//! H⁻ 束缚-自由截面。
//!
//! 重构自 TLUSTY `sbfhmi.f`

use crate::math::ylintp;

/// H⁻ 束缚-自由截面。
///
/// 计算负氢离子的束缚-自由吸收截面。
///
/// # 参数
///
/// * `fr` - 频率
///
/// # 返回值
///
/// H⁻ 束缚-自由截面 (cm²)。
///
/// # 备注
///
/// 数据来自 Mathisen (1984)，基于 Wishart (1979) 和 Broad & Reinhardt (1976)。
/// 阈值频率: 1.82365e14 Hz。
pub fn sbfhmi(fr: f64) -> f64 {
    // 波长数据 (nm)
    const WBF: [f64; 85] = [
        18.00, 19.60, 21.40, 23.60, 26.40, 29.80, 34.30,
        40.40, 49.10, 62.60, 111.30, 112.10, 112.67, 112.95, 113.05,
        113.10, 113.20, 113.23, 113.50, 114.40, 121.00, 139.00, 164.00,
        175.00, 200.00, 225.00, 250.00, 275.00, 300.00, 325.00, 350.00,
        375.00, 400.00, 425.00, 450.00, 475.00, 500.00, 525.00, 550.00,
        575.00, 600.00, 625.00, 650.00, 675.00, 700.00, 725.00, 750.00,
        775.00, 800.00, 825.00, 850.00, 875.00, 900.00, 925.00, 950.00,
        975.00, 1000.00, 1025.00, 1050.00, 1075.00, 1100.00, 1125.00, 1150.00,
        1175.00, 1200.00, 1225.00, 1250.00, 1275.00, 1300.00, 1325.00, 1350.00,
        1375.00, 1400.00, 1425.00, 1450.00, 1475.00, 1500.00, 1525.00, 1550.00,
        1575.00, 1600.00, 1610.00, 1620.00, 1630.00, 1643.91,
    ];

    // 截面数据 (10^18 cm²)
    const BF: [f64; 85] = [
        0.067, 0.088, 0.117, 0.155, 0.206, 0.283, 0.414,
        0.703, 1.24, 2.33, 11.60, 13.90, 24.30, 66.70, 95.00,
        56.60, 20.00, 14.60, 8.50, 7.10, 5.43, 5.91, 7.29,
        7.918, 9.453, 11.08, 12.75, 14.46, 16.19, 17.92, 19.65,
        21.35, 23.02, 24.65, 26.24, 27.77, 29.23, 30.62, 31.94,
        33.17, 34.32, 35.37, 36.32, 37.17, 37.91, 38.54, 39.07,
        39.48, 39.77, 39.95, 40.01, 39.95, 39.77, 39.48, 39.06,
        38.53, 37.89, 37.13, 36.25, 35.28, 34.19, 33.01, 31.72,
        30.34, 28.87, 27.33, 25.71, 24.02, 22.26, 20.46, 18.62,
        16.74, 14.85, 12.95, 11.07, 9.211, 7.407, 5.677, 4.052,
        2.575, 1.302, 0.8697, 0.4974, 0.1989, 0.0,
    ];

    const FR_THRESHOLD: f64 = 1.82365e14;

    if fr > FR_THRESHOLD {
        let wave = 2.99792458e17 / fr;
        ylintp(&WBF, &BF, wave) * 1e-18
    } else {
        0.0
    }
}

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

    #[test]
    fn test_sbfhmi_below_threshold() {
        // 低于阈值频率
        let result = sbfhmi(1e14);
        assert_relative_eq!(result, 0.0, epsilon = 1e-20);
    }

    #[test]
    fn test_sbfhmi_above_threshold() {
        // 高于阈值频率
        let result = sbfhmi(2e15);
        assert!(result > 0.0);
        assert!(result < 1e-16); // 截面量级
    }

    #[test]
    fn test_sbfhmi_visible() {
        // 可见光范围 (~5e14 Hz)
        let result = sbfhmi(5e14);
        assert!(result > 0.0);
    }

    #[test]
    fn test_sbfhmi_uv() {
        // 紫外范围
        let result = sbfhmi(1e15);
        assert!(result > 0.0);
    }
}