CHISQ.DIST

20 chisq_dist "CHISQ.DIST" (3, 3) [V "x", V "deg_freedom", V "cumulative"] |s, a, sv| {
    let [x, df] = nums::<2>(s, a)?;
    out(s, cft::chisq_dist_kernel(x, df, flag(s, a, 2)?))
}

pub fn chisq_pdf_kernel(x: f64, deg_freedom: f64) -> Result<f64, WorksheetErrorCode> {
    let x = validate_nonnegative_x(x)?;
    let k = truncate_positive_integer(deg_freedom)?;
    if x == 0.0 {
        return if k == 1.0 {
            // CDF limit is finite but density diverges to +inf; Excel returns #NUM!
            Err(WorksheetErrorCode::Num)
        } else if k == 2.0 {
            Ok(0.5)
        } else {
            Ok(0.0)
        };
    }
    let half_k = k / 2.0;
    Ok(crt_pow(x, half_k - 1.0) * crt_exp(-x / 2.0) / (crt_pow(2.0_f64, half_k) * gamma(half_k)))
}

pub fn chisq_dist_kernel(
    x: f64,
    deg_freedom: f64,
    cumulative: bool,
) -> Result<f64, WorksheetErrorCode> {
    let x = validate_nonnegative_x(x)?;
    let k = truncate_positive_integer(deg_freedom)?;
    if cumulative {
        // Inverse-problem identity, live Excel 16.0 b20228: CHISQ.DIST(x,1,TRUE)
        // == ERF.PRECISE(SQRT(x/2)) == GAMMA.DIST(x,0.5,2,TRUE) on 154/154
        // distinct x. Rival stagings SQRT(x)/SQRT(2) and SQRT(x)*(1/SQRT(2))
        // fail the same bank.
        // Inverse-problem identity, live Excel 16.0 b20228:
        // CHISQ.DIST(x,df,TRUE) == GAMMA.DIST(x, df/2, 2, TRUE) on 88/88
        // across df in {1,2,3,4,5,6,8,10}. 1-CHIDIST is not the CDF (49/88).
        // Routes df=1/2 through the landed ERF/EXPON gamma specials.
        crate::ported::beta_gamma_stats_family::gamma_dist_kernel(x, k / 2.0, 2.0, true).map_err(
            |e| match e {
                code => {
                    code
                }
                _ => WorksheetErrorCode::Value,
            },
        )
    } else {
        chisq_pdf_kernel(x, k)
    }
}