feat: Added NPV, Compound Interest and Payback Period

DIRECTORY.md

@@ -105,6 +105,9 @@
     * [Word Break](https://github.com/TheAlgorithms/Rust/blob/master/src/dynamic_programming/word_break.rs)
   * Financial
     * [Present Value](https://github.com/TheAlgorithms/Rust/blob/master/src/financial/present_value.rs)
+    * [Net Present Value](https://github.com/TheAlgorithms/Rust/blob/master/src/financial/npv.rs)
+    * [Compound Interest](https://github.com/TheAlgorithms/Rust/blob/master/src/financial/compound_interest.rs)
+    * [Payback Period](https://github.com/TheAlgorithms/Rust/blob/master/src/financial/payback.rs)
   * General
     * [Convex Hull](https://github.com/TheAlgorithms/Rust/blob/master/src/general/convex_hull.rs)
     * [Fisher Yates Shuffle](https://github.com/TheAlgorithms/Rust/blob/master/src/general/fisher_yates_shuffle.rs)

src/financial/compound_interest.rs

@@ -0,0 +1,23 @@
+// compound interest is given by A = P(1+r/n)^nt
+// where: A = Final Amount, P = Principal Amount, r = rate of interest,
+// n = number of times interest is compounded per year and t = time (in years)
+
+pub fn compound_interest(principal: f64, rate: f64, comp_per_year: u32, years: f64) -> f64 {
+    principal * (1.00 + rate / comp_per_year as f64).powf(comp_per_year as f64 * years)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_compound_interest() {
+        let principal = 1000.0;
+        let rate = 0.05; // 5% annual interest
+        let times_per_year = 4; // interest compounded quarterly
+        let years = 2.0; // 2 years tenure
+        let result = compound_interest(principal, rate, times_per_year, years);
+        assert!((result - 1104.486).abs() < 0.001); // expected value rounded up to 3 decimal
+                                                    // places
+    }
+}

src/financial/mod.rs

@@ -1,2 +1,8 @@
+mod compound_interest;
+mod npv;
+mod payback;
 mod present_value;
+pub use compound_interest::compound_interest;
+pub use npv::npv;
+pub use payback::payback;
 pub use present_value::present_value;

src/financial/npv.rs

@@ -0,0 +1,44 @@
+/// Calculates Net Present Value given a vector of cash flows and a discount rate.
+/// cash_flows: Vector of f64 representing cash flows for each period.
+/// rate: Discount rate as an f64 (e.g., 0.05 for 5%)
+
+pub fn npv(cash_flows: &[f64], rate: f64) -> f64 {
+    cash_flows
+        .iter()
+        .enumerate()
+        .map(|(t, &cf)| cf / (1.00 + rate).powi(t as i32))
+        .sum()
+}
+
+// tests
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_npv_basic() {
+        let cash_flows = vec![-1000.0, 300.0, 400.0, -50.0];
+        let rate = 0.10;
+        let result = npv(&cash_flows, rate);
+        // Calculated value ≈ -434.25
+        assert!((result - (-434.25)).abs() < 0.05); // Allow small margin of error
+    }
+
+    #[test]
+    fn test_npv_zero_rate() {
+        let cash_flows = vec![100.0, 200.0, -50.0];
+        let rate = 0.0;
+        let result = npv(&cash_flows, rate);
+        assert!((result - 250.0).abs() < 0.05);
+    }
+
+    #[test]
+    fn test_npv_empty() {
+        // For empty cash flows: NPV should be 0
+        let cash_flows: Vec<f64> = vec![];
+        let rate = 0.05;
+        let result = npv(&cash_flows, rate);
+        assert_eq!(result, 0.0);
+    }
+}

src/financial/payback.rs

@@ -0,0 +1,30 @@
+/// Returns the payback period in years
+/// If investment is not paid back, returns None.
+
+pub fn payback(cash_flow: &[f64]) -> Option<usize> {
+    let mut total = 0.00;
+    for (year, &cf) in cash_flow.iter().enumerate() {
+        total += cf;
+        if total >= 0.00 {
+            return Some(year);
+        }
+    }
+    None
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_payback() {
+        let cash_flows = vec![-1000.0, 300.0, 400.0, 500.0];
+        assert_eq!(payback(&cash_flows), Some(3)); // paid back in year 3
+    }
+
+    #[test]
+    fn test_no_payback() {
+        let cash_flows = vec![-1000.0, 100.0, 100.0, 100.0];
+        assert_eq!(payback(&cash_flows), None); // never paid back
+    }
+}