komp_ac/common/src/money.rs

// common/src/money.rs
//!
//! The one canonical currency spelling shared by the client and the server.
//!
//! A currency code is stored, sent and compared as its canonical uppercase
//! ISO-4217 alphabetic code. Both ends resolve it with
//! [`require_iso_currency_code`], so the client rejects exactly what the server
//! would reject, and each side renders the shared error as its own error type.

pub use rusty_money::iso;
pub use crate::proto::komp_ac::table_definition::MoneyRounding;

impl MoneyRounding {
    pub fn as_storage_str(self) -> &'static str {
        match self {
            Self::None => "none",
            Self::HalfUp => "half_up",
        }
    }
}

impl std::str::FromStr for MoneyRounding {
    type Err = serde::de::value::Error;

    fn from_str(value: &str) -> Result<Self, Self::Err> {
        match value {
            "none" => Ok(Self::None),
            "half_up" => Ok(Self::HalfUp),
            _ => Err(serde::de::Error::custom(format!("Invalid money rounding: {value}"))),
        }
    }
}

impl TryFrom<String> for MoneyRounding {
    type Error = serde::de::value::Error;

    fn try_from(value: String) -> Result<Self, Self::Error> {
        value.parse()
    }
}

pub mod rounding_serde {
    use super::MoneyRounding;
    use serde::{Deserialize, Deserializer, Serializer};

    pub fn serialize<S: Serializer>(value: &MoneyRounding, serializer: S) -> Result<S::Ok, S::Error> {
        serializer.serialize_str(value.as_storage_str())
    }

    pub fn deserialize<'de, D: Deserializer<'de>>(deserializer: D) -> Result<MoneyRounding, D::Error> {
        String::deserialize(deserializer)?.parse().map_err(serde::de::Error::custom)
    }
}

/// Resolves a canonical uppercase ISO-4217 alphabetic code to its currency.
///
/// Rejects anything not already canonical: lowercase (`eur`), surrounding
/// whitespace, and any length but three. Normalising here instead would let a
/// value be written in one spelling and compared in another.
pub fn require_iso_currency_code(currency_code: &str) -> Result<&'static iso::Currency, String> {
    iso::find(currency_code).ok_or_else(|| format!("Unknown ISO-4217 currency: {currency_code}"))
}

/// Returns every currency accepted by [`require_iso_currency_code`], ordered by
/// its canonical alphabetic code.
pub fn iso_currency_codes() -> Vec<&'static str> {
    let mut currencies = Vec::new();
    for first in b'A'..=b'Z' {
        for second in b'A'..=b'Z' {
            for third in b'A'..=b'Z' {
                let bytes = [first, second, third];
                let code = std::str::from_utf8(&bytes).expect("uppercase ASCII currency code");
                if let Some(currency) = iso::find(code) {
                    currencies.push(currency.iso_alpha_code);
                }
            }
        }
    }
    currencies
}

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

    #[test]
    fn rounding_storage_and_json_boundaries_are_strict() {
        #[derive(Debug, PartialEq, serde::Serialize, serde::Deserialize)]
        struct StoredPolicy {
            #[serde(with = "super::rounding_serde")]
            rounding: MoneyRounding,
        }

        for (text, policy) in [("none", MoneyRounding::None), ("half_up", MoneyRounding::HalfUp)] {
            assert_eq!(text.parse::<MoneyRounding>().unwrap(), policy);
            assert_eq!(MoneyRounding::try_from(text.to_string()).unwrap(), policy);
            assert_eq!(policy.as_storage_str(), text);
            let stored = StoredPolicy { rounding: policy };
            let json = serde_json::json!({ "rounding": text });
            assert_eq!(serde_json::to_value(&stored).unwrap(), json);
            assert_eq!(serde_json::from_value::<StoredPolicy>(json).unwrap(), stored);
        }
        for text in ["", "NONE", "HalfUp", "half-up", " half_up", "half_up ", "unknown"] {
            assert!(text.parse::<MoneyRounding>().is_err());
            assert!(serde_json::from_value::<StoredPolicy>(serde_json::json!({ "rounding": text })).is_err());
        }
        for value in [serde_json::Value::Null, serde_json::json!(0), serde_json::json!(1)] {
            assert!(serde_json::from_value::<StoredPolicy>(serde_json::json!({ "rounding": value })).is_err());
        }
        assert!(MoneyRounding::try_from(-1).is_err());
        assert!(MoneyRounding::try_from(2).is_err());
    }

    #[test]
    fn canonical_codes_resolve_to_their_currency() {
        assert_eq!(require_iso_currency_code("EUR").unwrap(), iso::EUR);
        assert_eq!(
            require_iso_currency_code("USD").unwrap().iso_alpha_code,
            "USD"
        );
    }

    #[test]
    fn non_canonical_or_unknown_codes_are_rejected() {
        for currency_code in [
            "",
            "E",
            "EU",
            "EURO",
            "eur",
            "Eur",
            " EUR",
            "EUR ",
            "E R",
            "E\u{20AC}R",
            "AAA",
            "123",
        ] {
            assert!(
                require_iso_currency_code(currency_code).is_err(),
                "unexpectedly accepted {currency_code:?}"
            );
        }
    }
}