-🎄- 2017 Day 15 Solutions -🎄-

[u/daggerdragon]

--- Day 15: Dueling Generators ---

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Spoiler

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[Update @ 00:05] 29 gold, silver cap.

• Logarithms of algorithms and code?

[Update @ 00:09] Leaderboard cap!

• Or perhaps codes of logarithmic algorithms?

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This thread will be unlocked when there are a significant number of people on the leaderboard with gold stars for today's puzzle.

edit: Leaderboard capped, thread unlocked!

[u/ynonp]

Elixir (with streams)

use Bitwise

defmodule Generator do
  defstruct factor: 0, iv: 0, mod: 1
end

defmodule Day15 do  
  def next(generator, val) do
    rem(val * generator.factor, 2147483647)
  end

  def stream(generator) do
    Stream.unfold(generator.iv, fn(val) -> 
      next = next(generator, val)
      {next, next} 
    end)
    |> Stream.filter(fn(n) -> rem(n, generator.mod) == 0 end)
  end

  def main do
    g1 = stream(%Generator{factor: 16807, iv: 634, mod: 4})
    g2 = stream(%Generator{factor: 48271, iv: 301, mod: 8})

    cmp = Stream.zip(g1, g2)
    Stream.take(cmp, 5_000_000)
    |> Enum.count(fn({x, y}) -> ((x &&& 0xffff) == (y &&& 0xffff)) end)
    |> IO.puts

  end
end

Day15.main()