create financial/price_plus_tax function

src/financial/mod.rs

@@ -0,0 +1,2 @@
+mod present_value;
+pub use present_value::present_value;

src/financial/present_value.rs

@@ -0,0 +1,91 @@
+/// 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.
+///
+/// -> Wikipedia reference: https://en.wikipedia.org/wiki/Present_value
+
+#[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(round(present_value))
+}
+
+fn round(value: f64) -> f64 {
+    (value * 100.0).round() / 100.0
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    macro_rules! test_present_value {
+        ($($name:ident: $inputs:expr,)*) => {
+            $(
+                #[test]
+                fn $name() {
+                    let ((discount_rate,cash_flows), expected) = $inputs;
+                    assert_eq!(present_value(discount_rate,cash_flows).unwrap(), expected);
+                }
+            )*
+        }
+    }
+
+    macro_rules! test_present_value_Err {
+        ($($name:ident: $inputs:expr,)*) => {
+            $(
+                #[test]
+                fn $name() {
+                    let ((discount_rate,cash_flows), expected) = $inputs;
+                    assert_eq!(present_value(discount_rate,cash_flows).unwrap_err(), expected);
+                }
+            )*
+        }
+    }
+
+    macro_rules! test_round {
+        ($($name:ident: $inputs:expr,)*) => {
+            $(
+                #[test]
+                fn $name() {
+                    let (input, expected) = $inputs;
+                    assert_eq!(round(input), expected);
+                }
+            )*
+        }
+    }
+
+    test_present_value! {
+        general_inputs1:((0.13, vec![10.0, 20.70, -293.0, 297.0]),4.69),
+        general_inputs2:((0.07, vec![-109129.39, 30923.23, 15098.93, 29734.0, 39.0]),-42739.63),
+        general_inputs3:((0.07, vec![109129.39, 30923.23, 15098.93, 29734.0, 39.0]), 175519.15),
+        zero_input:((0.0, vec![109129.39, 30923.23, 15098.93, 29734.0, 39.0]),  184924.55),
+
+    }
+
+    test_present_value_Err! {
+        negative_discount_rate:((-1.0, vec![10.0, 20.70, -293.0, 297.0]), PresentValueError::NegetiveDiscount),
+        empty_cash_flow:((1.0, vec![]), PresentValueError::EmptyCashFlow),
+
+    }
+    test_round! {
+            test1:(0.55434,  0.55),
+            test2:(10.453,  10.45),
+            test3:(1111_f64,  1111_f64),
+    }
+}

src/lib.rs

@@ -8,6 +8,7 @@ pub mod compression;
 pub mod conversions;
 pub mod data_structures;
 pub mod dynamic_programming;
+pub mod financial;
 pub mod general;
 pub mod geometry;
 pub mod graph;