weighted random choice

weighted_random_choice.coffee

module.exports = class WeightedRandomChoice
  constructor: (args) ->
    @_weightPairs = _.sortBy _.pairs(args), (pair) -> pair[1]
    @sum = _.reduce @_weightPairs, ((memo, pair) -> memo + pair[1]), 0

  check: (num, weight) -> num - weight <= 0
  random: -> _.random 1, @sum

  select: ->
    num = @random()
    for [key, weight] in @_weightPairs
      return key if @check num, weight
      num -= weight
    throw new Error('Invalid choice!')