main.rs

use std::env;

use pyo3::Python;

fn main() -> eyre::Result<()> {
    Python::with_gil(|py| {
        // workaround for https://github.com/PyO3/pyo3/issues/1896
        if cfg!(any(windows, target_os = "macos")) {
            if let Ok(virtual_env) = env::var("VIRTUAL_ENV") {
                let site_packages = if cfg!(windows) {
                    format!(r"{virtual_env}\Lib\site-packages")
                } else {
                    format!("{virtual_env}/lib/python3.10/site-packages")
                };
                py.import("sys")?
                    .getattr("path")?
                    .call_method1("append", (site_packages,))?;
            }
        }

        let np_round = py.import("numpy")?.getattr("round")?;
        let np_round = |v| np_round.call1((v,));

        for v in -10000..=10000 {
            let v = v as f64;
            for v in [v, v + 0.1, v + 0.4, v + 0.5, v + 0.6, v + 0.9] {
                let python = np_round(v)?.extract::<f64>()?;

                let rust = roundeven_by_llvm(v);
                assert_eq!(python, rust);

                let rust = roundeven_by_libm_rs(v);
                assert_eq!(python, rust);

                let rust = libm::rint(v);
                assert_eq!(python, rust);
            }
        }
        Ok(())
    })
}

fn roundeven_by_llvm(v: f64) -> f64 {
    let mut rounded = v.round();
    if (v - rounded).abs() == 0.5 {
        rounded = 2. * (v / 2.).round();
    }
    rounded
}

fn roundeven_by_libm_rs(v: f64) -> f64 {
    let mut rounded = libm::round(v);
    if libm::fabs(v - rounded) == 0.5 {
        rounded = 2. * libm::round(v / 2.);
    }
    rounded
}