AdventOfCode/2024/day9.exs

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Elixir
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2024-12-09 15:20:31 +00:00
#!/usr/bin/env elixir
defmodule Day9 do
def part1(input) do
{size, _, spaces, _, disk, files} =
Enum.reduce(input, {0, 0, 0, true, %{}, []}, fn len,
{idx, id, spaces, file?, disk, files} ->
if file? do
disk = Enum.into(idx..(idx + len - 1), disk, fn i -> {i, id} end)
files = if file?, do: for(_ <- 1..len, do: id) ++ files, else: files
{idx + len, id + 1, spaces, not file?, disk, files}
else
{idx + len, id, spaces + len, not file?, disk, files}
end
end)
for i <- 0..(size - spaces - 1), reduce: {0, files} do
{checksum, files} ->
case Map.fetch(disk, i) do
{:ok, block} ->
{checksum + block * i, files}
:error ->
[block | files] = files
{checksum + block * i, files}
end
end
|> elem(0)
end
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# Super inefficient, but this is the 2nd attempt after trying to be more efficient
# worked on the sample but not the actual input
def part2(input) do
{_, _, _, disk, files} =
Enum.reduce(input, {0, 0, true, %{}, []}, fn len, {idx, id, file?, disk, files} ->
if file? do
disk = Map.put(disk, idx, {:file, id, len})
files = [{idx, id, len} | files]
{idx + len, id + 1, not file?, disk, files}
else
disk = Map.put(disk, idx, {:space, len})
{idx + len, id, not file?, disk, files}
end
end)
Enum.reduce(files, disk, fn {idx, id, len}, disk ->
# This scans the disk every time. Could possibly optimise by storing the known
# eligible spaces separately
case Enum.find(0..(idx - 1), &match?({:space, gap} when gap >= len, disk[&1])) do
nil ->
disk
space_idx ->
{:space, spaces} = Map.fetch!(disk, space_idx)
disk = disk |> Map.delete(idx) |> Map.put(space_idx, {:file, id, len})
remainder_spaces = spaces - len
if remainder_spaces > 0 do
Map.put(disk, space_idx + len, {:space, remainder_spaces})
else
disk
end
end
end)
|> Enum.reduce(0, fn
{_idx, {:space, _}}, checksum ->
checksum
{idx, {:file, id, len}}, checksum ->
for idx <- idx..(idx + len - 1), reduce: checksum do
checksum -> checksum + idx * id
end
end)
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end
def input do
with [input_filename] <- System.argv(),
{:ok, input} <- File.read(input_filename) do
for <<c::binary-1 <- String.trim_trailing(input, "\n")>>, do: String.to_integer(c)
else
_ -> :error
end
end
#######################
# HERE BE BOILERPLATE #
#######################
def run do
case input() do
:error -> print_usage()
input -> run_parts_with_timer(input)
end
end
defp run_parts_with_timer(input) do
run_with_timer(1, fn -> part1(input) end)
run_with_timer(2, fn -> part2(input) end)
end
defp run_with_timer(part, fun) do
{time, result} = :timer.tc(fun)
IO.puts("Part #{part} (completed in #{format_time(time)}):\n")
IO.puts("#{inspect(result)}\n")
end
defp format_time(μsec) when μsec < 1_000, do: "#{μsec}μs"
defp format_time(μsec) when μsec < 1_000_000, do: "#{μsec / 1000}ms"
defp format_time(μsec), do: "#{μsec / 1_000_000}s"
defp print_usage do
IO.puts("Usage: elixir day9.exs input_filename")
end
end
Day9.run()