should-i-rent/crates/immo-core/src/interest_rate.rs

use std::fmt::Display;

use rust_decimal::Decimal;

#[derive(Debug, PartialEq, Eq, Clone, Copy, PartialOrd, Ord)]
pub struct InterestRate {
    rate_as_fraction: Decimal,
}

impl InterestRate {
    pub const ZERO: Self = InterestRate {
        rate_as_fraction: Decimal::ZERO,
    };

    pub fn from_percentage(rate_in_percent: Decimal) -> Self {
        InterestRate {
            rate_as_fraction: rate_in_percent / Decimal::ONE_HUNDRED,
        }
    }

    pub const fn from_fraction(rate_as_fraction: Decimal) -> Self {
        InterestRate { rate_as_fraction }
    }

    pub const fn as_fraction(self) -> Decimal {
        self.rate_as_fraction
    }

    pub fn as_percentage(self) -> Decimal {
        (self.rate_as_fraction * Decimal::ONE_HUNDRED).normalize()
    }
}

impl Display for InterestRate {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "{} %", self.as_percentage())
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn create_from_percentage_yields_correct_rate() {
        assert_eq!(
            InterestRate::from_percentage(Decimal::new(415, 2)).as_fraction(),
            Decimal::new(415, 4)
        );
    }

    #[test]
    fn create_from_fraction_yields_correct_rate() {
        assert_eq!(
            InterestRate::from_fraction(Decimal::new(415, 4)).as_fraction(),
            Decimal::new(415, 4)
        );
    }

    #[test]
    fn create_from_fraction_is_properly_output_as_percentage() {
        assert_eq!(
            InterestRate::from_fraction(Decimal::new(314, 4)).as_percentage(),
            Decimal::new(314, 2)
        )
    }

    #[test]
    fn display_works_as_expected_with_no_trailing_zeros() {
        let interest_rate = InterestRate::from_fraction(Decimal::new(3145, 5));
        assert_eq!(format!("{}", interest_rate), "3.145 %");
    }
}