Add return on investment to math

math/return_on_investment.c

@@ -0,0 +1,50 @@
+/**
+ * @file
+ * @brief Calculate [Return on Investment (ROI)](https://www.investopedia.com/terms/r/returnoninvestment.asp)
+ * @details
+ * Return on Investment (ROI) measures the profitability of an investment
+ * relative to its cost, expressed as a percentage.
+ *
+ * Formula: ROI = (Gain - Cost) / Cost * 100
+ *
+ * @author [InukaWijerathna](https://github.com/InukaWijerathna)
+ */
+
+#include <assert.h>  /// for assert()
+#include <stdio.h>   /// for IO operations
+
+/**
+ * @brief Calculate return on investment as a percentage
+ * @param gain_from_investment total value gained from the investment
+ * @param cost_of_investment total cost of the investment (must be > 0)
+ * @return ROI as a percentage, or -999.0 on invalid input
+ */
+double return_on_investment(double gain_from_investment,
+                            double cost_of_investment) {
+    if (cost_of_investment <= 0) {
+        fprintf(stderr, "Error: cost_of_investment must be > 0\n");
+        return -999.0;
+    }
+    return (gain_from_investment - cost_of_investment) / cost_of_investment *
+           100.0;
+}
+
+/**
+ * @brief Self-test implementations
+ */
+static void tests(void) {
+    assert(return_on_investment(1000.0, 500.0) == 100.0);
+    assert(return_on_investment(500.0, 500.0) == 0.0);
+    assert(return_on_investment(200.0, 500.0) == -60.0);
+    assert(return_on_investment(0.0, 500.0) == -100.0);
+    assert(return_on_investment(1000.0, 0.0) == -999.0);
+    printf("All tests passed!\n");
+}
+
+/**
+ * @brief Main function
+ */
+int main(void) {
+    tests();
+    return 0;
+}