merge double_factorial (TheAlgorithms#9431)

* merge double_factorial

* fix ruff error

* fix merge issues

* change test case

* fix import error

Author: AVAniketh0905

Diff for: maths/double_factorial.py

@@ -0,0 +1,60 @@
+def double_factorial_recursive(n: int) -> int:
+    """
+    Compute double factorial using recursive method.
+    Recursion can be costly for large numbers.
+
+    To learn about the theory behind this algorithm:
+    https://en.wikipedia.org/wiki/Double_factorial
+
+    >>> from math import prod
+    >>> all(double_factorial_recursive(i) == prod(range(i, 0, -2)) for i in range(20))
+    True
+    >>> double_factorial_recursive(0.1)
+    Traceback (most recent call last):
+        ...
+    ValueError: double_factorial_recursive() only accepts integral values
+    >>> double_factorial_recursive(-1)
+    Traceback (most recent call last):
+        ...
+    ValueError: double_factorial_recursive() not defined for negative values
+    """
+    if not isinstance(n, int):
+        raise ValueError("double_factorial_recursive() only accepts integral values")
+    if n < 0:
+        raise ValueError("double_factorial_recursive() not defined for negative values")
+    return 1 if n <= 1 else n * double_factorial_recursive(n - 2)
+
+
+def double_factorial_iterative(num: int) -> int:
+    """
+    Compute double factorial using iterative method.
+
+    To learn about the theory behind this algorithm:
+    https://en.wikipedia.org/wiki/Double_factorial
+
+    >>> from math import prod
+    >>> all(double_factorial_iterative(i) == prod(range(i, 0, -2)) for i in range(20))
+    True
+    >>> double_factorial_iterative(0.1)
+    Traceback (most recent call last):
+        ...
+    ValueError: double_factorial_iterative() only accepts integral values
+    >>> double_factorial_iterative(-1)
+    Traceback (most recent call last):
+        ...
+    ValueError: double_factorial_iterative() not defined for negative values
+    """
+    if not isinstance(num, int):
+        raise ValueError("double_factorial_iterative() only accepts integral values")
+    if num < 0:
+        raise ValueError("double_factorial_iterative() not defined for negative values")
+    value = 1
+    for i in range(num, 0, -2):
+        value *= i
+    return value
+
+
+if __name__ == "__main__":
+    import doctest
+
+    doctest.testmod()