pvlib-rust 0.1.6

//! DIRINT (Perez 1992) multiplier coefficient tensor, ported verbatim from
//! `pvlib-python::pvlib.irradiance._get_dirint_coeffs`.
//!
//! Shape: `[kt_prime_bin][zenith_bin][delta_kt_prime_bin][w_bin]`
//! with dimensions `[6][6][7][5]`.
//!
//! Bin edges (matching pvlib-python, 1-indexed there / 0-indexed here):
//!
//! * `kt_prime_bin`: `[0, 0.24), [0.24, 0.4), [0.4, 0.56), [0.56, 0.7), [0.7, 0.8), [0.8, 1]`
//! * `zenith_bin`: `[0, 25°), [25, 40°), [40, 55°), [55, 70°), [70, 80°), [80°, ∞)`
//! * `delta_kt'_bin`: `[0, 0.015), [0.015, 0.035), [0.035, 0.07), [0.07, 0.15), [0.15, 0.3), [0.3, 1]`,
//!   plus a fallback bin 6 (0-indexed) for `delta_kt' == -1`.
//! * `w_bin`: `[0, 1) cm, [1, 2), [2, 3), [3, ∞)`, plus fallback bin 4 for `w == -1`.
//!
//! Source coefficients: Perez, R., Ineichen, P., Maxwell, E., Seals, R. and
//! Zelenka, A. (1992). "Dynamic Global-to-Direct Irradiance Conversion
//! Models." ASHRAE Transactions-Research Series, pp. 354–369.

/// DIRINT coefficient tensor. A value of `f64::NAN` at `[i][j][k][l]` marks
/// an unassigned bin (treated as "no correction" by callers).
pub(crate) const DIRINT_COEFFS: [[[[f64; 5]; 7]; 6]; 6] = [
    // kt_prime_bin = 0  (0.00–0.24)
    [
        // zenith_bin = 0  (0–25°)
        [
            [0.385230, 0.385230, 0.385230, 0.462880, 0.317440],
            [0.338390, 0.338390, 0.221270, 0.316730, 0.503650],
            [0.235680, 0.235680, 0.241280, 0.157830, 0.269440],
            [0.830130, 0.830130, 0.171970, 0.841070, 0.457370],
            [0.548010, 0.548010, 0.478000, 0.966880, 1.036370],
            [0.548010, 0.548010, 1.000000, 3.012370, 1.976540],
            [0.582690, 0.582690, 0.229720, 0.892710, 0.569950],
        ],
        // zenith_bin = 1  (25–40°)
        [
            [0.131280, 0.131280, 0.385460, 0.511070, 0.127940],
            [0.223710, 0.223710, 0.193560, 0.304560, 0.193940],
            [0.229970, 0.229970, 0.275020, 0.312730, 0.244610],
            [0.090100, 0.184580, 0.260500, 0.687480, 0.579440],
            [0.131530, 0.131530, 0.370190, 1.380350, 1.052270],
            [1.116250, 1.116250, 0.928030, 3.525490, 2.316920],
            [0.090100, 0.237000, 0.300040, 0.812470, 0.664970],
        ],
        // zenith_bin = 2  (40–55°)
        [
            [0.587510, 0.130000, 0.400000, 0.537210, 0.832490],
            [0.306210, 0.129830, 0.204460, 0.500000, 0.681640],
            [0.224020, 0.260620, 0.334080, 0.501040, 0.350470],
            [0.421540, 0.753970, 0.750660, 3.706840, 0.983790],
            [0.706680, 0.373530, 1.245670, 0.864860, 1.992630],
            [4.864400, 0.117390, 0.265180, 0.359180, 3.310820],
            [0.392080, 0.493290, 0.651560, 1.932780, 0.898730],
        ],
        // zenith_bin = 3  (55–70°)
        [
            [0.126970, 0.126970, 0.126970, 0.126970, 0.126970],
            [0.810820, 0.810820, 0.810820, 0.810820, 0.810820],
            [3.241680, 2.500000, 2.291440, 2.291440, 2.291440],
            [4.000000, 3.000000, 2.000000, 0.975430, 1.965570],
            [12.494170, 12.494170, 8.000000, 5.083520, 8.792390],
            [21.744240, 21.744240, 21.744240, 21.744240, 21.744240],
            [3.241680, 12.494170, 1.620760, 1.375250, 2.331620],
        ],
        // zenith_bin = 4  (70–80°)
        [
            [0.126970, 0.126970, 0.126970, 0.126970, 0.126970],
            [0.810820, 0.810820, 0.810820, 0.810820, 0.810820],
            [3.241680, 2.500000, 2.291440, 2.291440, 2.291440],
            [4.000000, 3.000000, 2.000000, 0.975430, 1.965570],
            [12.494170, 12.494170, 8.000000, 5.083520, 8.792390],
            [21.744240, 21.744240, 21.744240, 21.744240, 21.744240],
            [3.241680, 12.494170, 1.620760, 1.375250, 2.331620],
        ],
        // zenith_bin = 5  (80°–∞)
        [
            [0.126970, 0.126970, 0.126970, 0.126970, 0.126970],
            [0.810820, 0.810820, 0.810820, 0.810820, 0.810820],
            [3.241680, 2.500000, 2.291440, 2.291440, 2.291440],
            [4.000000, 3.000000, 2.000000, 0.975430, 1.965570],
            [12.494170, 12.494170, 8.000000, 5.083520, 8.792390],
            [21.744240, 21.744240, 21.744240, 21.744240, 21.744240],
            [3.241680, 12.494170, 1.620760, 1.375250, 2.331620],
        ],
    ],
    // kt_prime_bin = 1  (0.24–0.4)
    [
        [
            [0.337440, 0.337440, 0.969110, 1.097190, 1.116080],
            [0.337440, 0.337440, 0.969110, 1.116030, 0.623900],
            [0.337440, 0.337440, 1.530590, 1.024420, 0.908480],
            [0.584040, 0.584040, 0.847250, 0.914940, 1.289300],
            [0.337440, 0.337440, 0.310240, 1.435020, 1.852830],
            [0.337440, 0.337440, 1.015010, 1.097190, 2.117230],
            [0.337440, 0.337440, 0.969110, 1.145730, 1.476400],
        ],
        [
            [0.300000, 0.300000, 0.700000, 1.100000, 0.796940],
            [0.219870, 0.219870, 0.526530, 0.809610, 0.649300],
            [0.386650, 0.386650, 0.119320, 0.576120, 0.685460],
            [0.746730, 0.399830, 0.470970, 0.986530, 0.785370],
            [0.575420, 0.936700, 1.649200, 1.495840, 1.335590],
            [1.319670, 4.002570, 1.276390, 2.644550, 2.518670],
            [0.665190, 0.678910, 1.012360, 1.199940, 0.986580],
        ],
        [
            [0.378870, 0.974060, 0.500000, 0.491880, 0.665290],
            [0.105210, 0.263470, 0.407040, 0.553460, 0.582590],
            [0.312900, 0.345240, 1.144180, 0.854790, 0.612280],
            [0.119070, 0.365120, 0.560520, 0.793720, 0.802600],
            [0.781610, 0.837390, 1.270420, 1.537980, 1.292950],
            [1.152290, 1.152290, 1.492080, 1.245370, 2.177100],
            [0.424660, 0.529550, 0.966910, 1.033460, 0.958730],
        ],
        [
            [0.310590, 0.714410, 0.252450, 0.500000, 0.607600],
            [0.975190, 0.363420, 0.500000, 0.400000, 0.502800],
            [0.175580, 0.196250, 0.476360, 1.072470, 0.490510],
            [0.719280, 0.698620, 0.657770, 1.190840, 0.681110],
            [0.426240, 1.464840, 0.678550, 1.157730, 0.978430],
            [2.501120, 1.789130, 1.387090, 2.394180, 2.394180],
            [0.491640, 0.677610, 0.685610, 1.082400, 0.735410],
        ],
        [
            [0.597000, 0.500000, 0.300000, 0.310050, 0.413510],
            [0.314790, 0.336310, 0.400000, 0.400000, 0.442460],
            [0.166510, 0.460440, 0.552570, 1.000000, 0.461610],
            [0.401020, 0.559110, 0.403630, 1.016710, 0.671490],
            [0.400360, 0.750830, 0.842640, 1.802600, 1.023830],
            [3.315300, 1.510380, 2.443650, 1.638820, 2.133990],
            [0.530790, 0.745850, 0.693050, 1.458040, 0.804500],
        ],
        [
            [0.597000, 0.500000, 0.300000, 0.310050, 0.800920],
            [0.314790, 0.336310, 0.400000, 0.400000, 0.237040],
            [0.166510, 0.460440, 0.552570, 1.000000, 0.581990],
            [0.401020, 0.559110, 0.403630, 1.016710, 0.898570],
            [0.400360, 0.750830, 0.842640, 1.802600, 3.400390],
            [3.315300, 1.510380, 2.443650, 1.638820, 2.508780],
            [0.204340, 1.157740, 2.003080, 2.622080, 1.409380],
        ],
    ],
    // kt_prime_bin = 2  (0.4–0.56)
    [
        [
            [1.242210, 1.242210, 1.242210, 1.242210, 1.242210],
            [0.056980, 0.056980, 0.656990, 0.656990, 0.925160],
            [0.089090, 0.089090, 1.040430, 1.232480, 1.205300],
            [1.053850, 1.053850, 1.399690, 1.084640, 1.233340],
            [1.151540, 1.151540, 1.118290, 1.531640, 1.411840],
            [1.494980, 1.494980, 1.700000, 1.800810, 1.671600],
            [1.018450, 1.018450, 1.153600, 1.321890, 1.294670],
        ],
        [
            [0.700000, 0.700000, 1.023460, 0.700000, 0.945830],
            [0.886300, 0.886300, 1.333620, 0.800000, 1.066620],
            [0.902180, 0.902180, 0.954330, 1.126690, 1.097310],
            [1.095300, 1.075060, 1.176490, 1.139470, 1.096110],
            [1.201660, 1.201660, 1.438200, 1.256280, 1.198060],
            [1.525850, 1.525850, 1.869160, 1.985410, 1.911590],
            [1.288220, 1.082810, 1.286370, 1.166170, 1.119330],
        ],
        [
            [0.600000, 1.029910, 0.859890, 0.550000, 0.813600],
            [0.604450, 1.029910, 0.859890, 0.656700, 0.928840],
            [0.455850, 0.750580, 0.804930, 0.823000, 0.911000],
            [0.526580, 0.932310, 0.908620, 0.983520, 0.988090],
            [1.036110, 1.100690, 0.848380, 1.035270, 1.042380],
            [1.048440, 1.652720, 0.900000, 2.350410, 1.082950],
            [0.817410, 0.976160, 0.861300, 0.974780, 1.004580],
        ],
        [
            [0.782110, 0.564280, 0.600000, 0.600000, 0.665740],
            [0.894480, 0.680730, 0.541990, 0.800000, 0.669140],
            [0.487460, 0.818950, 0.841830, 0.872540, 0.709040],
            [0.709310, 0.872780, 0.908480, 0.953290, 0.844350],
            [0.863920, 0.947770, 0.876220, 1.078750, 0.936910],
            [1.280350, 0.866720, 0.769790, 1.078750, 0.975130],
            [0.725420, 0.869970, 0.868810, 0.951190, 0.829220],
        ],
        [
            [0.791750, 0.654040, 0.483170, 0.409000, 0.597180],
            [0.566140, 0.948990, 0.971820, 0.653570, 0.718550],
            [0.648710, 0.637730, 0.870510, 0.860600, 0.694300],
            [0.637630, 0.767610, 0.925670, 0.990310, 0.847670],
            [0.736380, 0.946060, 1.117590, 1.029340, 0.947020],
            [1.180970, 0.850000, 1.050000, 0.950000, 0.888580],
            [0.700560, 0.801440, 0.961970, 0.906140, 0.823880],
        ],
        [
            [0.500000, 0.500000, 0.586770, 0.470550, 0.629790],
            [0.500000, 0.500000, 1.056220, 1.260140, 0.658140],
            [0.500000, 0.500000, 0.631830, 0.842620, 0.582780],
            [0.554710, 0.734730, 0.985820, 0.915640, 0.898260],
            [0.712510, 1.205990, 0.909510, 1.078260, 0.885610],
            [1.899260, 1.559710, 1.000000, 1.150000, 1.120390],
            [0.653880, 0.793120, 0.903320, 0.944070, 0.796130],
        ],
    ],
    // kt_prime_bin = 3  (0.56–0.7)
    [
        [
            [1.000000, 1.000000, 1.050000, 1.170380, 1.178090],
            [0.960580, 0.960580, 1.059530, 1.179030, 1.131690],
            [0.871470, 0.871470, 0.995860, 1.141910, 1.114600],
            [1.201590, 1.201590, 0.993610, 1.109380, 1.126320],
            [1.065010, 1.065010, 0.828660, 0.939970, 1.017930],
            [1.065010, 1.065010, 0.623690, 1.119620, 1.132260],
            [1.071570, 1.071570, 0.958070, 1.114130, 1.127110],
        ],
        [
            [0.950000, 0.973390, 0.852520, 1.092200, 1.096590],
            [0.804120, 0.913870, 0.980990, 1.094580, 1.042420],
            [0.737540, 0.935970, 0.999940, 1.056490, 1.050060],
            [1.032980, 1.034540, 0.968460, 1.032080, 1.015780],
            [0.900000, 0.977210, 0.945960, 1.008840, 0.969960],
            [0.600000, 0.750000, 0.750000, 0.844710, 0.899100],
            [0.926800, 0.965030, 0.968520, 1.044910, 1.032310],
        ],
        [
            [0.850000, 1.029710, 0.961100, 1.055670, 1.009700],
            [0.818530, 0.960010, 0.996450, 1.081970, 1.036470],
            [0.765380, 0.953500, 0.948260, 1.052110, 1.000140],
            [0.775610, 0.909610, 0.927800, 0.987800, 0.952100],
            [1.000990, 0.881880, 0.875950, 0.949100, 0.893690],
            [0.902370, 0.875960, 0.807990, 0.942410, 0.917920],
            [0.856580, 0.928270, 0.946820, 1.032260, 0.972990],
        ],
        [
            [0.750000, 0.857930, 0.983800, 1.056540, 0.980240],
            [0.750000, 0.987010, 1.013730, 1.133780, 1.038250],
            [0.800000, 0.947380, 1.012380, 1.091270, 0.999840],
            [0.800000, 0.914550, 0.908570, 0.999190, 0.915230],
            [0.778540, 0.800590, 0.799070, 0.902180, 0.851560],
            [0.680190, 0.317410, 0.507680, 0.388910, 0.646710],
            [0.794920, 0.912780, 0.960830, 1.057110, 0.947950],
        ],
        [
            [0.750000, 0.833890, 0.867530, 1.059890, 0.932840],
            [0.979700, 0.971470, 0.995510, 1.068490, 1.030150],
            [0.858850, 0.987920, 1.043220, 1.108700, 1.044900],
            [0.802400, 0.955110, 0.911660, 1.045070, 0.944470],
            [0.884890, 0.766210, 0.885390, 0.859070, 0.818190],
            [0.615680, 0.700000, 0.850000, 0.624620, 0.669300],
            [0.835570, 0.946150, 0.977090, 1.049350, 0.979970],
        ],
        [
            [0.689220, 0.809600, 0.900000, 0.789500, 0.853990],
            [0.854660, 0.852840, 0.938200, 0.923110, 0.955010],
            [0.938600, 0.932980, 1.010390, 1.043950, 1.041640],
            [0.843620, 0.981300, 0.951590, 0.946100, 0.966330],
            [0.694740, 0.814690, 0.572650, 0.400000, 0.726830],
            [0.211370, 0.671780, 0.416340, 0.297290, 0.498050],
            [0.843540, 0.882330, 0.911760, 0.898420, 0.960210],
        ],
    ],
    // kt_prime_bin = 4  (0.7–0.8)
    [
        [
            [1.054880, 1.075210, 1.068460, 1.153370, 1.069220],
            [1.000000, 1.062220, 1.013470, 1.088170, 1.046200],
            [0.885090, 0.993530, 0.942590, 1.054990, 1.012740],
            [0.920000, 0.950000, 0.978720, 1.020280, 0.984440],
            [0.850000, 0.908500, 0.839940, 0.985570, 0.962180],
            [0.800000, 0.800000, 0.810080, 0.950000, 0.961550],
            [1.038590, 1.063200, 1.034440, 1.112780, 1.037800],
        ],
        [
            [1.017610, 1.028360, 1.058960, 1.133180, 1.045620],
            [0.920000, 0.998970, 1.033590, 1.089030, 1.022060],
            [0.912370, 0.949930, 0.979770, 1.020420, 0.981770],
            [0.847160, 0.935300, 0.930540, 0.955050, 0.946560],
            [0.880260, 0.867110, 0.874130, 0.972650, 0.883420],
            [0.627150, 0.627150, 0.700000, 0.774070, 0.845130],
            [0.973700, 1.006240, 1.026190, 1.071960, 1.017240],
        ],
        [
            [1.028710, 1.017570, 1.025900, 1.081790, 1.024240],
            [0.924980, 0.985500, 1.014100, 1.092210, 0.999610],
            [0.828570, 0.934920, 0.994950, 1.024590, 0.949710],
            [0.900810, 0.901330, 0.928830, 0.979570, 0.913100],
            [0.761030, 0.845150, 0.805360, 0.936790, 0.853460],
            [0.626400, 0.546750, 0.730500, 0.850000, 0.689050],
            [0.957630, 0.985480, 0.991790, 1.050220, 0.987900],
        ],
        [
            [0.992730, 0.993880, 1.017150, 1.059120, 1.017450],
            [0.975610, 0.987160, 1.026820, 1.075440, 1.007250],
            [0.871090, 0.933190, 0.974690, 0.979840, 0.952730],
            [0.828750, 0.868090, 0.834920, 0.905510, 0.871530],
            [0.781540, 0.782470, 0.767910, 0.764140, 0.795890],
            [0.743460, 0.693390, 0.514870, 0.630150, 0.715660],
            [0.934760, 0.957870, 0.959640, 0.972510, 0.981640],
        ],
        [
            [0.965840, 0.941240, 0.987100, 1.022540, 1.011160],
            [0.988630, 0.994770, 0.976590, 0.950000, 1.034840],
            [0.958200, 1.018080, 0.974480, 0.920000, 0.989870],
            [0.811720, 0.869090, 0.812020, 0.850000, 0.821050],
            [0.682030, 0.679480, 0.632450, 0.746580, 0.738550],
            [0.668290, 0.445860, 0.500000, 0.678920, 0.696510],
            [0.926940, 0.953350, 0.959050, 0.876210, 0.991490],
        ],
        [
            [0.948940, 0.997760, 0.850000, 0.826520, 0.998470],
            [1.017860, 0.970000, 0.850000, 0.700000, 0.988560],
            [1.000000, 0.950000, 0.850000, 0.606240, 0.947260],
            [1.000000, 0.746140, 0.751740, 0.598390, 0.725230],
            [0.922210, 0.500000, 0.376800, 0.517110, 0.548630],
            [0.500000, 0.450000, 0.429970, 0.404490, 0.539940],
            [0.960430, 0.881630, 0.775640, 0.596350, 0.937680],
        ],
    ],
    // kt_prime_bin = 5  (0.8–1.0)
    [
        [
            [1.030000, 1.040000, 1.000000, 1.000000, 1.049510],
            [1.050000, 0.990000, 0.990000, 0.950000, 0.996530],
            [1.050000, 0.990000, 0.990000, 0.820000, 0.971940],
            [1.050000, 0.790000, 0.880000, 0.820000, 0.951840],
            [1.000000, 0.530000, 0.440000, 0.710000, 0.928730],
            [0.540000, 0.470000, 0.500000, 0.550000, 0.773950],
            [1.038270, 0.920180, 0.910930, 0.821140, 1.034560],
        ],
        [
            [1.041020, 0.997520, 0.961600, 1.000000, 1.035780],
            [0.948030, 0.980000, 0.900000, 0.950360, 0.977460],
            [0.950000, 0.977250, 0.869270, 0.800000, 0.951680],
            [0.951870, 0.850000, 0.748770, 0.700000, 0.883850],
            [0.900000, 0.823190, 0.727450, 0.600000, 0.839870],
            [0.850000, 0.805020, 0.692310, 0.500000, 0.788410],
            [1.010090, 0.895270, 0.773030, 0.816280, 1.011680],
        ],
        [
            [1.022450, 1.004600, 0.983650, 1.000000, 1.032940],
            [0.943960, 0.999240, 0.983920, 0.905990, 0.978150],
            [0.936240, 0.946480, 0.850000, 0.850000, 0.930320],
            [0.816420, 0.885000, 0.644950, 0.817650, 0.865310],
            [0.742960, 0.765690, 0.561520, 0.700000, 0.827140],
            [0.643870, 0.596710, 0.474460, 0.600000, 0.651200],
            [0.971740, 0.940560, 0.714880, 0.864380, 1.001650],
        ],
        [
            [0.995260, 0.977010, 1.000000, 1.000000, 1.035250],
            [0.939810, 0.975250, 0.939980, 0.950000, 0.982550],
            [0.876870, 0.879440, 0.850000, 0.900000, 0.917810],
            [0.873480, 0.873450, 0.751470, 0.850000, 0.863040],
            [0.761470, 0.702360, 0.638770, 0.750000, 0.783120],
            [0.734080, 0.650000, 0.600000, 0.650000, 0.715660],
            [0.942160, 0.919100, 0.770340, 0.731170, 0.995180],
        ],
        [
            [0.952560, 0.916780, 0.920000, 0.900000, 1.005880],
            [0.928620, 0.994420, 0.900000, 0.900000, 0.983720],
            [0.913070, 0.850000, 0.850000, 0.800000, 0.924280],
            [0.868090, 0.807170, 0.823550, 0.600000, 0.844520],
            [0.769570, 0.719870, 0.650000, 0.550000, 0.733500],
            [0.580250, 0.650000, 0.600000, 0.500000, 0.628850],
            [0.904770, 0.852650, 0.708370, 0.493730, 0.949030],
        ],
        [
            [0.911970, 0.800000, 0.800000, 0.800000, 0.956320],
            [0.912620, 0.682610, 0.750000, 0.700000, 0.950110],
            [0.653450, 0.659330, 0.700000, 0.600000, 0.856110],
            [0.648440, 0.600000, 0.641120, 0.500000, 0.695780],
            [0.570000, 0.550000, 0.598800, 0.400000, 0.560150],
            [0.475230, 0.500000, 0.518640, 0.339970, 0.520230],
            [0.743440, 0.592190, 0.603060, 0.316930, 0.794390],
        ],
    ],
];

/// Assign the kt' bin index (0-indexed, returns `None` for unassigned).
#[inline]
pub(crate) fn kt_prime_bin(kt_prime: f64) -> Option<usize> {
    if (0.0..0.24).contains(&kt_prime) { Some(0) }
    else if (0.24..0.4).contains(&kt_prime) { Some(1) }
    else if (0.4..0.56).contains(&kt_prime) { Some(2) }
    else if (0.56..0.7).contains(&kt_prime) { Some(3) }
    else if (0.7..0.8).contains(&kt_prime) { Some(4) }
    else if (0.8..=1.0).contains(&kt_prime) { Some(5) }
    else { None }
}

/// Assign the zenith bin index (degrees). Returns `None` for unassigned.
#[inline]
pub(crate) fn zenith_bin(zenith_deg: f64) -> Option<usize> {
    if !zenith_deg.is_finite() || zenith_deg < 0.0 { return None; }
    if zenith_deg < 25.0 { Some(0) }
    else if zenith_deg < 40.0 { Some(1) }
    else if zenith_deg < 55.0 { Some(2) }
    else if zenith_deg < 70.0 { Some(3) }
    else if zenith_deg < 80.0 { Some(4) }
    else { Some(5) }
}

/// Assign the precipitable-water bin index. `w == -1.0` → fallback bin 4
/// (used when dew-point data is unavailable, matching pvlib-python).
#[inline]
pub(crate) fn w_bin(w: f64) -> Option<usize> {
    if w == -1.0 { return Some(4); }
    if !w.is_finite() || w < 0.0 { return None; }
    if w < 1.0 { Some(0) }
    else if w < 2.0 { Some(1) }
    else if w < 3.0 { Some(2) }
    else { Some(3) }
}

/// Assign the delta_kt' bin index. `delta_kt' == -1.0` → fallback bin 6
/// (used when time-series context is not available, matching pvlib-python).
#[inline]
pub(crate) fn delta_kt_prime_bin(delta_kt_prime: f64) -> Option<usize> {
    if delta_kt_prime == -1.0 { return Some(6); }
    if !delta_kt_prime.is_finite() || delta_kt_prime < 0.0 { return None; }
    if delta_kt_prime < 0.015 { Some(0) }
    else if delta_kt_prime < 0.035 { Some(1) }
    else if delta_kt_prime < 0.07 { Some(2) }
    else if delta_kt_prime < 0.15 { Some(3) }
    else if delta_kt_prime < 0.3 { Some(4) }
    else if delta_kt_prime <= 1.0 { Some(5) }
    else { None }
}

/// Look up the DIRINT multiplier for a given set of bin indices.
/// Returns `None` (caller falls back to 1.0, i.e. no correction) if any
/// bin is unassigned.
#[inline]
pub(crate) fn coefficient(
    kt_prime: f64,
    zenith_deg: f64,
    delta_kt_prime: f64,
    w: f64,
) -> Option<f64> {
    let kt_bin = kt_prime_bin(kt_prime)?;
    let z_bin = zenith_bin(zenith_deg)?;
    let dkt_bin = delta_kt_prime_bin(delta_kt_prime)?;
    let w_b = w_bin(w)?;
    Some(DIRINT_COEFFS[kt_bin][z_bin][dkt_bin][w_b])
}