diff --git a/src/year2021/day13/Day13.kt b/src/year2021/day13/Day13.kt
new file mode 100644
index 0000000..a8f53a4
--- /dev/null
+++ b/src/year2021/day13/Day13.kt
@@ -0,0 +1,95 @@
+package year2021.day13
+
+import readInput
+
+enum class Along {
+    X, Y
+}
+
+data class Point(val x: Int, val y: Int)
+
+data class Instruction(val along: Along, val coordinate: Int) {
+    private fun isOver(point: Point): Boolean = when (along) {
+        Along.X -> point.x > coordinate
+        Along.Y -> point.y > coordinate
+    }
+
+    private fun flip(point: Point): Point = when (along) {
+        Along.X -> Point(2 * coordinate - point.x, point.y)
+        Along.Y -> Point(point.x, 2 * coordinate - point.y)
+    }
+
+    fun execute(points: Set<Point>): Set<Point> {
+        val pointsOverFold = points.filter { isOver(it) }
+        val pointsFlipped = pointsOverFold.map { flip(it) }
+        return points - pointsOverFold.toSet() + pointsFlipped.toSet()
+    }
+}
+
+data class Origami(private var points: Set<Point>, val instructions: List<Instruction>, private var i: Int = 0) {
+    val visiblePoints: Int
+        get() = points.size
+
+    val instructionsLeft: Boolean
+        get() = i < instructions.size
+
+    fun nextInstruction(): Origami {
+        if (i >= instructions.size) {
+            return this
+        }
+
+        points = instructions[i++].execute(points)
+        return this
+    }
+
+    override fun toString(): String =
+        (0..points.maxOf { it.y }).joinToString("\n") { y ->
+            (0..points.maxOf { it.x }).joinToString("") { x -> if (points.contains(Point(x, y))) "#" else " " }
+        }
+}
+
+fun String.toPoint(): Point {
+    val (x, y) = this.split(",").map(String::toInt)
+    return Point(x, y)
+}
+
+fun String.toInstruction(): Instruction {
+    val (_, _, description) = this.split(" ")
+    val (axis, coordinate) = description.split("=")
+    return Instruction(if (axis == "x") Along.X else Along.Y, coordinate.toInt())
+}
+
+fun readOrigami(input: List<String>): Origami =
+    input
+        .joinToString("\n")
+        .split("\n\n")
+        .map { it.split("\n") }
+        .let { (points, instructions) ->
+            Origami(
+                points.map(String::toPoint).toSet(),
+                instructions.map(String::toInstruction)
+            )
+        }
+
+fun part1(input: List<String>): Int =
+    readOrigami(input).nextInstruction().visiblePoints
+
+fun part2(input: List<String>): String {
+    val origami = readOrigami(input)
+    while (origami.instructionsLeft) {
+        origami.nextInstruction()
+    }
+    return origami.toString()
+}
+
+fun main() {
+    val sample = readInput(13, "sample")
+    val input = readInput(13, "input")
+
+    check(part1(sample) == 17)
+    println(part1(input))
+
+    println(part2(sample))
+    println()
+    println(part2(input))
+}
diff --git a/src/year2021/day13/sample.txt b/src/year2021/day13/sample.txt
new file mode 100644
index 0000000..32a8563
--- /dev/null
+++ b/src/year2021/day13/sample.txt
@@ -0,0 +1,21 @@
+6,10
+0,14
+9,10
+0,3
+10,4
+4,11
+6,0
+6,12
+4,1
+0,13
+10,12
+3,4
+3,0
+8,4
+1,10
+2,14
+8,10
+9,0
+
+fold along y=7
+fold along x=5
\ No newline at end of file