/// 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

}

}