weighted_choice.lua

function weighted_choice(choices)
  local total_weight = 0
  local thresholds = {}
  for choice, weight in pairs(choices) do
    add(thresholds, {total_weight, choice})
    total_weight += weight
  end

  local selection = rnd(total_weight)
  for i=1,#thresholds-1 do
    local tuple, next_tuple = thresholds[i], thresholds[i+1]
    local lower_bound, choice, upper_bound = tuple[1], tuple[2], next_tuple[1]
    if lower_bound <= selection and selection < upper_bound then
      return choice
    end
  end

  local choice = thresholds[#thresholds][2]
  return choice
end