Reverse Factorial Recursive
R
def reverse_factorial_recursive(value: int, current_divisor: int = 1) -> int:
"""
Return x such that x! == value, otherwise return -1.
The function divides `value` by 1, 2, 3, ... recursively. If the repeated
division reduces `value` exactly to 1, the factorial root x is
(current_divisor - 1). If the division ever has a remainder, no integer x
exists and the function returns -1.
Parameters
----------
value: The positive integer to test (candidate factorial value).
current_divisor: The current divisor used while reducing `value` (default is 1).
Returns
-------
The factorial root (x) if x! == value, otherwise -1.
Examples
--------
>>> reverse_factorial_recursive(120)
5
>>> reverse_factorial_recursive(24)
4
>>> reverse_factorial_recursive(150)
-1
>>> reverse_factorial_recursive(1)
1
>>> reverse_factorial_recursive(2)
2
"""
if not isinstance(value, int):
raise TypeError("value must be an integer")
if not isinstance(current_divisor, int):
raise TypeError("current_divisor must be an integer")
if value < 1:
raise ValueError("value must be a positive integer")
# Special-case: initial call with value == 1 should return 1 (since 1! = 1).
if value == 1 and current_divisor == 1:
return 1
# If value reduced to 1 during recursion, the factorial root is divisor - 1.
if value == 1:
return current_divisor - 1
# If not divisible by the current divisor, it's not a factorial number.
if value % current_divisor != 0:
return -1
return reverse_factorial_recursive(value // current_divisor, current_divisor + 1)
if __name__ == "__main__":
from doctest import testmod
testmod()