1 files changed, 38 insertions, 0 deletions
diff --git a/aoc-2021-kotlin/src/Day06.kt b/aoc-2021-kotlin/src/Day06.kt
new file mode 100644
index 0000000..4627cca
--- /dev/null
+++ b/aoc-2021-kotlin/src/Day06.kt
@@ -0,0 +1,38 @@
+object Day06 {
+    private fun calculateFishPopulation(input: String, days: Int): Long {
+        val fishCounts =
+            input
+                .trim()
+                .split(",")
+                .map(String::toInt)
+                .groupingBy { it }
+                .eachCount()
+                .mapValues { (_, v) -> v.toLong() }
+                .toMutableMap()
+
+        repeat(days) {
+            val readyToBirth = fishCounts.getOrDefault(0, 0)
+            repeat(8) {
+                fishCounts[it] = fishCounts.getOrDefault(it + 1, 0)
+            }
+            fishCounts.merge(6, readyToBirth, Long::plus)
+            fishCounts[8] = readyToBirth
+        }
+
+        return fishCounts.values.sum()
+    }
+
+    fun part1(input: String): Int = calculateFishPopulation(input, 80).toInt()
+
+    fun part2(input: String): Long = calculateFishPopulation(input, 256)
+}
+
+fun main() {
+    val testInput = readInputAsString("Day06_test")
+    check(Day06.part1(testInput) == 5934)
+    check(Day06.part2(testInput) == 26984457539)
+
+    val input = readInputAsString("Day06")
+    println(Day06.part1(input))
+    println(Day06.part2(input))
+}