Add return on investment algorithm to financial

Author: InukaWijerathna

financial/return_on_investment.py

@@ -0,0 +1,42 @@
+# https://www.investopedia.com/terms/r/returnoninvestment.asp
+
+from __future__ import annotations
+
+
+def return_on_investment(
+    gain_from_investment: float, cost_of_investment: float
+) -> float:
+    """
+    Return on Investment (ROI) measures the profitability of an investment
+    relative to its cost, expressed as a percentage.
+
+    Formula: ROI = (Gain - Cost) / Cost * 100
+
+    >>> return_on_investment(1000.0, 500.0)
+    100.0
+    >>> return_on_investment(500.0, 500.0)
+    0.0
+    >>> return_on_investment(200.0, 500.0)
+    -60.0
+    >>> return_on_investment(0.0, 500.0)
+    -100.0
+    >>> return_on_investment(1000.0, 0.0)
+    Traceback (most recent call last):
+        ...
+    ValueError: cost_of_investment must be > 0
+    >>> return_on_investment(1000.0, -100.0)
+    Traceback (most recent call last):
+        ...
+    ValueError: cost_of_investment must be > 0
+    """
+    if cost_of_investment <= 0:
+        raise ValueError("cost_of_investment must be > 0")
+    return round(
+        (gain_from_investment - cost_of_investment) / cost_of_investment * 100, 2
+    )
+
+
+if __name__ == "__main__":
+    import doctest
+
+    doctest.testmod()