Class: LoadedDie::Sampler

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

Overview

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

Constant Summary collapse

DEFAULT_RNG =

The default random number generator. It samples from a uniform distribution.

::Object.new

Instance Method Summary collapse

Constructor Details

#initialize(population) ⇒ Sampler

Creates a sampler from a population. The argument should return an enumerable (conventionally an instance of Enumerator) of arrays when sent a to_enum message with no arguments. (A normal instance of Hash qualifies.) Each array must have at least two elements: the first element is an individual that can be chosen and the second element is its weight – that is, the likelihood relative to the other weights that the individual will be chosen. Weights must convert to finite floats that are zero or positive.



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# File 'lib/loaded_die.rb', line 33

def initialize population
  @compiled = population.to_enum.inject [] do |accum, elem|
    individual = elem.fetch 0
    weight = elem.fetch 1
    weight_f = ::Kernel.Float weight
    unless 0.0 <= weight_f && weight_f.finite?
      ::Kernel.raise ::ArgumentError, "invalid weight #{weight_f}"
      break
    end
    prev_max =
      if last = accum.last
        last.fetch :maximum
      else
        0.0
      end
    accum << { :maximum => prev_max + weight_f, :individual => individual }
  end
  nil
end

Instance Method Details

#[](point) ⇒ Object

Returns the individual associated with the given point on a line. The point should convert to a float. 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 70

def [] point
  point_f = ::Kernel.Float point
  if 0.0 > point_f
    nil
  elsif choice = @compiled.detect { |segment|
    segment.fetch(:maximum) > point_f }
    choice.fetch :individual
  else
    nil
  end
end

#length ⇒ Object

Returns the sum of weights.



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# File 'lib/loaded_die.rb', line 56

def length
  if last = @compiled.last
    last.fetch :maximum
  else
    0.0
  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 (convertible to a float) greater than or equal to zero and less than the sum of weights.



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# File 'lib/loaded_die.rb', line 90

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