choice_randomly_by_ratio.coffee

# Choice randomly according to ratios
#
# e.g.
#
#   ratioMap=[[1.0, 'x'], [2.0, 'y'], [3.0, 'z']]
#   -> x=1/6, y=2/6, z=3/6
#
#   Can also write like this:
#   ratioMap={ x:1.0, y:2.0, z:3.0 }
#
@choiceRandomlyByRatio = (ratioMap) ->
  if not _.isArray(ratioMap) and _.isObject(ratioMap)
    ratioMap = _(ratioMap).map (v, k) -> [v, k]
  total = _.reduce(ratioMap, ((memo, v) -> memo + v[0]), 0)
  roll = Math.random() * total
  summing = 0
  for v in ratioMap
    summing += v[0]
    return v[1] if roll < summing
  throw new Error "Impossible case"