/* Copyright 2002-2025 CS GROUP
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    * contributor license agreements.  See the NOTICE file distributed with
    * this work for additional information regarding copyright ownership.
    * CS licenses this file to You under the Apache License, Version 2.0
    * (the "License"); you may not use this file except in compliance with
    * the License.  You may obtain a copy of the License at
    *
    *   http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
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   * See the License for the specific language governing permissions and
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  package org.orekit.models.earth.ionosphere.nequick;
  
  import org.hipparchus.util.FastMath;
  import org.hipparchus.util.SinCos;
  import org.orekit.time.DateComponents;
  import org.orekit.time.DateTimeComponents;
  import org.orekit.time.TimeComponents;
  
  /**
   * This class performs the computation of the parameters used by the NeQuick model.
   *
   * @author Bryan Cazabonne
   *
   * @see "European Union (2016). European GNSS (Galileo) Open Service-Ionospheric Correction
   *       Algorithm for Galileo Single Frequency Users. 1.2."
   * @see <a href="https://www.itu.int/rec/R-REC-P.531/en">ITU-R P.531</a>
   *
   * @since 10.1
   */
  public class NeQuickParameters {
  
      /** Solar zenith angle at day night transition, degrees. */
      private static final double X0 = 86.23292796211615;
  
      /** Current date time components.
       * @since 13.0
       */
      private final DateTimeComponents dateTime;
  
      /** Effective sunspot number.
       * @since 13.0
       */
      private final double azr;
  
      /** F2 layer critical frequency.
       * @since 13.0
       */
      private final double foF2;
  
      /** F2 layer maximum density. */
      private final double nmF2;
  
      /** F2 layer maximum density height [km]. */
      private final double hmF2;
  
      /** F1 layer maximum density height [km]. */
      private final double hmF1;
  
      /** E layer maximum density height [km]. */
      private final double hmE;
  
      /** F2 layer bottom thickness parameter [km]. */
      private final double b2Bot;
  
      /** F1 layer top thickness parameter [km]. */
      private final double b1Top;
  
      /** F1 layer bottom thickness parameter [km]. */
      private final double b1Bot;
  
      /** E layer top thickness parameter [km]. */
      private final double beTop;
  
      /** E layer bottom thickness parameter [km]. */
      private final double beBot;
  
      /** Layer amplitudes. */
      private final double[] amplitudes;
  
      /**
       * Build a new instance.
       * @param dateTime current date time components
       * @param flattenF2 F2 coefficients used by the F2 layer (flatten array)
       * @param flattenFm3 Fm3 coefficients used by the M(3000)F2 layer (flatten array)
       * @param latitude latitude of a point along the integration path, in radians
       * @param longitude longitude of a point along the integration path, in radians
       * @param az effective ionisation level
       * @param modip modip
       */
      public NeQuickParameters(final DateTimeComponents dateTime,
                               final double[] flattenF2, final double[] flattenFm3,
                               final double latitude, final double longitude, final double az, final double modip) {
          this(new FourierTimeSeries(dateTime, az, flattenF2, flattenFm3),
              latitude, longitude, modip);
     }
 
     /**
      * Build a new instance.
      * @param fourierTimeSeries Fourier time series for foF2 and M(3000)F2 layers
      * @param latitude latitude of a point along the integration path, in radians
      * @param longitude longitude of a point along the integration path, in radians
      * @param modip modip
      * @since 13.0.1
      */
     public NeQuickParameters(final FourierTimeSeries fourierTimeSeries,
                              final double latitude, final double longitude, final double modip) {
 
         this.dateTime = fourierTimeSeries.getDateTime();
         this.azr      = fourierTimeSeries.getAzr();
 
         // Date and Time components
         final DateComponents date = dateTime.getDate();
         final TimeComponents time = dateTime.getTime();
         // Hours
         final double hours  = time.getSecondsInUTCDay() / 3600.0;
         // Effective solar zenith angle in radians
         final double xeff = computeEffectiveSolarAngle(date.getMonth(), hours, latitude, longitude);
 
         // E layer maximum density height in km (Eq. 78)
         this.hmE = 120.0;
         // E layer critical frequency in MHz
         final double foE = computefoE(date.getMonth(), fourierTimeSeries.getAz(), xeff, latitude);
         // E layer maximum density in 10^11 m⁻³ (Eq. 36)
         final double nmE = 0.124 * foE * foE;
 
         // F2 layer critical frequency in MHz
         final double[] scL = FourierTimeSeries.sinCos(longitude, 8);
         this.foF2 = computefoF2(modip, fourierTimeSeries.getCf2Reference(), latitude, scL);
         // Maximum Usable Frequency factor
         final double mF2  = computeMF2(modip, fourierTimeSeries.getCm3Reference(), latitude, scL);
         // F2 layer maximum density in 10^11 m⁻³
         this.nmF2 = 0.124 * foF2 * foF2;
         // F2 layer maximum density height in km
         this.hmF2 = computehmF2(foE, mF2);
 
         // F1 layer critical frequency in MHz
         final double foF1 = computefoF1(foE);
         // F1 layer maximum density in 10^11 m⁻³
         final double nmF1;
         if (foF1 <= 0.0 && foE > 2.0) {
             final double foEpopf = foE + 0.5;
             nmF1 = 0.124 * foEpopf * foEpopf;
         } else {
             nmF1 = 0.124 * foF1 * foF1;
         }
         // F1 layer maximum density height in km
         this.hmF1 = 0.5 * (hmF2 + hmE);
 
         // Thickness parameters (Eq. 85 to 89)
         final double a = 0.01 * clipExp(-3.467 + 0.857 * FastMath.log(foF2 * foF2) + 2.02 * FastMath.log(mF2));
         this.b2Bot = 0.385 * nmF2 / a;
         this.b1Top = 0.3 * (hmF2 - hmF1);
         this.b1Bot = 0.5 * (hmF1 - hmE);
         this.beTop = FastMath.max(b1Bot, 7.0);
         this.beBot = 5.0;
 
         // Layer amplitude coefficients
         this.amplitudes = computeLayerAmplitudes(nmE, nmF1, foF1);
 
     }
 
     /**
      * Get current date time components.
      * @return current date time components
      * @since 13.0
      */
     public DateTimeComponents getDateTime() {
         return dateTime;
     }
 
     /**
      * Get effective sunspot number.
      * @return effective sunspot number
      * @since 13.0
      */
     public double getAzr() {
         return azr;
     }
 
     /**
      * Get F2 layer critical frequency.
      * @return F2 layer critical frequency
      * @since 13.0
      */
     public double getFoF2() {
         return foF2;
     }
 
      /**
      * Get the F2 layer maximum density.
      * @return nmF2
      */
     public double getNmF2() {
         return nmF2;
     }
 
     /**
      * Get the F2 layer maximum density height.
      * @return hmF2 in km
      */
     public double getHmF2() {
         return hmF2;
     }
 
     /**
      * Get the F1 layer maximum density height.
      * @return hmF1 in km
      */
     public double getHmF1() {
         return hmF1;
     }
 
     /**
      * Get the E layer maximum density height.
      * @return hmE in km
      */
     public double getHmE() {
         return hmE;
     }
 
     /**
      * Get the F2 layer thickness parameter (bottom).
      * @return B2Bot in km
      */
     public double getB2Bot() {
         return b2Bot;
     }
 
     /**
      * Get the F1 layer thickness parameter.
      * @param h current height (km)
      * @return B1 in km
      * @since 13.0
      */
     public double getBF1(final double h) {
         // Eq. 110
         return (h > hmF1) ? b1Top : b1Bot;
     }
 
     /**
      * Get the E layer thickness parameter.
      * @param h current height (km)
      * @return Be in km
      * @since 13.0
      */
     public double getBE(final double h) {
         // Eq. 109
         return (h > hmE) ? beTop : beBot;
     }
 
     /**
      * Get the F2, F1 and E layer amplitudes.
      * <p>
      * The resulting element is an array having the following form:
      * <ul>
      * <li>double[0] = A1 → F2 layer amplitude
      * <li>double[1] = A2 → F1 layer amplitude
      * <li>double[2] = A3 → E  layer amplitude
      * </ul>
      * @return layer amplitudes
      */
     public double[] getLayerAmplitudes() {
         return amplitudes.clone();
     }
 
     /**
      * This method computes the effective solar zenith angle.
      * <p>
      * The effective solar zenith angle is compute as a function of the
      * solar zenith angle and the solar zenith angle at day night transition.
      * </p>
      * @param month current month of the year
      * @param hours universal time (hours)
      * @param latitude in radians
      * @param longitude in radians
      * @return the effective solar zenith angle, radians
      */
     private double computeEffectiveSolarAngle(final int month,
                                               final double hours,
                                               final double latitude,
                                               final double longitude) {
         // Local time (Eq.4)
         final double lt = hours + longitude / FastMath.toRadians(15.0);
         // Day of year at the middle of the month (Eq. 20)
         final double dy = 30.5 * month - 15.0;
         // Time (Eq. 21)
         final double t = dy + (18 - hours) / 24;
         // Arguments am and al (Eq. 22 and 23)
         final double am = FastMath.toRadians(0.9856 * t - 3.289);
         final double al = am + FastMath.toRadians(1.916 * FastMath.sin(am) + 0.020 * FastMath.sin(2.0 * am) + 282.634);
         // Sine and cosine of solar declination (Eq. 24 and 25)
         final double sDec = 0.39782 * FastMath.sin(al);
         final double cDec = FastMath.sqrt(1. - sDec * sDec);
         // Solar zenith angle, deg (Eq. 26 and 27)
         final SinCos scLat   = FastMath.sinCos(latitude);
         final double coef    = (FastMath.PI / 12) * (12 - lt);
         final double cZenith = scLat.sin() * sDec + scLat.cos() * cDec * FastMath.cos(coef);
         final double angle   = FastMath.atan2(FastMath.sqrt(1.0 - cZenith * cZenith), cZenith);
         final double x       = FastMath.toDegrees(angle);
         // Effective solar zenith angle (Eq. 28)
         final double xeff = join(90.0 - 0.24 * clipExp(20.0 - 0.2 * x), x, 12.0, x - X0);
         return FastMath.toRadians(xeff);
     }
 
     /**
      * This method computes the E layer critical frequency at a given location.
      * @param month current month
      * @param az ffective ionisation level
      * @param xeff effective solar zenith angle in radians
      * @param latitude latitude in radians
      * @return the E layer critical frequency at a given location in MHz
      */
     private double computefoE(final int month, final double az,
                               final double xeff, final double latitude) {
         // The latitude has to be converted in degrees
         final double lat = FastMath.toDegrees(latitude);
         // Square root of the effective ionisation level
         final double sqAz = FastMath.sqrt(az);
         // seas parameter (Eq. 30 to 32)
         final int seas;
         if (month == 1 || month == 2 || month == 11 || month == 12) {
             seas = -1;
         } else if (month == 3 || month == 4 || month == 9 || month == 10) {
             seas = 0;
         } else {
             seas = 1;
         }
         // Latitudinal dependence (Eq. 33 and 34)
         final double ee = clipExp(0.3 * lat);
         final double seasp = seas * ((ee - 1.0) / (ee + 1.0));
         // Critical frequency (Eq. 35)
         final double coef = 1.112 - 0.019 * seasp;
         return FastMath.sqrt(coef * coef * sqAz * FastMath.pow(FastMath.cos(xeff), 0.6) + 0.49);
 
     }
 
     /**
      * Computes the F2 layer height of maximum electron density.
      * @param foE E layer layer critical frequency in MHz
      * @param mF2 maximum usable frequency factor
      * @return hmF2 in km
      */
     private double computehmF2(final double foE, final double mF2) {
         // Ratio
         final double fo = foF2 / foE;
         final double ratio = join(fo, 1.75, 20.0, fo - 1.75);
 
         // deltaM parameter
         double deltaM = -0.012;
         if (foE >= 1e-30) {
             deltaM += 0.253 / (ratio - 1.215);
         }
 
         // hmF2 Eq. 80
         final double mF2Sq = mF2 * mF2;
         final double temp  = FastMath.sqrt((0.0196 * mF2Sq + 1) / (1.2967 * mF2Sq - 1.0));
         return ((1490.0 * mF2 * temp) / (mF2 + deltaM)) - 176.0;
 
     }
 
     /**
      * This method computes the F2 layer critical frequency.
      * @param modip modified DIP latitude, in degrees
      * @param cf2 Fourier time series for foF2
      * @param latitude latitude in radians
      * @param scL sines/cosines array of longitude argument
      * @return the F2 layer critical frequency, MHz
      */
     private double computefoF2(final double modip, final double[] cf2,
                                final double latitude, final double[] scL) {
 
         // Legendre grades (Eq. 63)
         final int[] q = new int[] {
             12, 12, 9, 5, 2, 1, 1, 1, 1
         };
 
         double frequency = cf2[0];
 
         // ModipGrid coefficients Eq. 57
         final double sinMODIP = FastMath.sin(FastMath.toRadians(modip));
         final double[] m = new double[12];
         m[0] = 1.0;
         for (int i = 1; i < q[0]; i++) {
             m[i] = sinMODIP * m[i - 1];
             frequency += m[i] * cf2[i];
         }
 
         // latitude and longitude terms
         int index = 12;
         final double cosLat1 = FastMath.cos(latitude);
         double cosLatI = cosLat1;
         for (int i = 1; i < q.length; i++) {
             final double c = cosLatI * scL[2 * i - 1];
             final double s = cosLatI * scL[2 * i - 2];
             for (int j = 0; j < q[i]; j++) {
                 frequency += m[j] * c * cf2[index++];
                 frequency += m[j] * s * cf2[index++];
             }
             cosLatI *= cosLat1; // Eq. 58
         }
 
         return frequency;
 
     }
 
     /**
      * This method computes the Maximum Usable Frequency factor.
      * @param modip modified DIP latitude, in degrees
      * @param cm3 Fourier time series for M(3000)F2
      * @param latitude latitude in radians
      * @param scL sines/cosines array of longitude argument
      * @return the Maximum Usable Frequency factor
      */
     private double computeMF2(final double modip, final double[] cm3,
                               final double latitude, final double[] scL) {
 
         // Legendre grades (Eq. 71)
         final int[] r = new int[] {
             7, 8, 6, 3, 2, 1, 1
         };
 
         double m3000 = cm3[0];
 
         // ModipGrid coefficients Eq. 57
         final double sinMODIP = FastMath.sin(FastMath.toRadians(modip));
         final double[] m = new double[12];
         m[0] = 1.0;
         for (int i = 1; i < 12; i++) {
             m[i] = sinMODIP * m[i - 1];
             if (i < 7) {
                 m3000 += m[i] * cm3[i];
             }
         }
 
         // latitude and longitude terms
         int index = 7;
         final double cosLat1 = FastMath.cos(latitude);
         double cosLatI = cosLat1;
         for (int i = 1; i < r.length; i++) {
             final double c = cosLatI * scL[2 * i - 1];
             final double s = cosLatI * scL[2 * i - 2];
             for (int j = 0; j < r[i]; j++) {
                 m3000 += m[j] * c * cm3[index++];
                 m3000 += m[j] * s * cm3[index++];
             }
             cosLatI *= cosLat1; // Eq. 58
         }
 
         return m3000;
 
     }
 
     /**
      * This method computes the F1 layer critical frequency.
      * <p>
      * This computation performs the algorithm exposed in Annex F
      * of the reference document.
      * </p>
      * @param foE the E layer critical frequency, MHz
      * @return the F1 layer critical frequency, MHz
      */
     private double computefoF1(final double foE) {
         final double temp  = join(1.4 * foE, 0.0, 1000.0, foE - 2.0);
         final double temp2 = join(0.0, temp, 1000.0, foE - temp);
         final double value = join(temp2, 0.85 * temp2, 60.0, 0.85 * foF2 - temp2);
         if (value < 1.0E-6) {
             return 0.0;
         } else {
             return value;
         }
     }
 
     /**
      * This method allows the computation of the F2, F1 and E layer amplitudes.
      * <p>
      * The resulting element is an array having the following form:
      * <ul>
      * <li>double[0] = A1 → F2 layer amplitude
      * <li>double[1] = A2 → F1 layer amplitude
      * <li>double[2] = A3 → E  layer amplitude
      * </ul>
      * </p>
      * @param nmE E layer maximum density in 10^11 m⁻³
      * @param nmF1 F1 layer maximum density in 10^11 m⁻³
      * @param foF1 F1 layer critical frequency in MHz
      * @return a three components array containing the layer amplitudes
      */
     private double[] computeLayerAmplitudes(final double nmE, final double nmF1, final double foF1) {
         // Initialize array
         final double[] amplitude = new double[3];
 
         // F2 layer amplitude (Eq. 90)
         final double a1 = 4.0 * nmF2;
         amplitude[0]   = a1;
 
         // F1 and E layer amplitudes (Eq. 91 to 98)
         if (foF1 < 0.5) {
             amplitude[1] = 0.0;
             amplitude[2] = 4.0 * (nmE - epst(a1, hmF2, b2Bot, hmE));
         } else {
             double a2a = 0.0;
             double a3a = 4.0 * nmE;
             for (int i = 0; i < 5; i++) {
                 a2a = 4.0 * (nmF1 - epst(a1, hmF2, b2Bot, hmF1) - epst(a3a, hmE, beTop, hmF1));
                 a2a = join(a2a, 0.8 * nmF1, 1.0, a2a - 0.8 * nmF1);
                 a3a = 4.0 * (nmE - epst(a2a, hmF1, b1Bot, hmE) - epst(a1, hmF2, b2Bot, hmE));
             }
             amplitude[1] = a2a;
             amplitude[2] = join(a3a, 0.05, 60.0, a3a - 0.005);
         }
 
         return amplitude;
     }
 
     /**
      * A clipped exponential function.
      * <p>
      * This function, describe in section F.2.12.2 of the reference document, is
      * recommanded for the computation of exponential values.
      * </p>
      * @param power power for exponential function
      * @return clipped exponential value
      */
     private double clipExp(final double power) {
         if (power > 80.0) {
             return 5.5406E34;
         } else if (power < -80) {
             return 1.8049E-35;
         } else {
             return FastMath.exp(power);
         }
     }
 
     /**
      * Allows smooth joining of functions f1 and f2
      * (i.e. continuous first derivatives) at origin.
      * <p>
      * This function, describe in section F.2.12.1 of the reference document, is
      * recommended for computational efficiency.
      * </p>
      * @param dF1 first function
      * @param dF2 second function
      * @param dA width of transition region
      * @param dX x value
      * @return the computed value
      */
     double join(final double dF1, final double dF2, final double dA, final double dX) {
         final double ee = clipExp(dA * dX);
         return (dF1 * ee + dF2) / (ee + 1.0);
     }
 
     /**
      * The Epstein function.
      * <p>
      * This function, describe in section 2.5.1 of the reference document, is used
      * as a basis analytical function in NeQuick for the construction of the ionospheric layers.
      * </p>
      * @param x x parameter
      * @param y y parameter
      * @param z z parameter
      * @param w w parameter
      * @return value of the epstein function
      */
     private double epst(final double x, final double y,
                         final double z, final double w) {
         final double ex  = clipExp((w - y) / z);
         final double opex = 1.0 + ex;
         return x * ex / (opex * opex);
 
     }
 
 }