create present value module

add documentation in readme.md

src/financial/README.md

@@ -1 +1,5 @@
 ## Financial Algorithms
+### [Present-Value Algorithm](./present_value.rs)
+From [Wikipedia][present-value-wiki]: In economics and finance, present value (PV), also known as present discounted value, is the value of an expected income stream determined as of the date of valuation. The present value is usually less than the future value because money has interest-earning potential, a characteristic referred to as the time value of money, except during times of negative interest rates, when the present value will be equal or more than the future value
+
+[present-value-wiki]: https://en.wikipedia.org/wiki/Present_value

src/financial/mod.rs

@@ -1,3 +1,5 @@
 mod price_plus_tax;
-
+mod present_value;
 pub use price_plus_tax::price_plus_tax;
+pub use present_value::present_value;
+

src/financial/present_value.rs

@@ -0,0 +1,52 @@
+#[derive(PartialEq, Eq,Debug)]
+pub enum PresentValueError{
+    NegetiveDiscount,
+    EmptyCashFlow
+}
+
+
+pub fn present_value(discount_rate: f64, cash_flows: Vec<f64>) -> Result<f64, PresentValueError> {
+    if discount_rate < 0.0 {
+        return Err(PresentValueError::NegetiveDiscount);
+    }
+    if cash_flows.is_empty() {
+        return Err(PresentValueError::EmptyCashFlow);
+    }
+
+    let present_value = cash_flows.iter().enumerate()
+        .map(|(i, &cash_flow)| cash_flow / (1.0 + discount_rate).powi(i as i32))
+        .sum::<f64>();
+
+    Ok((present_value * 100.0).round() / 100.0)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_present_value() {
+
+        assert_eq!(4.69,present_value(0.13, vec![10.0, 20.70, -293.0, 297.0]).unwrap());
+
+        assert_eq!( -42739.63,present_value(0.07, vec![-109129.39, 30923.23, 15098.93, 29734.0,39.0]).unwrap());
+
+        assert_eq!(175519.15,present_value(0.07, vec![109129.39, 30923.23, 15098.93, 29734.0,39.0]).unwrap());
+    }
+
+    #[test]
+    fn test_present_value_negative_discount_rate() {
+        assert_eq!(PresentValueError::NegetiveDiscount,present_value(-1.0, vec![10.0, 20.70, -293.0, 297.0]).unwrap_err());
+
+    }
+
+    #[test]
+    fn test_present_value_empty_cash_flow() {
+        assert_eq!(PresentValueError::EmptyCashFlow,present_value(1.0, vec![]).unwrap_err());
+    }
+
+    #[test]
+    fn test_present_value_zero_discount_rate() {
+        assert_eq!(184924.55,present_value(0.0, vec![109129.39, 30923.23, 15098.93, 29734.0,39.0]).unwrap());
+    }
+}