use rust_decimal::prelude::*;
use rust_decimal::{Decimal, RoundingStrategy};

use crate::error::ApiError;
use crate::models::Currency;

/// Convert integer minor units between currencies via base rates.
/// rate_to_base = units of currency per 1 unit of base. Rounds half-up
/// to whole minor units of the target currency.
pub fn convert_minor(amount_minor: i64, from: &Currency, to: &Currency) -> Result<i64, ApiError> {
    if amount_minor < 0 {
        return Err(ApiError::BadRequest("amount_minor must be >= 0".into()));
    }
    let amount = Decimal::from(amount_minor);
    let from_scale = Decimal::from(10i64.pow(from.exponent as u32));
    let to_scale = Decimal::from(10i64.pow(to.exponent as u32));
    // minor(from) -> major(from) -> major(base) -> minor(to)
    let base_major = amount / from_scale / from.rate_to_base;
    let target = (base_major * to.rate_to_base * to_scale)
        .round_dp_with_strategy(0, RoundingStrategy::MidpointAwayFromZero);
    target
        .to_i64()
        .ok_or_else(|| ApiError::BadRequest("amount out of range".into()))
}

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

    fn usd() -> Currency {
        Currency {
            code: "USD".into(),
            name: serde_json::json!({"en": "US Dollar"}),
            symbol: "$".into(),
            exponent: 2,
            is_base: true,
            rate_to_base: dec!(1),
            enabled: true,
        }
    }

    fn jpy() -> Currency {
        Currency {
            code: "JPY".into(),
            name: serde_json::json!({"en": "Yen"}),
            symbol: "¥".into(),
            exponent: 0,
            is_base: false,
            rate_to_base: dec!(150),
            enabled: true,
        }
    }

    #[test]
    fn same_currency_is_identity() {
        assert_eq!(convert_minor(199, &usd(), &usd()).unwrap(), 199);
    }

    #[test]
    fn rejects_negative() {
        assert!(convert_minor(-1, &usd(), &usd()).is_err());
    }

    #[test]
    fn converts_via_base_half_up() {
        // 1.00 USD -> 150 JPY at rate 150
        assert_eq!(convert_minor(100, &usd(), &jpy()).unwrap(), 150);
    }
}