reservoir.exs

defmodule R do
  def sample_file_inefficient(path, count) do
    s = File.stream!(path, [], :line)
    sample = Enum.take(s, count) |> List.to_tuple

    Stream.drop(s, count)
    |> Stream.with_index
    |> Enum.reduce(sample, fn {line, index}, sample ->
      case random(index) do
        r when r < count -> put_elem(sample, r, line)
        _ -> sample
      end
    end)
    |> Tuple.to_list
  end

  defp random(n) do
    :random.uniform(n+1)-1
  end
end

R.sample_file_inefficient("enwiki/enwiki-latest-all-titles-in-ns0", 10)
|> IO.inspect

reservoir.py

import random
SAMPLE_COUNT = 10

# Force the value of the seed so the results are repeatable
random.seed(12345)

sample_titles = []
for index, line in enumerate(open("enwiki/enwiki-latest-all-titles-in-ns0")):
        # Generate the reservoir
        if index < SAMPLE_COUNT:
                sample_titles.append(line)
        else:
                # Randomly replace elements in the reservoir
                # with a decreasing probability.
                # Choose an integer between 0 and index (inclusive)
                r = random.randint(0, index)
                if r < SAMPLE_COUNT:
                        sample_titles[r] = line
print sample_titles

results

$ du -h enwiki/enwiki-latest-all-titles-in-ns0
218M    enwiki/enwiki-latest-all-titles-in-ns0

$ time python reservoir.py
['UFMA\n', 'Ark-La-Tex\n', 'Carolyn_George\n', 'Libby_Potter\n', 'Housepainters\n', 'Fayette_County_Courthouse_(Iowa)\n', 'Hussein_Darbouk\n', 'Ribeira_Palace\n', 'National_Road_574\n', 'Aphalangia\n']
python reservoir.py  23.71s user 0.08s system 99% cpu 23.789 total

$ time elixir reservoir.exs
["Congratulations_(album)\n", "2001_in_King_of_the_Cage\n", "Genero_chico\n",
 "GSAT-5P\n", "Limavady_United_F_C\n", "Jeroboam_Rothschild\n",
 "Rose_Hill,_Fairfax_County,_VA\n", "Agent_Helix_(G.I._Joe)\n",
 "Domain_Athletic_Centre\n", "Pedro_de_cordoba\n"]
elixir reservoir.exs  42.44s user 0.43s system 100% cpu 42.490 total