fastscalarproduct.java

import java.util.*;
class fastscalarproduct {

    public final static float scalarProduct(float[]  v1, float[]  v2) {
        return v1[0] * v2[0]  + v1[1] * v2[1] + v1[2] * v2[2] + v1[3] * v2[3];
    }

    public final static float fastscalarProduct(float[]  v1, float b1, float[]  v2, float b2) {

        return ( v1[0] + v2[1] ) * ( v2[0]  + v1[1]) + ( v1[2] + v2[3] ) * ( v1[3] + v2[2]) -b1 - b2;
    }

    public static void main(String[]  args) {
        int N = 10000;
        float[][] vectors = new float[N][5];
        Random rand = new Random();
        float[] buffers = new float[N];
        for (int k = 0; k<N; ++k) {
            for(int l = 0; l<4; ++l) {
                vectors[k][l] = rand.nextFloat();
            }
            buffers[k] = vectors[k][0] * vectors[k][1] + vectors[k][2] * vectors[k][3];
        }
        float sumofproducts = 0;
        for(int k1 = 0; k1< N; ++k1) {
            for(int k2 = 0; k2< N; ++k2) {
                sumofproducts += scalarProduct(vectors[k1],vectors[k2]);
            }
        }
        float sumofproducts2 = 0;
        for(int k1 = 0; k1< N; ++k1) {
            for(int k2 = 0; k2< N; ++k2) {
                sumofproducts2 += fastscalarProduct(vectors[k1],buffers[k1],vectors[k2],buffers[k2]);
            }
        }
        long bef = System.currentTimeMillis();
        for(int k1 = 0; k1< N; ++k1) {
            for(int k2 = 0; k2< N; ++k2) {
                sumofproducts += scalarProduct(vectors[k1],vectors[k2]);
            }
        }
        long aft1 = System.currentTimeMillis();
        for(int k1 = 0; k1< N; ++k1) {
            for(int k2 = 0; k2< N; ++k2) {
                sumofproducts2 += fastscalarProduct(vectors[k1],buffers[k1],vectors[k2],buffers[k2]);
            }
        }
        long aft = System.currentTimeMillis();
        System.out.println((aft1-bef)+" "+(aft-aft1));
        System.out.println(sumofproducts+" "+sumofproducts2);

    }
}