feat(maths): add harmonic mean algorithm

Author: benarch

maths/harmonic_mean.py

@@ -0,0 +1,61 @@
+"""
+The Harmonic Mean of n numbers is defined as n divided by the sum of the
+reciprocals of those numbers. It is used to calculate average rates and ratios.
+https://en.wikipedia.org/wiki/Harmonic_mean
+"""
+
+from typing import Union
+
+
+def compute_harmonic_mean(*args: Union[int, float]) -> float:
+    """
+    Return the harmonic mean of the argument numbers.
+
+    The harmonic mean is particularly useful for rates (e.g., speed, density).
+
+    >>> compute_harmonic_mean(1, 2, 4)
+    1.7142857142857142
+    >>> compute_harmonic_mean(2, 4)
+    2.6666666666666665
+    >>> compute_harmonic_mean('a', 4)
+    Traceback (most recent call last):
+        ...
+    TypeError: Not a Number
+    >>> compute_harmonic_mean(1, 0)
+    Traceback (most recent call last):
+        ...
+    ValueError: Cannot compute harmonic mean with zero values
+    >>> compute_harmonic_mean(1, -1)
+    Traceback (most recent call last):
+        ...
+    ValueError: Cannot compute harmonic mean with non-positive values
+    >>> compute_harmonic_mean()
+    Traceback (most recent call last):
+        ...
+    ValueError: At least one number is required
+    >>> compute_harmonic_mean(4, 4, 4)
+    4.0
+    >>> compute_harmonic_mean(1, 2)
+    1.3333333333333333
+    """
+    if len(args) == 0:
+        raise ValueError("At least one number is required")
+
+    reciprocal_sum = 0
+    for number in args:
+        if not isinstance(number, (int, float)):
+            raise TypeError("Not a Number")
+        if number == 0:
+            raise ValueError("Cannot compute harmonic mean with zero values")
+        if number < 0:
+            raise ValueError("Cannot compute harmonic mean with non-positive values")
+        reciprocal_sum += 1 / number
+
+    return len(args) / reciprocal_sum
+
+
+if __name__ == "__main__":
+    from doctest import testmod
+
+    testmod(name="compute_harmonic_mean")
+    print(compute_harmonic_mean(1, 2, 4))