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()