There are two provided implementations for the number_of_paths function. The ‘hardcoded’ version is correct, but the simplified version is not.

pg. 176

def number_of_paths(n):
  return 0 if n <= 0
  return 1 if n == 1
  return 2 if n == 2
  return 4 if n == 3
  return number_of_paths(n - 1) + number_of_paths(n - 2) + number_of_paths(n - 3)
end

pg. 177

def number_of_paths(n):
  return 0 if n < 0
  return 1 if n == 1 || n == 0
  return number_of_paths(n - 1) + number_of_paths(n - 2) + number_of_paths(n - 3)
end

Page 177’s implementation returns 1 instead of 0 for number_of_paths(0).