Add improved day 6 code with comments

day6/p2improved.nim

@@ -0,0 +1,39 @@
+import os, sequtils
+import std/strutils
+
+const SIM_DAYS = 256
+const FISH_DAYS = 8
+
+proc main() =
+  let fileName = paramStr(1)
+
+  let f = fileName.open(fmRead)
+
+  var lanternFishSim: array[0..FISH_DAYS, int]
+
+  # get all of the initial counts of fish
+  for fishDay in f.readLine().split(",").mapIt(it.parseInt):
+    lanternFishSim[fishDay] += 1
+
+  f.close()
+
+
+  for i in 0 ..< SIM_DAYS:
+
+    let spawningFish = lanternFishSim[0]
+
+    # decrease all fish by one
+    for day in 0 .. FISH_DAYS-1:
+      lanternFishSim[day] = lanternFishSim[day+1]
+
+    # set the newly spawned fish
+    lanternFishSim[FISH_DAYS] = spawningFish
+
+    # 6 day left fish now only have the 7 day fish from
+    # the last round. We need to add the spawning fish
+    # to that count because they are refreshed at 6 days
+    lanternFishSim[6] += spawningFish
+
+  echo "NumFish: ", lanternFishSim.foldl(a + b)
+
+main()