//! H2⁻ 自由-自由吸收系数计算。
//!
//! 重构自 SYNSPEC `synspec54.f` 中的 `h2minus` 子程序。
//!
//! 数据来源: K L Bell 1980, J. Phys. B: At. Mol. Phys. 13 1859, Table 1
//! 单位: 10^26 cm^4/dyn^-1

use crate::tlusty::math::interpolation::locate;
use crate::synspec::math::{CL, BOLK};

// ============================================================================
// 静态数据表
// ============================================================================

/// theta = 5040/T(K) 网格点 (9 个)
const FFTHET: [f64; 9] = [0.5, 0.8, 1.0, 1.2, 1.6, 2.0, 2.8, 3.6, 10.0];

/// lambda (Angstroms) 网格点 (18 个)
const FFLAMB: [f64; 18] = [
    151883.0, 113913.0, 91130.0, 60753.0,
    45565.0, 36452.0, 30377.0, 22783.0,
    18226.0, 15188.0, 11391.0, 9113.0, 7594.0,
    6509.0, 5696.0, 5063.0, 4142.0, 3505.0,
];

/// kappa 表 (18 x 9)，按列优先存储 (Fortran 布局)
const NTHET: usize = 9;
const NLAMB: usize = 18;

/// FFkapp 表，按 Fortran 列优先存储: FFkapp[theta_idx * NLAMB + lamb_idx]
/// 即 FFkapp(i,j) = FFKAPP[j * 18 + i]，其中 i=lambda, j=theta (0-based)
const FFKAPP: [f64; NLAMB * NTHET] = [
    // 列 1 (theta=0.5): 18 个 lambda 值
    7.16e+01, 4.03e+01, 2.58e+01, 1.15e+01, 6.47e+00,
    4.15e+00, 2.89e+00, 1.63e+00, 1.05e+00, 7.36e-01,
    4.20e-01, 2.73e-01, 1.92e-01, 1.43e-01, 1.10e-01,
    8.70e-02, 5.84e-02, 4.17e-02,
    // 列 2 (theta=0.8)
    9.23e+01, 5.20e+01, 3.33e+01, 1.48e+01, 8.37e+00,
    5.38e+00, 3.76e+00, 2.14e+00, 1.39e+00, 9.75e-01,
    5.64e-01, 3.71e-01, 2.64e-01, 1.98e-01, 1.54e-01,
    1.24e-01, 8.43e-02, 6.10e-02,
    // 列 3 (theta=1.0)
    1.01e+02, 5.70e+01, 3.65e+01, 1.63e+01, 9.20e+00,
    5.92e+00, 4.14e+00, 2.36e+00, 1.54e+00, 1.09e+00,
    6.35e-01, 4.22e-01, 3.03e-01, 2.30e-01, 1.80e-01,
    1.46e-01, 1.01e-01, 7.34e-02,
    // 列 4 (theta=1.2)
    1.08e+02, 6.08e+01, 3.90e+01, 1.74e+01, 9.84e+00,
    6.35e+00, 4.44e+00, 2.55e+00, 1.66e+00, 1.18e+00,
    6.97e-01, 4.67e-01, 3.39e-01, 2.59e-01, 2.06e-01,
    1.67e-01, 1.17e-01, 8.59e-02,
    // 列 5 (theta=1.6)
    1.18e+02, 6.65e+01, 4.27e+01, 1.91e+01, 1.08e+01,
    6.99e+00, 4.91e+00, 2.84e+00, 1.87e+00, 1.34e+00,
    8.06e-01, 5.52e-01, 4.08e-01, 3.17e-01, 2.55e-01,
    2.10e-01, 1.49e-01, 1.11e-01,
    // 列 6 (theta=2.0)
    1.26e+02, 7.08e+01, 4.54e+01, 2.04e+01, 1.16e+01,
    7.50e+00, 5.28e+00, 3.07e+00, 2.04e+00, 1.48e+00,
    9.09e-01, 6.33e-01, 4.76e-01, 3.75e-01, 3.05e-01,
    2.53e-01, 1.82e-01, 1.37e-01,
    // 列 7 (theta=2.8)
    1.38e+02, 7.76e+01, 4.98e+01, 2.24e+01, 1.28e+01,
    8.32e+00, 5.90e+00, 3.49e+00, 2.36e+00, 1.74e+00,
    1.11e+00, 7.97e-01, 6.13e-01, 4.92e-01, 4.06e-01,
    3.39e-01, 2.49e-01, 1.87e-01,
    // 列 8 (theta=3.6)
    1.47e+02, 8.30e+01, 5.33e+01, 2.40e+01, 1.38e+01,
    9.02e+00, 6.44e+00, 3.90e+00, 2.68e+00, 2.01e+00,
    1.32e+00, 9.63e-01, 7.51e-01, 6.09e-01, 5.07e-01,
    4.27e-01, 3.16e-01, 2.40e-01,
    // 列 9 (theta=10.0) — 线性外推
    2.19e+02, 1.26e+02, 8.13e+01, 3.68e+01, 2.18e+01,
    1.46e+01, 1.08e+01, 7.18e+00, 5.24e+00, 4.17e+00,
    3.00e+00, 2.29e+00, 1.86e+00, 1.55e+00, 1.32e+00,
    1.13e+00, 8.52e-01, 6.64e-01,
];

// ============================================================================
// h2minus - H2⁻ 自由-自由吸收
// ============================================================================

/// 计算 H2⁻ 自由-自由吸收系数。
///
/// # 参数
///
/// - `t` - 温度 (K)
/// - `anh2` - H2 分子数密度
/// - `ane` - 电子数密度
/// - `fr` - 频率 (Hz)
///
/// # 返回
///
/// H2⁻ 自由-自由吸收系数 `oph2m`
pub fn h2minus(t: f64, anh2: f64, ane: f64, fr: f64) -> f64 {
    // theta = 5040 / T
    let theta = 5040.0 / t;

    // 在温度数组中定位 (0-indexed)
    // locate 返回 j 使得 FFTHET[j] <= theta < FFTHET[j+1]
    let j = locate(&FFTHET, theta);

    // 波长 (Angstroms): lambda = c / fr * 1e8
    let flamb = CL * 1.0e8 / fr;

    // 在波长数组中定位 (0-indexed)
    let i = locate(&FFLAMB, flamb);

    // 双线性插值
    // 注意: FFTHET 是递增的，FFLAMB 是递减的
    let fkappa = if j >= NTHET - 1 {
        // theta >= FFTHET[NTHET-1]，保持恒定 (高温端)
        let i_clamped = i.min(NLAMB - 2);
        let y1 = ffkapp_at(i_clamped, NTHET - 1);
        let y2 = ffkapp_at(i_clamped + 1, NTHET - 1);
        let tt = (flamb - FFLAMB[i_clamped]) / (FFLAMB[i_clamped + 1] - FFLAMB[i_clamped]);
        (1.0 - tt) * y1 + tt * y2
    } else if !(FFLAMB[NLAMB - 1]..=FFLAMB[0]).contains(&flamb) {
        // 超出波长表范围 (FFLAMB 递减: [0] 最大, [NLAMB-1] 最小)
        0.0
    } else {
        // 表内双线性插值
        let y1 = ffkapp_at(i, j);
        let y2 = ffkapp_at(i + 1, j);
        let y3 = ffkapp_at(i + 1, j + 1);
        let y4 = ffkapp_at(i, j + 1);

        // tt: 波长方向插值 (FFLAMB 递减)
        let tt = (flamb - FFLAMB[i]) / (FFLAMB[i + 1] - FFLAMB[i]);
        // uu: 温度方向插值 (FFTHET 递增)
        let uu = (theta - FFTHET[j]) / (FFTHET[j + 1] - FFTHET[j]);

        (1.0 - tt) * (1.0 - uu) * y1
            + tt * (1.0 - uu) * y2
            + tt * uu * y3
            + (1.0 - tt) * uu * y4
    };

    // 电子压力
    let pe = ane * BOLK * t;

    // 最终吸收系数
    anh2 * 1.0e-26 * pe * fkappa
}

/// 从 FFkapp 表中获取值 (处理边界)
/// 索引: FFkapp(i,j) = FFKAPP[j * NLAMB + i]，其中 i=lambda, j=theta (0-based)
fn ffkapp_at(i: usize, j: usize) -> f64 {
    let i_clamped = i.min(NLAMB - 1);
    let j_clamped = j.min(NTHET - 1);
    FFKAPP[j_clamped * NLAMB + i_clamped]
}

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

    #[test]
    fn test_h2minus_basic() {
        // 典型恒星大气参数
        let t = 5000.0; // K
        let anh2 = 1.0e15; // H2 数密度
        let ane = 1.0e13; // 电子数密度
        let fr = 1.0e14; // Hz (红外)

        let oph2m = h2minus(t, anh2, ane, fr);
        assert!(oph2m > 0.0, "oph2m 应为正值: {}", oph2m);
    }

    #[test]
    fn test_h2minus_high_temperature() {
        // 高温情况
        let t = 10000.0;
        let anh2 = 1.0e14;
        let ane = 1.0e12;
        let fr = 3.0e14;

        let oph2m = h2minus(t, anh2, ane, fr);
        assert!(oph2m >= 0.0, "oph2m 应非负: {}", oph2m);
    }

    #[test]
    fn test_h2minus_low_temperature() {
        // 低温情况 (theta 大)
        let t = 3000.0;
        let anh2 = 1.0e16;
        let ane = 1.0e14;
        let fr = 5.0e14;

        let oph2m = h2minus(t, anh2, ane, fr);
        assert!(oph2m >= 0.0, "oph2m 应非负: {}", oph2m);
    }

    #[test]
    fn test_h2minus_scaling() {
        // 吸收系数应与 anh2 和 ane 成正比
        let t = 6000.0;
        let fr = 2.0e14;

        let oph2m1 = h2minus(t, 1.0e14, 1.0e12, fr);
        let oph2m2 = h2minus(t, 2.0e14, 1.0e12, fr);
        let oph2m3 = h2minus(t, 1.0e14, 2.0e12, fr);

        // 双倍 anh2 → 双倍 opacity
        assert!(
            (oph2m2 / oph2m1 - 2.0).abs() < 0.01,
            "anh2 线性性: {} vs {}",
            oph2m2,
            oph2m1
        );
        // 双倍 ane → 双倍 opacity (pe 线性)
        assert!(
            (oph2m3 / oph2m1 - 2.0).abs() < 0.01,
            "ane 线性性: {} vs {}",
            oph2m3,
            oph2m1
        );
    }
}