fill_up_to.rb

MIN_COUNT = 2

# Naïve implementation, just return whatever number I need
def sum_up_a(existing)
  return 2 if existing == 0
  return 1 if existing == 1
  return 0
end

sum_up_a(0)   # => 2
sum_up_a(1)   # => 1
sum_up_a(2)   # => 0
sum_up_a(3)   # => 0
sum_up_a(10)  # => 0


# slightly improved with less conditionals
def sum_up_b(existing)
  return 0 if existing >= MIN_COUNT
  MIN_COUNT - existing
end

sum_up_b(0)   # => 2
sum_up_b(1)   # => 1
sum_up_b(2)   # => 0
sum_up_b(3)   # => 0
sum_up_b(10)  # => 0


# my favorite: return the biggest number either 0 or the difference
# Works well because difference is only positive for numbers where I 
# need some extra rows
def sum_up_c(existing)
  [MIN_COUNT - existing, 0].max 
end

sum_up_c(0)   # => 2
sum_up_c(1)   # => 1
sum_up_c(2)   # => 0
sum_up_c(3)   # => 0
sum_up_c(10)  # => 0