day1.ex

# --- Day 1: Chronal Calibration ---
# "We've detected some temporal anomalies," one of Santa's Elves at the Temporal
# Anomaly Research and Detection Instrument Station tells you. She sounded pretty
# worried when she called you down here. "At 500-year intervals into the past,
# someone has been changing Santa's history!"
#
# "The good news is that the changes won't propagate to our time stream for
# another 25 days, and we have a device" - she attaches something to your wrist -
# "that will let you fix the changes with no such propagation delay. It's
# configured to send you 500 years further into the past every few days; that was
# the best we could do on such short notice."
#
# "The bad news is that we are detecting roughly fifty anomalies throughout time;
# the device will indicate fixed anomalies with stars. The other bad news is
# that we only have one device and you're the best person for the job! Good lu--"
# She taps a button on the device and you suddenly feel like you're falling. To
# save Christmas, you need to get all fifty stars by December 25th.
#
# Collect stars by solving puzzles. Two puzzles will be made available on each
# day in the advent calendar; the second puzzle is unlocked when you complete
# the first. Each puzzle grants one star. Good luck!
#
# After feeling like you've been falling for a few minutes, you look at the
# device's tiny screen. "Error: Device must be calibrated before first use.
# Frequency drift detected. Cannot maintain destination lock." Below the
# message, the device shows a sequence of changes in frequency (your puzzle
# input). A value like +6 means the current frequency increases by 6; a value
# like -3 means the current frequency decreases by 3.
#
# For example, if the device displays frequency changes of +1, -2, +3, +1, then
# starting from a frequency of zero, the following changes would occur:
#
# Current frequency  0, change of +1; resulting frequency  1.
# Current frequency  1, change of -2; resulting frequency -1.
# Current frequency -1, change of +3; resulting frequency  2.
# Current frequency  2, change of +1; resulting frequency  3.
# In this example, the resulting frequency is 3.
#
# Here are other example situations:
#
# +1, +1, +1 results in  3
# +1, +1, -2 results in  0
# -1, -2, -3 results in -6
#
# Starting with a frequency of zero, what is the resulting frequency after all
# of the changes in frequency have been applied?


# --- Part Two ---
# You notice that the device repeats the same frequency change list over and
# over. To calibrate the device, you need to find the first frequency it
# reaches twice.
#
# For example, using the same list of changes above, the device would loop as follows:
#
# - Current frequency  0, change of +1; resulting frequency  1.
# - Current frequency  1, change of -2; resulting frequency -1.
# - Current frequency -1, change of +3; resulting frequency  2.
# - Current frequency  2, change of +1; resulting frequency  3.
# - (At this point, the device continues from the start of the list.)
# - Current frequency  3, change of +1; resulting frequency  4.
# - Current frequency  4, change of -2; resulting frequency  2, which has already been seen.
#
# In this example, the first frequency reached twice is 2. Note that your
# device might need to repeat its list of frequency changes many times before a
# duplicate frequency is found, and that duplicates might be found while in the
# middle of processing the list.
#
# Here are other examples:
#
# +1, -1 first reaches 0 twice.
# +3, +3, +4, -2, -4 first reaches 10 twice.
# -6, +3, +8, +5, -6 first reaches 5 twice.
# +7, +7, -2, -7, -4 first reaches 14 twice.
# What is the first frequency your device reaches twice?

defmodule Mix.Tasks.Day1 do
  use Mix.Task

  def run(_args) do
    { :ok, input } = File.read("../inputs/day1.txt")
    frequencies = input
                  |> String.split
                  |> Enum.map(&String.to_integer/1)

    final_frequency = frequencies |> Enum.sum
    IO.puts("Hey Commander! I gave my best and according to my calculatiosn the the final frequency is #{final_frequency}!")

    IO.puts("This will take a while..")
    repeated_frequency = detect_repeated_frequency(frequencies)
    IO.puts("Hey Chief. Try `#{repeated_frequency}` for the second part.")
  end


  @doc ~S"""
  Repetitively iterates over the given frequency list and returns the first
  repeated frequency in the process.

  ## Examples

      iex> Mix.Tasks.Day1.detect_repeated_frequency([+1 -1])
      0

      iex> Mix.Tasks.Day1.detect_repeated_frequency([+3, +3, +4, -2, -4])
      10

      iex> Mix.Tasks.Day1.detect_repeated_frequency([-6, +3, +8, +5, -6])
      5

      iex> Mix.Tasks.Day1.detect_repeated_frequency([+7, +7, -2, -7, -4])
      14
  """

  # TODO: Use MapSet instead of List
  def detect_repeated_frequency(list) do
    detect_repeated_frequency(list, 0, [])
  end

  def detect_repeated_frequency(list, index, frequencies) do
    nindex = if index >= length(list) do
      0
    else
      index
    end

    current_frequency = List.last(frequencies) || 0
    frequency_change = Enum.at(list, nindex)
    next_frequency = current_frequency + frequency_change

    if Enum.member?(frequencies, next_frequency) do
      next_frequency
    else
      detect_repeated_frequency(list, nindex+1, frequencies ++ [next_frequency])
    end
  end
end