Class: LoadedDie::Sampler

Inherits:
Object
  • Object
show all
Defined in:
lib/loaded_die.rb

Overview

Objects of this class can choose randomly from a set of options (called individuals here). The options can have different probabilities of being chosen.

Defined Under Namespace

Classes: Segment

Constant Summary collapse

DEFAULT_RNG =

The default random number generator.

::Object.new

Instance Method Summary collapse

Constructor Details

#initialize(population) ⇒ Sampler

Creates a sampler from a population. The argument should be an enumerable of two-element arrays (a hash qualifies). The first element of each array is an individual that can be chosen; the second element is its weight – that is, the likelihood (relative to the other weights) that the individual will be chosen. Weights must be positive numbers.



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# File 'lib/loaded_die.rb', line 40

def initialize(population)
  @compiled = population.inject [] do |accum, (individual, weight)|
    if weight <= 0
      raise ::ArgumentError, "non-positive weight #{weight}"
    end
    prev_max =
      if last = accum.last
        last.maximum
      else
        0
      end
    accum << Segment.new(prev_max + weight, individual)
  end
  nil
end

Instance Method Details

#[](point) ⇒ Object

Returns the individual for the given point. The point should be a number. If it is greater than or equal to zero and less than the sum of weights, this returns the corresponding individual. If it is outside those bounds, this returns nil.



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# File 'lib/loaded_die.rb', line 73

def [](point)
  if point < 0
    nil
  elsif choice = @compiled.detect { |segment| point < segment.maximum }
    choice.individual
  else
    nil
  end
end

#length ⇒ Object

Returns the sum of weights.



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# File 'lib/loaded_die.rb', line 59

def length
  if last = @compiled.last
    last.maximum
  else
    0
  end
end

#sample(options = {}) ⇒ Object

Returns a randomly-chosen individual. The argument is a hash, empty by default. If it contains a value for the :random key, that value will be used as the random number generator instead of the default. This RNG will be sent a “rand” message with one argument, the sum of weights, and should return a number greater than or equal to zero and less than the sum of weights.



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# File 'lib/loaded_die.rb', line 91

def sample(options = {})
  rng = options.fetch(:random) { DEFAULT_RNG }
  point = rng.rand(length)
  self[point]
end