#.#....#.#####
+
+#.##. => .
+#.#.. => .
+###.# => .
+..#.# => .
+....# => .
+.#### => .
+##.## => #
+###.. => #
+.###. => #
+...#. => .
+..... => .
+##..# => .
+.#.#. => #
+.#.## => #
+##.#. => .
+##... => .
+##### => #
+#...# => .
+..##. => .
+..### => .
+.#... => #
+.##.# => .
+#.... => .
+.#..# => .
+.##.. => #
+...## => #
+#.### => .
+#..#. => .
+..#.. => #
+#.#.# => #
+####. => #
+#..## => .
diff --git a/src/main/resources/day/12/test.txt b/src/main/resources/day/12/test.txt
new file mode 100644
index 0000000..02cf6df
--- /dev/null
+++ b/src/main/resources/day/12/test.txt
@@ -0,0 +1,3 @@
+initial state: #..#
+...#. => #
+.#... => #
\ No newline at end of file