diff --git a/advent-of-code/2023/day_04/.gitignore b/advent-of-code/2023/day_04/.gitignore
new file mode 100644
index 0000000..261ed2a
--- /dev/null
+++ b/advent-of-code/2023/day_04/.gitignore
@@ -0,0 +1,43 @@
+.gradle
+gradlew*
+gradle
+build/
+!**/src/main/**/build/
+!**/src/test/**/build/
+
+### IntelliJ IDEA ###
+.idea
+*.iws
+*.iml
+*.ipr
+out/
+!**/src/main/**/out/
+!**/src/test/**/out/
+
+### Kotlin ###
+.kotlin
+
+### Eclipse ###
+.apt_generated
+.classpath
+.factorypath
+.project
+.settings
+.springBeans
+.sts4-cache
+bin/
+!**/src/main/**/bin/
+!**/src/test/**/bin/
+
+### NetBeans ###
+/nbproject/private/
+/nbbuild/
+/dist/
+/nbdist/
+/.nb-gradle/
+
+### VS Code ###
+.vscode/
+
+### Mac OS ###
+.DS_Store
diff --git a/advent-of-code/2023/day_04/README.md b/advent-of-code/2023/day_04/README.md
new file mode 100644
index 0000000..5554c8a
--- /dev/null
+++ b/advent-of-code/2023/day_04/README.md
@@ -0,0 +1,103 @@
+# Advent of Code 2023 day 4 solution in Kotlin
+
+## Day 4: Scratchcards
+
+[Task page](https://adventofcode.com/2023/day/4)
+
+The gondola takes you up. Strangely, though, the ground doesn't seem to be coming with you;
+you're not climbing a mountain. As the circle of Snow Island recedes below you,
+an entire new landmass suddenly appears above you! The gondola carries you to the surface of the new island
+and lurches into the station.
+
+As you exit the gondola, the first thing you notice is that the air here is much warmer than it was on Snow Island.
+It's also quite humid. Is this where the water source is?
+
+The next thing you notice is an Elf sitting on the floor across the station in what
+seems to bea pile of colorful square cards.
+
+"Oh! Hello!" The Elf excitedly runs over to you. "How may I be of service?" You ask about water sources.
+
+"I'm not sure; I just operate the gondola lift. That does sound like something we'd have, though - this is
+Island Island, after all! I bet the gardener would know. He's on a different island, though - er,
+the small kind surrounded by water, not the floating kind. We really need to come up with a better naming scheme.
+Tell you what: if you can help me with something quick, I'll let you borrow my boat and you can go visit the gardener.
+I got all these scratchcards as a gift, but I can't figure out what I've won."
+
+The Elf leads you over to the pile of colorful cards. There, you discover dozens of scratchcards,
+all with their opaque covering already scratched off. Picking one up, it looks like each card has
+two lists of numbers separated by a vertical bar (|): a list of winning numbers and then a list of numbers you have.
+You organize the information into a table (your puzzle input).
+
+As far as the Elf has been able to figure out, you have to figure out which of the numbers you have appear in the list
+of winning numbers. The first match makes the card worth one point and each match after the first doubles
+the point value of that card.
+
+For example:
+
+```
+Card 1: 41 48 83 86 17 | 83 86  6 31 17  9 48 53
+Card 2: 13 32 20 16 61 | 61 30 68 82 17 32 24 19
+Card 3:  1 21 53 59 44 | 69 82 63 72 16 21 14  1
+Card 4: 41 92 73 84 69 | 59 84 76 51 58  5 54 83
+Card 5: 87 83 26 28 32 | 88 30 70 12 93 22 82 36
+Card 6: 31 18 13 56 72 | 74 77 10 23 35 67 36 11
+```
+
+In the above example, card 1 has five winning numbers (41, 48, 83, 86, and 17) and eight numbers you have
+(83, 86, 6, 31, 17, 9, 48, and 53). Of the numbers you have, four of them (48, 83, 17, and 86) are winning numbers!
+That means card 1 is worth 8 points (1 for the first match, then doubled three times for each of the three matches
+after the first).
+
+
+- Card 2 has two winning numbers (32 and 61), so it is worth 2 points.
+- Card 3 has two winning numbers (1 and 21), so it is worth 2 points.
+- Card 4 has one winning number (84), so it is worth 1 point.
+- Card 5 has no winning numbers, so it is worth no points.
+- Card 6 has no winning numbers, so it is worth no points.
+
+So, in this example, the Elf's pile of scratchcards is worth 13 points.
+
+Take a seat in the large pile of colorful cards. How many points are they worth in total?
+
+
+## Part Two
+
+Just as you're about to report your findings to the Elf, one of you realizes that the rules have actually been printed
+on the back of every card this whole time.
+
+There's no such thing as "points". Instead, scratchcards only cause you to win more scratchcards equal
+to the number of winning numbers you have.
+
+Specifically, you win copies of the scratchcards below the winning card equal to the number of matches.
+So, if card 10 were to have 5 matching numbers, you would win one copy each of cards 11, 12, 13, 14, and 15.
+
+Copies of scratchcards are scored like normal scratchcards and have the same card number as the card they copied.
+So, if you win a copy of card 10 and it has 5 matching numbers, it would then win a copy of the same cards
+that the original card 10 won: cards 11, 12, 13, 14, and 15. This process repeats until none of the copies cause you
+to win any more cards. (Cards will never make you copy a card past the end of the table.)
+
+This time, the above example goes differently:
+
+```
+Card 1: 41 48 83 86 17 | 83 86  6 31 17  9 48 53
+Card 2: 13 32 20 16 61 | 61 30 68 82 17 32 24 19
+Card 3:  1 21 53 59 44 | 69 82 63 72 16 21 14  1
+Card 4: 41 92 73 84 69 | 59 84 76 51 58  5 54 83
+Card 5: 87 83 26 28 32 | 88 30 70 12 93 22 82 36
+Card 6: 31 18 13 56 72 | 74 77 10 23 35 67 36 11
+```
+
+- Card 1 has four matching numbers, so you win one copy each of the next four cards: cards 2, 3, 4, and 5.
+- Your original card 2 has two matching numbers, so you win one copy each of cards 3 and 4.
+- Your copy of card 2 also wins one copy each of cards 3 and 4.
+- Your four instances of card 3 (one original and three copies) have two matching numbers, so you win four copies each of cards 4 and 5.
+- Your eight instances of card 4 (one original and seven copies) have one matching number, so you win eight copies of card 5.
+- Your fourteen instances of card 5 (one original and thirteen copies) have no matching numbers and win no more cards.
+- Your one instance of card 6 (one original) has no matching numbers and wins no more cards.
+
+Once all of the originals and copies have been processed, you end up with 1 instance of card 1, 2 instances of card 2,
+4 instances of card 3, 8 instances of card 4, 14 instances of card 5, and 1 instance of card 6. In total,
+this example pile of scratchcards causes you to ultimately have 30 scratchcards!
+
+Process all of the original and copied scratchcards until no more scratchcards are won. Including the original set
+of scratchcards, how many total scratchcards do you end up with?
diff --git a/advent-of-code/2023/day_04/build.gradle.kts b/advent-of-code/2023/day_04/build.gradle.kts
new file mode 100644
index 0000000..6397ac9
--- /dev/null
+++ b/advent-of-code/2023/day_04/build.gradle.kts
@@ -0,0 +1,18 @@
+plugins {
+    kotlin("jvm") version "2.0.21"
+}
+
+group = "space.comfycamp"
+version = "1.0"
+
+repositories {
+    mavenCentral()
+}
+
+dependencies {
+    testImplementation(kotlin("test"))
+}
+
+tasks.test {
+    useJUnitPlatform()
+}
diff --git a/advent-of-code/2023/day_04/gradle.properties b/advent-of-code/2023/day_04/gradle.properties
new file mode 100644
index 0000000..7fc6f1f
--- /dev/null
+++ b/advent-of-code/2023/day_04/gradle.properties
@@ -0,0 +1 @@
+kotlin.code.style=official
diff --git a/advent-of-code/2023/day_04/settings.gradle.kts b/advent-of-code/2023/day_04/settings.gradle.kts
new file mode 100644
index 0000000..dee997c
--- /dev/null
+++ b/advent-of-code/2023/day_04/settings.gradle.kts
@@ -0,0 +1,2 @@
+rootProject.name = "day_04"
+
diff --git a/advent-of-code/2023/day_04/src/main/kotlin/Cards.kt b/advent-of-code/2023/day_04/src/main/kotlin/Cards.kt
new file mode 100644
index 0000000..213a994
--- /dev/null
+++ b/advent-of-code/2023/day_04/src/main/kotlin/Cards.kt
@@ -0,0 +1,79 @@
+package space.comfycamp
+
+import kotlin.math.min
+
+/** Part 1. */
+fun countPoints(lines: List<String>): Int {
+    val cards = parseCards(lines)
+    return cards.sumOf { it.getPoints() }
+}
+
+/** Part 2. */
+fun countCards(lines: List<String>): Int {
+    val cards = parseCards(lines)
+
+    for ((idx, card) in cards.withIndex()) {
+        val maxIdx = min(idx+card.countMatchingNumbers(), cards.size-1)
+        for (i in idx+1..maxIdx) {
+            cards[i].instances += card.instances
+        }
+    }
+
+    return cards.sumOf { it.instances }
+}
+
+private fun parseCards(lines: List<String>): MutableList<Card> {
+    val cards = mutableListOf<Card>()
+
+    for (line in lines) {
+        val (_, numbers) = line.split(": ")
+        if (numbers.isEmpty()) continue
+
+        val (winningNumbers, selectedNumbers) = numbers.split(" | ")
+        cards.add(Card(
+            strToInts(winningNumbers),
+            strToInts(selectedNumbers),
+        ))
+    }
+
+    return cards
+}
+
+private fun strToInts(s: String): MutableList<Int> {
+    return s
+        .split(" ")
+        .filter{it.isNotEmpty()}
+        .map{it.toInt()}
+        .toMutableList()
+}
+
+data class Card(
+    val winningNumbers: MutableList<Int> = mutableListOf(),
+    val selectedNumbers: MutableList<Int> = mutableListOf(),
+    var instances: Int = 1,
+) {
+    fun countMatchingNumbers(): Int {
+        return selectedNumbers
+            .filter{winningNumbers.contains(it)}
+            .size
+    }
+
+    fun getPoints(): Int {
+        val count = countMatchingNumbers()
+        return if (count == 0) 0 else intPow(2, count-1)
+    }
+}
+
+/**
+ * I haven't found a function to raise an integer to a power.
+ * You should not use powers less than 0 :)
+ */
+fun intPow(base: Int, exponent: Int): Int {
+    var result = 1
+
+    for (i in 1..exponent) {
+        result *= base
+    }
+
+    return result
+}
diff --git a/advent-of-code/2023/day_04/src/main/kotlin/Main.kt b/advent-of-code/2023/day_04/src/main/kotlin/Main.kt
new file mode 100644
index 0000000..400f8c9
--- /dev/null
+++ b/advent-of-code/2023/day_04/src/main/kotlin/Main.kt
@@ -0,0 +1,12 @@
+package space.comfycamp
+
+fun main() {
+    val resource = object{}.javaClass.getResource("/input.txt")!!
+    val lines = resource.readText().trim().lines()
+
+    val res1 = countPoints(lines)
+    println("Part 1: $res1")
+
+    val res2 = countCards(lines)
+    println("Part 2: $res2")
+}
diff --git a/advent-of-code/2023/kotlin/src/main/resources/day-04/input.txt b/advent-of-code/2023/day_04/src/main/resources/input.txt
similarity index 100%
rename from advent-of-code/2023/kotlin/src/main/resources/day-04/input.txt
rename to advent-of-code/2023/day_04/src/main/resources/input.txt
diff --git a/advent-of-code/2023/day_04/src/test/kotlin/CardsTest.kt b/advent-of-code/2023/day_04/src/test/kotlin/CardsTest.kt
new file mode 100644
index 0000000..f567389
--- /dev/null
+++ b/advent-of-code/2023/day_04/src/test/kotlin/CardsTest.kt
@@ -0,0 +1,25 @@
+import space.comfycamp.countCards
+import space.comfycamp.countPoints
+import kotlin.test.assertEquals
+import kotlin.test.Test
+
+class CardsTest {
+    val lines = listOf(
+        "Card 1: 41 48 83 86 17 | 83 86  6 31 17  9 48 53",
+        "Card 2: 13 32 20 16 61 | 61 30 68 82 17 32 24 19",
+        "Card 3:  1 21 53 59 44 | 69 82 63 72 16 21 14  1",
+        "Card 4: 41 92 73 84 69 | 59 84 76 51 58  5 54 83",
+        "Card 5: 87 83 26 28 32 | 88 30 70 12 93 22 82 36",
+        "Card 6: 31 18 13 56 72 | 74 77 10 23 35 67 36 11",
+    )
+
+    @Test
+    fun testCountPoints() {
+        assertEquals(13, countPoints(lines))
+    }
+
+    @Test
+    fun testCountCards() {
+        assertEquals(30, countCards(lines))
+    }
+}
diff --git a/advent-of-code/2023/kotlin/src/main/kotlin/MyMath.kt b/advent-of-code/2023/kotlin/src/main/kotlin/MyMath.kt
deleted file mode 100644
index 787e840..0000000
--- a/advent-of-code/2023/kotlin/src/main/kotlin/MyMath.kt
+++ /dev/null
@@ -1,13 +0,0 @@
-/**
- * I haven't found a function to raise an integer to a power.
- * You should not use powers less than 0 :)
- */
-fun intPow(base: Int, exponent: Int): Int {
-    var result = 1
-
-    for (i in 1..exponent) {
-        result *= base
-    }
-
-    return result
-}
diff --git a/advent-of-code/2023/kotlin/src/main/kotlin/Solution04.kt b/advent-of-code/2023/kotlin/src/main/kotlin/Solution04.kt
deleted file mode 100644
index 608fc4b..0000000
--- a/advent-of-code/2023/kotlin/src/main/kotlin/Solution04.kt
+++ /dev/null
@@ -1,74 +0,0 @@
-class Solution04 {
-    /** Part 1. */
-    fun getPointCount(lines: List<String>): Int {
-        val cards = parseCards(lines)
-        return cards
-            .map{it.getPoints()}
-            .reduce{acc, points -> acc + points}
-    }
-
-    /** Part 2. */
-    fun getCardCount(lines: List<String>): Int {
-        val cards = parseCards(lines)
-
-        for ((idx, card) in cards.withIndex()) {
-            // This is the last card.
-            if (idx == cards.size - 1) break
-
-            val matchingNumberCount = card.getMatchingNumberCount()
-            if (matchingNumberCount == 0) continue
-
-            for (i in idx+1..idx+matchingNumberCount) {
-                if (i >= cards.size) break
-
-                cards[i].instances += card.instances
-            }
-        }
-
-        return cards
-            .map{it.instances}
-            .reduce{acc, instances -> acc + instances}
-    }
-
-    private fun parseCards(lines: List<String>): MutableList<Card> {
-        val cards = mutableListOf<Card>()
-
-        for (line in lines) {
-            val (_, numbers) = line.split(": ")
-            if (numbers.isEmpty()) continue
-
-            val (winningNumbers, selectedNumbers) = numbers.split(" | ")
-            cards.add(Card(
-                strToInts(winningNumbers),
-                strToInts(selectedNumbers),
-            ))
-        }
-
-        return cards
-    }
-
-    private fun strToInts(s: String): MutableList<Int> {
-        return s
-            .split(" ")
-            .filter{it.isNotEmpty()}
-            .map{it.toInt()}
-            .toMutableList()
-    }
-}
-
-data class Card(
-    val winningNumbers: MutableList<Int> = mutableListOf(),
-    val selectedNumbers: MutableList<Int> = mutableListOf(),
-    /** Used only in part 2. */
-    var instances: Int = 1,
-) {
-    fun getMatchingNumberCount(): Int {
-        return selectedNumbers
-            .filter{winningNumbers.contains(it)}
-            .size
-    }
-    fun getPoints(): Int {
-        val count = getMatchingNumberCount()
-        return if (count == 0) 0 else intPow(2, count-1)
-    }
-}
diff --git a/advent-of-code/2023/kotlin/src/main/resources/day-04/test.txt b/advent-of-code/2023/kotlin/src/main/resources/day-04/test.txt
deleted file mode 100644
index 9bdb874..0000000
--- a/advent-of-code/2023/kotlin/src/main/resources/day-04/test.txt
+++ /dev/null
@@ -1,6 +0,0 @@
-Card 1: 41 48 83 86 17 | 83 86  6 31 17  9 48 53
-Card 2: 13 32 20 16 61 | 61 30 68 82 17 32 24 19
-Card 3:  1 21 53 59 44 | 69 82 63 72 16 21 14  1
-Card 4: 41 92 73 84 69 | 59 84 76 51 58  5 54 83
-Card 5: 87 83 26 28 32 | 88 30 70 12 93 22 82 36
-Card 6: 31 18 13 56 72 | 74 77 10 23 35 67 36 11
diff --git a/advent-of-code/2023/kotlin/src/test/kotlin/Solution04Test.kt b/advent-of-code/2023/kotlin/src/test/kotlin/Solution04Test.kt
deleted file mode 100644
index 2e858e1..0000000
--- a/advent-of-code/2023/kotlin/src/test/kotlin/Solution04Test.kt
+++ /dev/null
@@ -1,32 +0,0 @@
-import kotlin.test.Test
-import kotlin.test.assertEquals
-
-class Solution04Test {
-    @Test
-    fun testGetPointCount() {
-        val text = ResourceReader().readFile("day-04/test.txt")
-        val res = Solution04().getPointCount(text)
-        assertEquals(13, res)
-    }
-
-    @Test
-    fun solvePart1() {
-        val text = ResourceReader().readFile("day-04/input.txt")
-        val res = Solution04().getPointCount(text)
-        println(res)
-    }
-
-    @Test
-    fun testGetCardCount() {
-        val text = ResourceReader().readFile("day-04/test.txt")
-        val res = Solution04().getCardCount(text)
-        assertEquals(30, res)
-    }
-
-    @Test
-    fun solvePart2() {
-        val text = ResourceReader().readFile("day-04/input.txt")
-        val res = Solution04().getCardCount(text)
-        println(res)
-    }
-}