class MoreMath::ChiSquareDistribution

This class is used to compute the Chi-Square Distribution.

def initialize(df)

Creates a ChiSquareDistribution for +df+ degrees of freedom. 
def initialize(df)
  @df = df
  @df_half = @df / 2.0
end

def inverse_probability(p)

NormalDistribution for the probability +p+.
Returns the inverse cumulative probability value of the 
def inverse_probability(p)
  case
  when p <= 0, p >= 1
    0.0
  else
    begin
      bisect = NewtonBisection.new { |x| probability(x) - p }
      range = bisect.bracket 0.5..10
      bisect.solve(range, 1_000_000)
    rescue
      0 / 0.0
    end
  end
end

def probability(x)

for the value +x+.
Returns the cumulative probability (p-value) of the ChiSquareDistribution 
def probability(x)
  if x < 0
    0.0
  else
    gammaP_regularized(x / 2, @df_half)
  end
end