CPC/src/currency.rs

use crate::extra_units;
use crate::units::UnitType;
use fastnum::decimal::Context;
use fastnum::D128;
use serde_json::Value;

const EXCHANGE_RATE_API_USD_LATEST: &str = "https://api.exchangerate-api.com/v4/latest/USD";

fn parse_d128_from_json_number(v: &Value) -> Result<D128, String> {
	let s = match v {
		Value::Number(n) => n.to_string(),
		_ => return Err("Expected JSON number".to_string()),
	};
	D128::from_str(&s, Context::default())
		.map_err(|_| format!("Invalid decimal number: {s}"))
}

/// Registers/updates currencies from the ExchangeRate API JSON response (base USD).
///
/// The JSON is expected to look like `{"base":"USD","rates":{"EUR":0.852,...}}`.
///
/// Internally we store a "weight" as USD-per-unit, so conversions use the existing
/// `from.weight()/to.weight()` logic.
pub fn register_currencies_from_exchange_rate_api_json(json: &str) -> Result<usize, String> {
	let v: Value = serde_json::from_str(json).map_err(|e| format!("Invalid JSON: {e}"))?;

	let base = v
		.get("base")
		.and_then(|b| b.as_str())
		.ok_or("Missing 'base'")?;
	if base != "USD" {
		return Err(format!("Expected base USD but got {base}"));
	}

	let rates = v
		.get("rates")
		.and_then(|r| r.as_object())
		.ok_or("Missing 'rates' object")?;

	let mut count = 0usize;
	for (code, rate_value) in rates {
		// Skip weird/empty keys defensively
		if code.len() != 3 {
			continue;
		}
		let code_upper = code.to_ascii_uppercase();
		let code_lower = code.to_ascii_lowercase();

		let rate = parse_d128_from_json_number(rate_value)?;
		if rate.is_zero() {
			continue;
		}
		// weight = USD per 1 {CODE} = 1 / (CODE per 1 USD)
		let weight = D128::ONE / rate;

		extra_units::upsert_unit(
			&code_lower,
			&code_upper,
			&code_upper,
			UnitType::Currency,
			weight,
		);
		count += 1;
	}

	Ok(count)
}

#[cfg(not(target_arch = "wasm32"))]
fn cache_path() -> std::path::PathBuf {
	let mut p = std::env::temp_dir();
	p.push("cpc_currency_rates_usd.json");
	p
}

#[cfg(not(target_arch = "wasm32"))]
pub fn init_currencies_blocking() -> Result<usize, String> {
	// 1) Load cached (if any) so currencies work offline.
	let mut cached_count = 0usize;
	let cache_path = cache_path();
	if let Ok(s) = std::fs::read_to_string(&cache_path) {
		if let Ok(n) = register_currencies_from_exchange_rate_api_json(&s) {
			cached_count = n;
		}
	}

	// 2) Try refresh from network.
	let resp = ureq::get(EXCHANGE_RATE_API_USD_LATEST)
		.call()
		.map_err(|e| format!("Currency fetch failed: {e}"))?;
	let body = resp
		.into_string()
		.map_err(|e| format!("Currency fetch read failed: {e}"))?;

	let n = register_currencies_from_exchange_rate_api_json(&body)?;
	// Best-effort cache write
	let _ = std::fs::write(&cache_path, body);

	Ok(std::cmp::max(cached_count, n))
}

#[cfg(test)]
mod tests {
	use super::*;
	use crate::extra_units;
	use crate::units::{convert, Unit};
	use crate::Number;
	use fastnum::dec128 as d;

	#[test]
	fn register_and_convert_currency() {
		extra_units::clear_for_tests();

		let json = r#"{
			"base": "USD",
			"rates": {
				"USD": 1,
				"EUR": 0.5
			}
		}"#;

		let n = register_currencies_from_exchange_rate_api_json(json).unwrap();
		assert_eq!(n, 2);

		let usd = extra_units::lookup_unit("usd").unwrap();
		let eur = extra_units::lookup_unit("eur").unwrap();

		let one_usd = Number::new(d!(1), usd);
		let eur_amount = convert(one_usd, eur).unwrap();
		assert_eq!(eur_amount.value, d!(0.5));

		let one_eur = Number::new(d!(1), eur);
		let usd_amount = convert(one_eur, usd).unwrap();
		assert_eq!(usd_amount.value, d!(2));

		// Display name should be currency code
		assert_eq!(eur_amount.unit.singular(), "EUR");
		assert_eq!(eur_amount.unit.plural(), "EUR");

		// Sanity: it's still a Unit
		let _ = Unit::NoUnit;
	}
}