correlate.java

public static double correlate(double[] a, double[] b) {
    if (a.length != b.length) {
        return Double.NaN;
    }
    double partial[] = new double[4];
    int i = 0;
    for ( ; i + 3 < a.length; i += 4) {
        partial[0] += a[i+0] * b[i+0];
        partial[1] += a[i+1] * b[i+1];
        partial[2] += a[i+2] * b[i+2];
        partial[3] += a[i+3] * b[i+3];
    }
    double correlation = partial[0] + partial[1] + partial[2] + partial[3];
    for ( ; i < a.length; i++) {
        correlation += a[i] * b[i];
    }
    return correlation;
}