An incomplete probability function, with a manual example of some prospective code to work on it.

code

class ProbabilityDensity < Array

  def probability
    Rational(1,count)
  end

  def to_density
    if block_given?
      each do |v|
        if v.respond_to? :probability
          v.to_density do |(subv,subp)|
            yield [ subv, probability * subp ]
          end
        else
          yield [ v, probability ]
        end
      end
    else
      Enumerator.new(self, :to_density)
    end
  end

  def []=(index, value)
    super(index, ProbabilityDensity.new([value]))
  end

  def <<(value)
    super(ProbabilityDensity.new([value]))
  end

  def to_s
    to_density.to_a.to_s
  end
  alias_method :inspect, :to_s
end


# Imagine a dice expression: 1d4!p r4

# first we set up the basic die roll:
v = ProbabilityDensity.new
v << 1
v << 2
v << 3
v << 4

# Now we handle the !p. Code will replace the 4, because it would be rolled
# (the example below is a simulation of what the code will do)
v[3] << 5
v[3] << 6
v[3] << 7
v
# This will iterate up to the cap for !p. Let's assume the cap is 2
v[3][3] << 8
v[3][3] << 9
v[3][3] << 10
v

# Now we pass it to the reroll function, which iterates over each value.
# When it finds the 4, it deletes it
v[3].delete(4)

# And now we have:
v

output

probability.rb(main):037:0* # Imagine a dice expression: 1d4!p r4
probability.rb(main):038:0* 
probability.rb(main):039:0* # first we set up the basic die roll:
probability.rb(main):040:0* v = ProbabilityDensity.new
=> []
probability.rb(main):041:0> v << 1
=> [[1, (1/1)]]
probability.rb(main):042:0> v << 2
=> [[1, (1/2)], [2, (1/2)]]
probability.rb(main):043:0> v << 3
=> [[1, (1/3)], [2, (1/3)], [3, (1/3)]]
probability.rb(main):044:0> v << 4
=> [[1, (1/4)], [2, (1/4)], [3, (1/4)], [4, (1/4)]]
probability.rb(main):045:0> 
probability.rb(main):046:0* # Now we handle the !p. Code will replace the 4, because it would be rolled
probability.rb(main):047:0* # (the example below is a simulation of what the code will do)
probability.rb(main):048:0* v[3] << 5
=> [[4, (1/2)], [5, (1/2)]]
probability.rb(main):049:0> v[3] << 6
=> [[4, (1/3)], [5, (1/3)], [6, (1/3)]]
probability.rb(main):050:0> v[3] << 7
=> [[4, (1/4)], [5, (1/4)], [6, (1/4)], [7, (1/4)]]
probability.rb(main):051:0> v
=> [[1, (1/4)], [2, (1/4)], [3, (1/4)], [4, (1/16)], [5, (1/16)], [6, (1/16)], [7, (1/16)]]
probability.rb(main):052:0> # This will iterate up to the cap for !p. Let's assume the cap is 2
probability.rb(main):053:0* v[3][3] << 8
=> [[7, (1/2)], [8, (1/2)]]
probability.rb(main):054:0> v[3][3] << 9
=> [[7, (1/3)], [8, (1/3)], [9, (1/3)]]
probability.rb(main):055:0> v[3][3] << 10
=> [[7, (1/4)], [8, (1/4)], [9, (1/4)], [10, (1/4)]]
probability.rb(main):056:0> v
=> [[1, (1/4)], [2, (1/4)], [3, (1/4)], [4, (1/16)], [5, (1/16)], [6, (1/16)], [7, (1/64)], [8, (1/64)], [9, (1/64)], [10, (1/64)]]
probability.rb(main):057:0> 
probability.rb(main):058:0* # Now we pass it to the reroll function, which iterates over each value.
probability.rb(main):059:0* # When it finds the 4, it deletes it
probability.rb(main):060:0* v[3].delete(4)
=> 4
probability.rb(main):061:0> 
probability.rb(main):062:0* # And now we have:
probability.rb(main):063:0* v
=> [[1, (1/4)], [2, (1/4)], [3, (1/4)], [5, (1/12)], [6, (1/12)], [7, (1/48)], [8, (1/48)], [9, (1/48)], [10, (1/48)]]
probability.rb(main):064:0> 
probability.rb(main):065:0* 
probability.rb(main):065:0>