Very fast accurate approximation to Math.exp in java using floating point bit hacks.

fastexp.java

/* fast floating point exp function
 * must initialize table with buildexptable before using
 
Based on 
 A Fast, Compact Approximation of the Exponential Function
 Nicol N. Schraudolph 1999

Adapted to single precision to improve speed and added adjustment table to improve accuracy.
Alrecenk 2014

 * i = ay + b
 * a = 2^(mantissa bits) / ln(2)   ~ 12102203
 * b = (exponent bias) * 2^ ( mantissa bits) ~ 1065353216
 */
public static float fastexp(float x){
	final int temp = (int)(12102203 * x + 1065353216) ;
	return Float.intBitsToFloat(temp)*expadjust[(temp>>15)&0xff] ;
}

static float expadjust[] ;
//build correction table to improve result in region of interest
//if region of interest is large enough then improves result everywhere
public static void buildexptable(double min, double max, double step){
	expadjust = new float[256];
	int amount[] = new int[256] ;
	//calculate what adjustments should have been for values in region
	for(double x=min; x < max;x+=step){
		double exp = Math.exp(x);
		int temp = (int)(12102203 * x + 1065353216) ;
		int index = (temp>>15)&0xff ;
	    double fexp = Float.intBitsToFloat(temp);
		expadjust[index]+= exp/fexp ;
		amount[index]++;
	}
	//average them out to get adjustment table
	for(int k=0;k<amount.length;k++){
		expadjust[k]/=amount[k];
	}
}