Util.java

package aaronsantos.culturetests;

public class Util
{
  
  // Cell length in meters
  static final float l = 1;
  
  // Cell area in meters^2
  static final float A = l*l;
  
  public static double fastpow(final double a, final double b)
  {
      final long tmp = Double.doubleToLongBits(a);
      final long tmp2 = (long)(b * (tmp - 4606921280493453312L)) + 4606921280493453312L;
      return Double.longBitsToDouble(tmp2);
  }
  
  public static double random(float low, float high)
  {
      return (high - low) * Math.random() + low;
  }
  
  // http://en.wikipedia.org/wiki/Poisson_distribution#Generating_Poisson-distributed_random_variables
  public static double poissonRandom(float expectedValue)
  {
    float l = (float)fastpow(Math.E, -expectedValue);
    int k = 0;
    float p = 1;
    do
    {
      k++;
      p*=Math.random();
    }
    while (p > l);
    return k - 1;
  }
  /**
   * Calculate pressure in Pa given height in m.
   * @see http://en.wikipedia.org/wiki/Atmospheric_pressure
   */
  public static float pressurePaByHeightM(float h)
  {
    return  101.325f*(float)fastpow(1.0-0.0065*h/288.15, (8.80665*0.0289)/(8.3144/0.0065));
  }
  
  /**
   * Atmospheric temperature by height in meters.
   * @see http://www.kansasflyer.org/index.asp?nav=Avi&sec=Alti&tab=Theory&pg=2
   * @param h Height in meters above sea level.
   * @param lattitude [0,1), 0 is most northern, 1 is most southern.
   * @return The atmospheric temperature at this height in Kelvin.
   */
  public static float temperatureByHeightAndLattitudeAndTime(float h, float v, float minutesElapsedInDay)
  {
    return temperatureByHeightAndLattitudeAndTime(h, v, 0, minutesElapsedInDay);
  }
  
  public static float temperatureByHeightAndLattitudeAndTime(float h, float v, float humidity, float minutesElapsedInDay)
  {
    // temperature in C.
    float Tc = 15 + (-6.5f * (h-3200)/1000);
    
    float lattitudeAdjustmentC = (float)(80.0f*Math.abs(v-0.5)-15);
    
    // Under clouds? Let the clouds block the light, cooler temperature.
    float humidityAdjustment = 0;//humidity> 100 ? 20*humidity/100:0;
    
    // Adjust for day/night temperatures +/- 7C
    float diurnalAdjustment = 0;
    float temperatureVariance = -7;
    diurnalAdjustment = (float)(temperatureVariance * Math.sin(minutesElapsedInDay*2*Math.PI/720));
    
    // Temperature in K.
    // Don't return a negative number, absolute zero and all.
    return Math.max(0.01f, (Tc+273-lattitudeAdjustmentC-humidityAdjustment)+diurnalAdjustment);
  }
  
  /**
   * Calculate the partial pressure of water vapor required to saturate air.
   * Shamelessly copied from
   * http://www.nco.ncep.noaa.gov/pmb/codes/nwprod/rap.v1.0.6/sorc/rap_gsi.fd/gsdcloud/adaslib.f90
   * @param Ps the air pressure in Pascals
   * @param T the air temperature in Kelvin
   * @return The partial pressure of water vapor required to saturate the air in Pascals
   */
  public static float waterVaporSaturationThreshold(float Ps, float T)
  {
    final float satfwa = 1.0007f;
    final float satfwb = 3.46E-8f;
    final float satewa = 611.21f;
    final float satewb = 17.502f;
    final float satewc = 32.18f;
    
    final float f = satfwa + satfwb * Ps;
    final float fesl = (float)(f * satewa * fastpow((float)Math.E, satewb*(T-273.15)/(T-satewc)));
    return fesl;
  }
  
  /**
   * Calculate the mass of water vapor in grams that occupies a given volume
   * with a given pressure and temperature.
   * @param V The volume occupied by the water vapor in m^3.
   * @param P The partial pressure exerted by the vapor in Pascals.
   * @param T The temperature of the vapor in Kelvin.
   * @return Mass of water in grams.
   */
  public static float waterVaporPartialPressureToMass(float V, float P, float T)
  {
    // R is the ideal gas constant and has the value 8.314 J·K^−1·mol^−1.
    float R = 8.314f;
    
    //PV=nRT
    // or PV/RT = n
    // n is in moles
    float n = P*V/(R*T);
    
    // Moar mass of water 18.01528(33) g/mol
    float mm = 18.01528f;
    
    // moles * grams/mole = grams.
    float mass = n * mm;
    
    // return mass in grams
    return (mass);
  }
  
  /**
   * Convert grams of water into height of water
   * @param g Amount of water in grams
   * @param A The area the water takes up.
   * @return The height the water would take up in meters
   */
  public static float waterMassToWaterHeight(float g, float A)
  {
    return (g/(A*100*100*100));
  }
  
  
  /**
   * Convert height of water into mass of water
   * @param h The height of the water in meters
   * @param A The area the water takes up.
   * @return The mass of the water in grams.
   */
  public static float waterHeightToWaterMass(float h, float A)
  {
    return (h*(A*100*100*100));
  }
}