//! Van der Waals broadening parameter evaluation.
//!
//! Translated from SYNSPEC54.FOR function GVDW(IL,ILIST,ID) at line 19468.
//!
//! Supports two modes:
//! - Standard expression (`ivdwli == 0`)
//! - EXOMOL form with H2 and He broadening (`ivdwli > 0`)

/// Parameters for Van der Waals broadening calculation.
pub struct GvdwParams<'a> {
    /// Line index
    pub il: usize,
    /// Line list index
    pub ilist: usize,
    /// Depth index
    pub id: usize,
    /// Van der Waals damping parameter (standard mode)
    pub gwm: f64,
    /// Van der Waals coefficient at depth
    pub vdwc: &'a [f64],
    /// Mode of evaluation per line list (0 = standard, >0 = EXOMOL)
    pub ivdwli: &'a [i32],
    /// Temperature at each depth
    pub temp: &'a [f64],
    /// He number density at each depth (from rrr array)
    pub anhe: f64,
    /// H2 number density at each depth
    pub anh2: &'a [f64],
    /// EXOMOL H2 broadening exponent
    pub gexph2: f64,
    /// EXOMOL H2 broadening width
    pub gvdwh2: f64,
    /// EXOMOL He broadening exponent
    pub gexphe: f64,
    /// EXOMOL He broadening width
    pub gvdwhe: f64,
}

/// Van der Waals broadening parameter.
///
/// Computes the Van der Waals broadening parameter for spectral line profiles.
/// Supports both the standard classical expression and the EXOMOL form
/// (broadening by H2 and He).
///
/// # Arguments
/// * `params` - Calculation parameters
///
/// # Returns
/// Van der Waals broadening parameter
pub fn gvdw(params: &GvdwParams) -> f64 {
    // Standard classical expression
    if params.ivdwli[params.ilist] == 0 {
        return params.gwm * params.vdwc[params.id];
    }

    // EXOMOL form - broadening by H2 and He
    // con = 1e-6 * c * k  (cgs)
    let con = 4.1388e-12;
    let t = params.temp[params.id];

    con * t
        * ((296.0 / t).powf(params.gexph2) * params.gvdwh2 * params.anh2[params.id]
            + (296.0 / t).powf(params.gexphe) * params.gvdwhe * params.anhe)
}

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

    #[test]
    fn test_gvdw_standard_mode() {
        let vdwc = vec![1.0, 2.0, 3.0];
        let ivdwli = vec![0];
        let temp = vec![5000.0, 6000.0, 7000.0];
        let anh2 = vec![1e10, 1e10, 1e10];

        let params = GvdwParams {
            il: 0,
            ilist: 0,
            id: 1,
            gwm: 0.5,
            vdwc: &vdwc,
            ivdwli: &ivdwli,
            temp: &temp,
            anhe: 1e10,
            anh2: &anh2,
            gexph2: 0.0,
            gvdwh2: 0.0,
            gexphe: 0.0,
            gvdwhe: 0.0,
        };

        // Standard: gwm * vdwc[id] = 0.5 * 2.0 = 1.0
        let result = gvdw(&params);
        assert!((result - 1.0).abs() < 1e-15);
    }

    #[test]
    fn test_gvdw_exomol_mode() {
        let vdwc = vec![1.0; 3];
        let ivdwli = vec![1];
        let temp = vec![5000.0; 3];
        let anh2 = vec![1e12; 3];

        let params = GvdwParams {
            il: 0,
            ilist: 0,
            id: 0,
            gwm: 1.0,
            vdwc: &vdwc,
            ivdwli: &ivdwli,
            temp: &temp,
            anhe: 1e11,
            anh2: &anh2,
            gexph2: 0.5,
            gvdwh2: 1e-9,
            gexphe: 0.3,
            gvdwhe: 5e-10,
        };

        let result = gvdw(&params);
        assert!(result > 0.0);
        assert!(result.is_finite());
    }

    #[test]
    fn test_gvdw_exomol_temperature_dependence() {
        let vdwc = vec![1.0; 2];
        let ivdwli = vec![1];
        let anh2 = vec![1e12; 2];

        let params_low = GvdwParams {
            il: 0,
            ilist: 0,
            id: 0,
            gwm: 1.0,
            vdwc: &vdwc,
            ivdwli: &ivdwli,
            temp: &[3000.0, 10000.0],
            anhe: 1e11,
            anh2: &anh2,
            gexph2: 0.5,
            gvdwh2: 1e-9,
            gexphe: 0.3,
            gvdwhe: 5e-10,
        };

        let params_high = GvdwParams {
            id: 1,
            ..params_low
        };

        let r_low = gvdw(&params_low);
        let r_high = gvdw(&params_high);
        // Both should be positive
        assert!(r_low > 0.0);
        assert!(r_high > 0.0);
    }
}