Day 4 - Advent of Code 2015

Mix.install([:kino, :benchee])

:ok

Links

• Advent of Code (AoC)
• AoC Puzzle Input
• tylerj GitHub

Prompt

--- Day 4: The Ideal Stocking Stuffer ---

Santa needs help mining some AdventCoins (very similar to bitcoins) to use as gifts for all the economically forward-thinking little girls and boys.

To do this, he needs to find MD5 hashes which, in hexadecimal, start with at least five zeroes. The input to the MD5 hash is some secret key (your puzzle input, given below) followed by a number in decimal. To mine AdventCoins, you must find Santa the lowest positive number (no leading zeroes: 1, 2, 3, ...) that produces such a hash.

For example:

• If your secret key is abcdef, the answer is 609043, because the MD5 hash of abcdef609043 starts with five zeroes (000001dbbfa...), and it is the lowest such number to do so.
• If your secret key is pqrstuv, the lowest number it combines with to make an MD5 hash starting with five zeroes is 1048970; that is, the MD5 hash of pqrstuv1048970 looks like 000006136ef....

Your puzzle input is bgvyzdsv.

To mine AdventCoins, you must find Santa the lowest positive number that produces such a hash.

--- Part Two ---

Now find one that starts with six zeroes.

Although it hasn't changed, you can still get your puzzle input.

Input

input = Kino.Input.textarea("Please paste your input file:")

input = input |> Kino.Input.read()

"bgvyzdsv"

Solution

(input <> to_string(6))
|> :erlang.md5()
|> Base.encode16(case: :lower)

"b464a797636cb5862b776679f22b1223"

defmodule Day04 do
  def find_lowest_suffix(prefix, suffix \\ 1, desired_prefix) do
    hash =
      (prefix <> to_string(suffix))
      |> :erlang.md5()
      |> Base.encode16(case: :lower)

    if String.starts_with?(hash, desired_prefix),
      do: suffix,
      else: find_lowest_suffix(prefix, suffix + 1, desired_prefix)
  end
end

{:module, Day04, <<70, 79, 82, 49, 0, 0, 9, ...>>, {:find_lowest_suffix, 3}}

To do this, he needs to find MD5 hashes which, in hexadecimal, start with at least five zeroes. To mine AdventCoins, you must find Santa the lowest positive number that produces such a hash.

Your puzzle answer was 254575.

Day04.find_lowest_suffix(input, "00000")

254575

Now find one that starts with six zeroes.

Your puzzle answer was 1038736.

Day04.find_lowest_suffix(input, "000000")

1038736

Both parts of this puzzle are complete! They provide two gold stars: **

At this point, you should return to your Advent calendar and try another puzzle.

If you still want to see it, you can get your puzzle input.

Tests

ExUnit.start(auto_run: false)

defmodule Day04Test do
  use ExUnit.Case, async: false

  setup_all do
    [prefix: "00000"]
  end

  describe "find_lowest_suffix/1" do
    test "returns expected value", %{prefix: prefix} do
      assert Day04.find_lowest_suffix("abcdef", prefix) == 609_043
      assert Day04.find_lowest_suffix("pqrstuv", prefix) == 1_048_970
    end
  end
end

ExUnit.run()

.
Finished in 0.9 seconds (0.00s async, 0.9s sync)
1 test, 0 failures

Randomized with seed 367531

%{excluded: 0, failures: 0, skipped: 0, total: 1}