Day 24

1 files changed, 96 insertions, 0 deletions

diff --git a/2015/day24.exs b/2015/day24.exs
new file mode 100644
index 0000000..576b8e8
--- /dev/null
+++ b/2015/day24.exs
@@ -0,0 +1,96 @@
+packages =
+  File.stream!("inputs/day24.txt")
+  |> Enum.map(fn line ->
+    {num, ""} = String.trim(line) |> Integer.parse()
+    num
+  end)
+
+defmodule SleighPacker do
+  def canMakeSecondPile(_, 0) do
+    true
+  end
+
+  def canMakeSecondPile([], _) do
+    false
+  end
+
+  def canMakeSecondPile([next | remainingPackages], remainingWeight) do
+    canMakeSecondPile(remainingPackages, remainingWeight - next) ||
+      canMakeSecondPile(remainingPackages, remainingWeight)
+  end
+
+  def packagesCanBeSplitEvenly(rejectedPackages, piles) do
+    weightPerPile = div(Enum.sum(rejectedPackages), piles)
+    # This gave me the right answer for part 2, but that seems to just be a
+    # lucky detail. You probably need to make sure that you can make all of the
+    # remaining piles until it's the last two being split.
+    canMakeSecondPile(rejectedPackages, weightPerPile)
+  end
+
+  def findBestSmallestPile(remainingPackages, 0, rejectedPackages, piles) do
+    if packagesCanBeSplitEvenly(remainingPackages ++ rejectedPackages, piles - 1) do
+      []
+    else
+      nil
+    end
+  end
+
+  def findBestSmallestPile([], _, _, _) do
+    nil
+  end
+
+  def findBestSmallestPile([next | remainingPackages], remainingWeight, rejectedPackages, piles)
+      when next > remainingWeight do
+    findBestSmallestPile(remainingPackages, remainingWeight, [next | rejectedPackages], piles)
+  end
+
+  def findBestSmallestPile([next | remainingPackages], remainingWeight, rejectedPackages, piles) do
+    withNextTail =
+      findBestSmallestPile(remainingPackages, remainingWeight - next, rejectedPackages, piles)
+
+    withNext =
+      if withNextTail == nil do
+        nil
+      else
+        [next | withNextTail]
+      end
+
+    withoutNext =
+      findBestSmallestPile(remainingPackages, remainingWeight, [next | rejectedPackages], piles)
+
+    cond do
+      withNext == nil && withoutNext == nil ->
+        nil
+
+      withNext == nil ->
+        withoutNext
+
+      withoutNext == nil ->
+        withNext
+
+      length(withNext) == length(withoutNext) ->
+        if Enum.product(withNext) < Enum.product(withoutNext) do
+          withNext
+        else
+          withoutNext
+        end
+
+      length(withNext) < length(withoutNext) ->
+        withNext
+
+      true ->
+        withoutNext
+    end
+  end
+
+  def packTheSleigh(packages, piles) do
+    weightPerPile = div(Enum.sum(packages), piles)
+    findBestSmallestPile(packages, weightPerPile, [], piles)
+  end
+end
+
+threePack = SleighPacker.packTheSleigh(packages, 3)
+IO.puts("Product with three piles: #{Enum.product(threePack)}")
+
+fourPack = SleighPacker.packTheSleigh(packages, 4)
+IO.puts("Product with four piles: #{Enum.product(fourPack)}")