reservoir.exs

defmodule Enum2 do
  import Enum

  def sample(collection),
    do: hd(sample(collection, 1))

  # http://en.wikipedia.org/wiki/Reservoir_sampling
  def sample(collection, count) do
    results = take(collection, count)

    collection
      |> Stream.drop(count)
      |> Stream.with_index 
      |> reduce(results, fn {value, index}, acc ->
        case :random.uniform(index+1)-1 do
          random when random < count -> List.replace_at(acc, random, value)
          _ -> acc
        end
      end)
  end
end

arr = 0..1000000 |> Enum.to_list

IO.puts "slow"
IO.inspect(arr |> Enum.shuffle |> Enum.take(100))
IO.puts "fast"
IO.inspect Enum2.sample(arr, 100)
IO.puts "fast"
IO.inspect Enum2.sample(arr)