rust-roche 0.4.4

Rust translation of Tom Marsh's cpp-roche package for modelling Roche distorted stars/binary systems.
Documentation 
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use crate::errors::RocheError;

///
/// vtrans computes two velocity transforms, (1) a straight transform
/// from rotating to inertial frame and (2) an inertial frame velocity
/// in the disc.
///
/// When translating to inertial, the accretor velocity is added.
/// If you want the velocity relative to this you must add
/// mu = q/(1+q) to tvy before using it.
///
/// Arguments:
///
/// * `q`: mass ratio M2/M1
/// * `transform_type`: integer representing velocity transform.
///   1: rotating frame -> inertial frame
///   2: rotating frame to inertial frame velocity of disc
///   3: rotating frame (i.e. gived out what you entered)
/// * `x`: x-coordinate where transform is performed for
/// * `y`: y-coordinate where transform is performed for
/// * `vx`: velocity in x-axis in the rotating frame to transform
/// * `vy`: velocity in y-axis in the rotating frame to transform
///
/// Returns:
///
/// * `vx`: velocity in x-axis transformed to the chosen frame
/// * `vy`: velocity in y-axis tranformed to the chosen frame
///
pub fn vel_transform(
    q: f64,
    transform_type: i32,
    x: f64,
    y: f64,
    vx: f64,
    vy: f64,
) -> Result<(f64, f64), RocheError> {
    let mu: f64 = q / (1.0 + q);
    let rad: f64 = (x * x + y * y).sqrt();
    let vkep: f64 = 1.0 / ((1.0 + q) * rad).sqrt();

    match transform_type {
        1 => Ok((vx - y, vy + x - mu)),
        2 => Ok((-vkep * y / rad, vkep * x / rad - mu)),
        3 => Ok((vx, vy)),
        _ => Err(RocheError::ParameterError(format!(
            "{} is not a valid transform_type.",
            transform_type
        ))),
    }
}