SigmodLookup.java

package suskun.nn;

public class SigmoidLookup {
    public static final float SCALE_FACTOR = 50.0f;
    public static final int LOOKUP_SIZE = 1024;
    int[] lookup;

    static final float SIGMOID_QUANTIZATION_MULTIPLIER = 255.0f;

    static final int SIGMOID_QUANTIZATION_MULTIPLIER_UCHAR = 255;

    static final int SIGMOID_HALF_LOOKUP_SIZE = LOOKUP_SIZE/2;

    public SigmoidLookup() {
        int size = LOOKUP_SIZE;  // table lookup size 1088=64*20 is arbitrary but can be
        // divided to 64
        lookup = new int[LOOKUP_SIZE];

        // when a sigmoid is quantized with 255, numbers below around -5.4 becomes 0,
        // numbers over around +5.4 becomes 254
        for (int i = -SIGMOID_HALF_LOOKUP_SIZE; i < SIGMOID_HALF_LOOKUP_SIZE; ++i) {
            float k = i / SCALE_FACTOR;
            float sigmoid = (float) (1.0f / (1 + Math.exp(-k)));
            int q = Math.round(sigmoid * SIGMOID_QUANTIZATION_MULTIPLIER);
            lookup[i + size / 2] = q;
        }
    }

    int get(float input) {
        int k = Math.round(input * SCALE_FACTOR);
        if (k <= -SIGMOID_HALF_LOOKUP_SIZE) return 0;
        if (k >= SIGMOID_HALF_LOOKUP_SIZE)
            return SIGMOID_QUANTIZATION_MULTIPLIER_UCHAR;
        return lookup[k + SIGMOID_HALF_LOOKUP_SIZE];
    }

    static int sigmoid(float input) {
        return (int) Math.round(1f/(1+Math.exp(-input))*255);
    }

    public static void main(String[] args) {

        SigmoidLookup sl = new SigmoidLookup();

        for(int i =-10; i<10;i++) {
            for(int j =0;j<100;j++) {
                float k = i + j/ 100f;
                System.out.println(k+" - "+ sl.get(k) + " - " + sigmoid(k));
            }
        }

    }


}