Function doubling_time 

pub fn doubling_time(rate: f64) -> FinanceResult<f64>

Exact periods to double under discrete compounding: ln(2) / ln(1 + rate).

Errors

FinanceError::InvalidRate if rate <= 0, or non-finite / unsolvable rates.

Examples

use finance_solution::{doubling_time, rule_of_72, FinanceError};

let exact = doubling_time(0.08).unwrap();
let approx = rule_of_72(0.08).unwrap();
assert!((exact - approx).abs() < 0.5);
assert!((exact - 9.0065).abs() < 1e-3);

match doubling_time(-0.05) {
    Err(FinanceError::InvalidRate { rate }) => assert!(rate < 0.0),
    other => panic!("expected InvalidRate, got {other:?}"),
}