Create random arithmetic operations using numbers between 0 and 20

gistfile1.rb

# returns a random operation that
# result in n. eg:
# randomOperation(20) => [4, '*', 5]
# randomOperation(10) => [20, '/', 2]
class RandomOperationGenerator
  def initialize(min = 0, max = 20)
    raise "min must be less than max" unless min < max
    @min = min
    @max = max
  end

  def random_operation(n)
    raise "n must be in range [#{min}, #{max}]" unless n >= min && n <= max
    
    r = rand(0)
   
    # with large numbers, favour multiplication and addition
    if n >= min + (max - min) / 2 
      distribution = [[4, :addition], [2, :multiplication], [1, :subtraction], [1, :division]]
    else
      distribution = [[1, :addition], [1, :multiplication], [4, :subtraction], [4, :division]]
    end
  
    operation = choose_from_distribution(distribution)
    operations.fetch(operation).call(n)
  end
   
  private
  attr_reader :min, :max

  def operations
    symbols = [:addition, :multiplication, :subtraction, :division]
    
    Hash[symbols.map { |func| [func, proc { |n| self.send(func, n) } ] } ]
  end

  def addition(n)
    a = rand(n - min).floor + min
    [a, '+', n - a]
  end
   
  def subtraction(n)
    b = rand(max + 1 - n)
    [n + b, '-', b]
  end
   
  def division(n)
    return [0, '/', rand(max)] if n.zero?
    b = 1 + rand((max/n).ceil - min) + min
    [n * b, '/', b]
  end
   
  def multiplication(n)
    if n == 0 
      a, b = [rand(max), 0].shuffle
    else
      a = ((min > 0 ? min : 1)..n).select { |f| (n % f).zero? }.shuffle.first
      b = n / a
    end
    [a, '*', b]
  end

  def choose_from_distribution(distribution)
    width = distribution.map(&:first).inject(&:+)
    r = rand(0) * width
 
    distribution.each do |d|
      r -= d.first
      return d.last if r <= 0
    end
  end
end

if __FILE__ == $0
  100.times do 
    min = 0
    max = 99
    n = rand(max - min + 1) + min
    op = RandomOperationGenerator.new(min, max).random_operation(n)
    puts "#{op[0].to_s.rjust(2, ' ')} #{op[1]} #{op[2].to_s.rjust(2, ' ')} = #{n.to_s.rjust(2, ' ')}"
  end
end