nsb 0.1.0

Night-sky background evaluation and observation planning for ground-based astronomy.
Documentation 
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// SPDX-License-Identifier: AGPL-3.0-only
// Copyright (C) 2026 Vallés Puig, Ramon
//! Shared atmospheric assumptions for site-aware NSB components.
//!
//! These conditions are intentionally component-neutral. Moonlight and airglow
//! both consume the same pressure, Rayleigh, and aerosol assumptions selected by
//! a resolved site profile.

use crate::{NsbError, Result};

use siderust::atmosphere::{
    rayleigh_optical_depth_bodhaine99, AtmosphereProfile, MieParams, DEFAULT_SCALE_HEIGHT,
};
use siderust::coordinates::centers::Geodetic;
use siderust::coordinates::frames::ECEF;
use siderust::qtty::{Hectopascals, Kilometers, Nanometers, OpticalDepths};

/// Atmospheric inputs shared by NSB components that model scattering.
///
/// Observer altitude is deliberately not stored here. Site geometry remains
/// tied to the [`Geodetic`] observer passed to the component or evaluator.
#[derive(Debug, Clone, Copy, PartialEq)]
#[non_exhaustive]
pub struct AtmosphericConditions {
    /// Surface pressure used for Rayleigh scattering.
    pub surface_pressure: Hectopascals,
    /// Rayleigh atmospheric scale height.
    pub rayleigh_scale_height: Kilometers,
    /// Aerosol optical-depth and phase-function parameters.
    pub mie_params: MieParams,
}

/// Bodhaine Rayleigh optical depth for conditions whose pressure is already local.
///
/// `AtmosphericConditions::surface_pressure` represents the local atmospheric
/// column selected by the site profile. Siderust's Bodhaine helper also applies
/// an exponential reduction from observer altitude, so supplying both local
/// pressure and the query altitude would reduce the Rayleigh column twice.
pub(crate) fn rayleigh_optical_depth_local_pressure(
    wavelength: Nanometers,
    atmosphere: AtmosphericConditions,
) -> OpticalDepths {
    rayleigh_optical_depth_bodhaine99(
        wavelength,
        atmosphere.surface_pressure,
        Kilometers::new(0.0),
        atmosphere.rayleigh_scale_height,
    )
}

impl AtmosphericConditions {
    /// Convert a Siderust profile while intentionally discarding its altitude.
    pub fn from_profile_without_altitude(profile: AtmosphereProfile) -> Self {
        Self {
            surface_pressure: profile.surface_pressure,
            rayleigh_scale_height: profile.rayleigh_scale_height,
            mie_params: profile.mie_params,
        }
    }

    /// Generic clear-sky conditions for an arbitrary location.
    ///
    /// Pressure is estimated from the supplied altitude and the aerosol
    /// parameters use the generic Paranal-like clear-sky values available from
    /// Siderust. This is not a named-site calibration.
    pub fn generic_clear_sky(location: Geodetic<ECEF>) -> Self {
        let altitude_m = location.height.value().max(0.0);
        let pressure = 1013.25 * (-altitude_m / 8_400.0).exp();
        Self {
            surface_pressure: Hectopascals::new(pressure),
            rayleigh_scale_height: DEFAULT_SCALE_HEIGHT,
            mie_params: MieParams::PARANAL,
        }
    }

    /// Clear-sky Mie/Rayleigh defaults with an explicit surface pressure.
    ///
    /// Use this when constructing caller-defined planning profiles that fix
    /// pressure independently of the query observer altitude.
    pub fn clear_sky_with_pressure(surface_pressure: Hectopascals) -> Result<Self> {
        let conditions = Self {
            surface_pressure,
            rayleigh_scale_height: DEFAULT_SCALE_HEIGHT,
            mie_params: MieParams::PARANAL,
        };
        conditions.validate()?;
        Ok(conditions)
    }

    /// Paranal-like average clear-sky conditions from Siderust's built-in profile.
    pub fn paranal_average() -> Self {
        Self::from_profile_without_altitude(AtmosphereProfile::EL_PARANAL)
    }

    pub(crate) fn validate(&self) -> Result<()> {
        validate_positive_finite(self.surface_pressure.value(), "surface pressure")?;
        validate_positive_finite(self.rayleigh_scale_height.value(), "Rayleigh scale height")?;
        let mie = self.mie_params;
        if !mie.tau0.value().is_finite() || mie.tau0.value() < 0.0 {
            return Err(NsbError::OutOfRange(
                "Mie optical depth must be finite and nonnegative".to_owned(),
            ));
        }
        if !mie.alpha.is_finite() {
            return Err(NsbError::OutOfRange(
                "Mie wavelength exponent must be finite".to_owned(),
            ));
        }
        validate_positive_finite(mie.lambda_ref.value(), "Mie reference wavelength")
    }
}

fn validate_positive_finite(value: f64, field: &str) -> Result<()> {
    if !value.is_finite() || value <= 0.0 {
        return Err(NsbError::OutOfRange(format!(
            "{field} must be finite and greater than zero"
        )));
    }
    Ok(())
}