weighted_sample.rb

require 'ostruct'

alternatives = [
  OpenStruct.new(name: "A1", weight: 1),
  OpenStruct.new(name: "A2", weight: 1),
  OpenStruct.new(name: "A3", weight: 1),
  OpenStruct.new(name: "A4", weight: 1),
  OpenStruct.new(name: "B1", weight: 1),
  OpenStruct.new(name: "B2", weight: 1),
  OpenStruct.new(name: "B3", weight: 1),
  OpenStruct.new(name: "B4", weight: 1),
  OpenStruct.new(name: "C1", weight: 1),
  OpenStruct.new(name: "C2", weight: 1),
  OpenStruct.new(name: "C3", weight: 1),
  OpenStruct.new(name: "C4", weight: 1)
]
experiment = OpenStruct.new(alternatives: alternatives)

groups = {
  "A1" => 0,
  "A2" => 0,
  "A3" => 0,
  "A4" => 0,
  "B1" => 0,
  "B2" => 0,
  "B3" => 0,
  "B4" => 0,
  "C1" => 0,
  "C2" => 0,
  "C3" => 0,
  "C4" => 0
}

def choose_alternative(experiment)
  weights = experiment.alternatives.map(&:weight)
  total = weights.inject(:+)
  point = rand * total

  experiment.alternatives.zip(weights).each do |n, w|
    return n if w >= point
    point -= w
  end
end

1_000_000.times do
  chosen = choose_alternative(experiment)
  groups[chosen.name] += 1
end

puts "*" * 80
%w(A B C).each do |group|
  puts "#{group}: #{groups.select { |g, count| g[0] == group }.values.inject(:+)}"
end

# ------------------------------
# 1_000 times
#
# A: 336
# B: 364
# C: 300

# 10_000 times
# A: 3332
# B: 3371
# C: 3297

# 100_000 times
#
# A: 33486
# B: 33382
# C: 33132

# 1_000_000 times
#
# A: 333213
# B: 333712
# C: 333075