diff --git a/2021/six.nim b/2021/six.nim
new file mode 100644
index 0000000..8134e77
--- /dev/null
+++ b/2021/six.nim
@@ -0,0 +1,30 @@
+import std/[strutils, sequtils, tables, strformat]
+import ./common
+
+const DEFAULT_DAYS = 80
+const CYCLE_DAYS = 7
+const MATURING_DAYS = 2
+
+proc lanternFish(inputLines: seq[string], days = DEFAULT_DAYS): int =
+  var maturingFish: seq[(int, int)]
+  var fish = newSeq[int](CYCLE_DAYS)
+  for i in inputLines[0].split(',').mapIt(it.parseInt()):
+    inc fish[i]
+  for day in 0..days+1:
+    let daySeq = day mod CYCLE_DAYS
+    maturingFish.add((MATURING_DAYS+1, fish[daySeq]))
+    for v in maturingFish.mitems(): dec v[0]
+    if maturingFish[0][0] == 0:
+      fish[daySeq] += maturingFish[0][1]
+      maturingFish.delete(0)
+  result = fish.foldl(a + b)
+
+let input = 6.loadInput()
+time("day 6 part 1"): echo input.lanternFish()
+time("day 6 part 2"): echo input.lanternFish(256)
+
+when not defined(release):
+  let testInput = @["3,4,3,1,2"]
+  doAssert testInput.lanternFish(18) == 26
+  doAssert testInput.lanternFish() == 5934
+  doAssert testInput.lanternFish(256) == 26984457539