diff --git a/pom.xml b/pom.xml
index f69c17b..f03aaf9 100644
--- a/pom.xml
+++ b/pom.xml
@@ -90,6 +90,14 @@
                     </execution>
                 </executions>
             </plugin>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-compiler-plugin</artifactId>
+                <configuration>
+                    <source>8</source>
+                    <target>8</target>
+                </configuration>
+            </plugin>
         </plugins>
     </build>
 
diff --git a/src/main/kotlin/com/basdado/adventofcode/PrimeSieve.kt b/src/main/kotlin/com/basdado/adventofcode/PrimeSieve.kt
new file mode 100644
index 0000000..7d0d57a
--- /dev/null
+++ b/src/main/kotlin/com/basdado/adventofcode/PrimeSieve.kt
@@ -0,0 +1,198 @@
+package com.basdado.adventofcode
+
+import java.util.function.Consumer
+import kotlin.math.ln
+import kotlin.math.sqrt
+
+/**
+ * Prime number generator based on the sieve of Eratosthenes.
+ *
+ * @author Bas Dado
+ */
+class PrimeSieve(private var until: Int) : Iterable<Int?> {
+
+    private var sieve: BoolArray
+
+    override fun iterator(): MutableIterator<Int> {
+        return PrimeSieveIterator(sieve)
+    }
+
+    /**
+     * @param n The number to check
+     * @return True iff n is even
+     */
+    private fun isEven(n: Int): Boolean {
+        return n % 2 == 0
+    }
+
+    /**
+     * @param n The number to check
+     * @return Checks if the number is in the sieve, since we skip even numbers and numbers smaller than 3.
+     */
+    fun hasIndex(n: Int): Boolean {
+        return !isEven(n) && n >= 3
+    }
+
+    /**
+     * @return True iff n is prime
+     */
+    fun isPrime(n: Int): Boolean {
+        return if (n == 2) true else if (!hasIndex(n)) false else if (n <= until) sieve[getIndex(n)] else if (n <= until * until) {
+            for (prime in this) if (n % prime == 0) return false
+            true
+        } else {
+            throw IndexOutOfBoundsException("With the current sieve size it's impossible to determine whether n is prime")
+        }
+    }
+
+    /**
+     * @param n The number to check
+     * @return The index of n in the sieve. Gives a lower bound index if n is not in the sieve
+     */
+    private fun getIndex(n: Int): Int {
+        return (n - 3) / 2
+    }
+
+    /**
+     * @param i
+     * @return The number at index i in the Sieve
+     */
+    private fun getNumberAtIndex(i: Int): Int {
+        return i * 2 + 3
+    }
+
+    /**
+     * @param until The number to check against
+     * @return Expected number of primes below "until", based on the "Prime Number Theorem",
+     */
+    fun getExpectedPrimeCount(until: Double): Int {
+        return (until / (ln(until) - 1.0)).toInt()
+    }
+
+    /**
+     * Marks of multiples of n in the sieve, starting at the square of n
+     * @param sieve The sieve in which the numbers should be marked of
+     * @param n
+     * @param i
+     */
+    private fun markMultiples(sieve: BoolArray, n: Int, i: Int) {
+        var cur = i + (i + 1) * n // Start at the square of the number.
+        while (cur < sieve.size()) { // Since we're multiplying two odd numbers, the result is always odd, and thus in our sieve.
+            sieve[cur] = false
+            cur += n
+        }
+    }
+
+    /**
+     * Builds a prime sieve up to the current "until" value.
+     * @return
+     */
+    private fun generateSieve(): BoolArray { // Initialize the sieve assuming all numbers are prime
+        val sieve = BoolArray(getIndex(until) + 1, true)
+        val sqrtUntil = (sqrt(until.toDouble()) + 1).toInt()
+        val iSqrtUntil = getIndex(sqrtUntil)
+        for (i in 0..iSqrtUntil) { // If the current number is prime, mark all multiples of it as being not prime:
+            if (sieve[i]) { // n is the current number that is considered
+                val n = getNumberAtIndex(i)
+                //MarkMultiples(sieve, n, until);
+                markMultiples(sieve, n, i)
+            }
+        }
+        return sieve
+    }
+
+    inner class PrimeSieveIterator(private val sieve: BoolArray) : MutableIterator<Int> {
+
+        private var i: Int = -1
+        private val lastPrimeIndex: Int
+
+        private fun getLastPrimeIndex(): Int {
+            for (i in sieve.size() - 1 downTo 1) {
+                if (sieve[i]) return i
+            }
+            return 0
+        }
+
+        override fun hasNext(): Boolean {
+            return i < lastPrimeIndex
+        }
+
+        override fun next(): Int {
+            if (i == -1) {
+                i++
+                return 2
+            }
+            while (!sieve[i]) i++
+            return getNumberAtIndex(i++)
+        }
+
+        override fun remove() {
+            throw UnsupportedOperationException()
+        }
+
+        override fun forEachRemaining(action: Consumer<in Int>) {
+            while (hasNext()) {
+                action.accept(next())
+            }
+        }
+
+        init {
+            lastPrimeIndex = getLastPrimeIndex()
+        }
+    }
+
+    class BoolArray(size: Int, initial: Boolean) {
+
+        private val data: IntArray
+        private val size: Int
+
+        private fun getWordIndex(i: Int): Int {
+            return i shr 5
+        }
+
+        private fun getBitIndex(i: Int): Int {
+            return i and 0x1F
+        }
+
+        /**
+         * @param i
+         * @return The bit mask that masks the bit corresponding to index i
+         */
+        private fun getWordMask(i: Int): Int {
+            return 1 shl getBitIndex(i)
+        }
+
+        /**
+         * @param i The index of the wanted boolean
+         * @return The value of the boolean stored at index i
+         */
+        operator fun get(i: Int): Boolean {
+            return data[getWordIndex(i)] and getWordMask(i) != 0
+        }
+
+        /**
+         * Sets the boolean at index i to the value given in value.
+         * @param i
+         * @param value
+         */
+        operator fun set(i: Int, value: Boolean) {
+            if (value) data[getWordIndex(i)] = data[getWordIndex(i)] or getWordMask(i) else data[getWordIndex(i)] =
+                data[getWordIndex(i)] and getWordMask(i).inv()
+        }
+
+        fun size(): Int {
+            return size
+        }
+
+        init {
+            data = IntArray(getWordIndex(size - 1) + 1)
+            // Default int value is 0, so we only set it if the initial value is true
+            if (initial) for (i in data.indices) data[i] = 0.inv()
+            this.size = size
+        }
+    }
+
+    init {
+        sieve = generateSieve()
+    }
+}
\ No newline at end of file
diff --git a/src/main/kotlin/com/basdado/adventofcode/day12/Day12.kt b/src/main/kotlin/com/basdado/adventofcode/day12/Day12.kt
index 5776c57..194edc1 100644
--- a/src/main/kotlin/com/basdado/adventofcode/day12/Day12.kt
+++ b/src/main/kotlin/com/basdado/adventofcode/day12/Day12.kt
@@ -1,10 +1,11 @@
 package com.basdado.adventofcode.day12
 
+import com.basdado.adventofcode.PrimeSieve
 import com.basdado.adventofcode.lines
-import java.lang.Long.compare
 import java.util.stream.Collectors
 import java.util.stream.LongStream
 import kotlin.math.abs
+import kotlin.math.sqrt
 
 const val DAY12_INPUT_PATH = "/day/12/input.txt"
 
@@ -15,12 +16,14 @@ fun main() {
 
 object Day12 {
 
+    val primes = PrimeSieve(1000000)
+
     fun puzzle1() {
         val positions = parseInput()
         val velocities = Array(positions.size) { Vector3l.ZERO }
 
         repeat(1000) {
-            println("$it: ${positions.toList()}")
+//            println("$it: pos: ${positions.toList()}, vel: ${velocities.toList()}")
 //            println("$it: energy: ${energy(positions, velocities)}")
             applyGravity(positions, velocities)
             applyVelocity(positions, velocities)
@@ -37,24 +40,68 @@ object Day12 {
 
         val initialPositions = positions.clone()
         val initialVelocities = velocities.clone()
-        val initialRelativePositions = relative(positions)
 
-        println(LongStream.iterate(0) { it + 1 }
-            .peek {
-                applyGravity(positions, velocities)
-                applyVelocity(positions, velocities)
-                if (it % 100000L == 0L) println("$it: pos: ${positions.toList()}, vel: ${velocities.toList()}")
-            }
-            .dropWhile { !(positions.contentEquals(initialRelativePositions) && velocities.contentEquals(initialVelocities)) }
-            .findFirst()!!
-        )
+        var periodPerAxis = Vector3l(0L, 0L, 0L)
 
         // Oh wait, X, Y and Z are completely independent, so we could just simulate the universe per component, and
         // then find the lowest number that's divisible by all three numbers
+        LongStream.iterate(1) { it + 1 }
+            .peek { t ->
+                applyGravity(positions, velocities)
+                applyVelocity(positions, velocities)
+
+                var xEqual = true
+                var yEqual = true
+                var zEqual = true
+                for (i in positions.indices) {
+                    xEqual = xEqual && positions[i].x == initialPositions[i].x && velocities[i].x == initialVelocities[i].x
+                    yEqual = yEqual && positions[i].y == initialPositions[i].y && velocities[i].y == initialVelocities[i].y
+                    zEqual = zEqual && positions[i].z == initialPositions[i].z && velocities[i].z == initialVelocities[i].z
+                }
+                if (xEqual && periodPerAxis.x == 0L) periodPerAxis = periodPerAxis.copy(x = t)
+                if (yEqual && periodPerAxis.y == 0L) periodPerAxis = periodPerAxis.copy(y = t)
+                if (zEqual && periodPerAxis.z == 0L) periodPerAxis = periodPerAxis.copy(z = t)
+//                if (t % 100000L == 0L) println("$t: $periodPerAxis, pos: ${positions.toList()}, vel: ${velocities.toList()}")
+            }
+            .dropWhile { periodPerAxis.x == 0L || periodPerAxis.y == 0L || periodPerAxis.z == 0L }
+            .findFirst()
+        println(periodPerAxis)
+
+        // Now all we need to do is find the LCM (Lowest Common Multiple) of the periods per axis:
+        println(lcm(longArrayOf(periodPerAxis.x, periodPerAxis.y, periodPerAxis.z)))
         
 
     }
 
+    private fun lcm(numbers: LongArray, tryUntilMultiplier: Long = 1024): Long {
+
+        val factorizations = numbers.map { factors(it) }
+        return factorizations
+            .flatMap { it.keys }.distinct()
+            .map { p -> Pair(p, factorizations.map { it[p] ?: 0 }.max()!!) }
+            .map { p -> Math.pow(p.first.toDouble(), p.second.toDouble()).toLong() }
+            .reduce { x, y -> x * y }
+    }
+
+    private fun factors(n: Long): Map<Long, Long> {
+
+        var rem = n
+        val res = mutableMapOf<Long, Long>()
+
+        for (prime in primes) {
+            var primeCount = 0L
+            while (rem % prime.toLong() == 0L) {
+                rem /= prime
+                primeCount++
+            }
+            if (primeCount > 0L) {
+                res[prime.toLong()] = primeCount
+            }
+        }
+
+        return res
+    }
+
     private fun energy(positions: Array<Vector3l>, velocities: Array<Vector3l>): Long {
         return positions.indices.map { i -> positions[i].manhattan() * velocities[i].manhattan() }.sum()
     }
diff --git a/src/main/resources/day/12/ex2.txt b/src/main/resources/day/12/ex2.txt
new file mode 100644
index 0000000..c0edb40
--- /dev/null
+++ b/src/main/resources/day/12/ex2.txt
@@ -0,0 +1,4 @@
+<x=-8, y=-10, z=0>
+<x=5, y=5, z=10>
+<x=2, y=-7, z=3>
+<x=9, y=-8, z=-3>
\ No newline at end of file