day15.ex

defmodule day15 do
  use bitwise
  use agent
  
  def start_agents(init_a, init_b) do
    {:ok, a} = agent.start_link fn -> %{cur: init_a, factor: 16807, cond: &(rem(&1, 4) == 0)} end
    {:ok, b} = agent.start_link fn -> %{cur: init_b, factor: 48271, cond: &(rem(&1, 8) == 0)} end
    %{a: a, b: b}
  end

  def next(state) do
    newstate = rem(state.cur * state.factor, 2147483647)
    {newstate, %{cur: newstate, factor: state.factor, cond: state.cond}}
  end

  def get_next_pair(agent) do
    {agent.get_and_update(agent.a, &next/1),
     agent.get_and_update(agent.b, &next/1)}
  end

  def get_next_16(agent) do
    a = agent.get_and_update(agent.a, &next/1)
    b = agent.get_and_update(agent.b, &next/1)
    {a &&& 0xffff, b &&& 0xffff}
  end

  def lastbyte(num) do
    integer.to_string(num, 2)
    |> string.slice(-16..-1)
  end

  def judge(agent, iterations, hits \\ 0)
  def judge(_agent, iterations, hits) when iterations == 0, do: hits 
  def judge(agent, iterations, hits) do
    {a,b} = get_next_16(agent)
    if a == b do
      judge(agent, iterations - 1, hits + 1)
    else
      judge(agent, iterations - 1, hits)
    end
  end
    
  def p2next(state) do
    newstate = rem(state.cur * state.factor, 2147483647)
    if state.cond.(newstate) do
      {newstate, %{cur: newstate, factor: state.factor, cond: state.cond}}
    else
      p2next(%{cur: newstate, factor: state.factor, cond: state.cond})
    end
  end

  def p2_get_next_pair(agent) do
    {agent.get_and_update(agent.a, &p2next/1),
     agent.get_and_update(agent.b, &p2next/1)}
  end

  def p2_get_next_16(agent) do
    a = agent.get_and_update(agent.a, &p2next/1)
    b = agent.get_and_update(agent.b, &p2next/1)
    {a &&& 0xffff, b &&& 0xffff}
  end

  def p2judge(agent, iterations, hits \\ 0)
  def p2judge(_agent, iterations, hits) when iterations == 0, do: hits 
  def p2judge(agent, iterations, hits) do
    {a,b} = p2_get_next_pair(agent)
    if (a &&& 0xffff) == (b &&& 0xffff) do
      p2judge(agent, iterations - 1, hits + 1)
    else
      p2judge(agent, iterations - 1, hits)
    end
  end
end

agents = day15.start_agents(516,190)
#day15.judge(agents, 40_000_000)
#|> io.inspect

day15.p2judge(agents, 5_000_000)
|> io.inspect