/* 
 * Low-autocorrelation problem of binary sequences
 * 'Brute-force' method
 * Rick van der Zwet, Oct 2009
 * Licence: BSD
 */
#include <sysexits.h>
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
int main(int argc, char *argv[]) {

const int SIZE = 20;
int * signal = malloc(SIZE * sizeof(int));
int e_total, e_sub;
// Temp counters
int i,k;
long int n;
float f_y;

long int best_number = 0;
float best_value = 0;

void print_array(long int number, float result) {
    for(i = 0; i < SIZE; i++) {
        if (number & (1 << i)) {
            printf("+");
        } else {
            printf("-");
        }
    }
    printf(" = %f\n", result);
}

// All values to be determined
for (n = 0; n < pow(2,SIZE); n++) {
    // Make array ready
    for (i = 0; i < SIZE; i++) {
        signal[i] =  (n & (1 << i)) ? 1 : -1;
    }

    // Calculate E(y)
    e_total = 0;
    for(k = 1; k < SIZE; k++) {
        e_sub = 0;
        for(i = 0; i < SIZE - k; i++) {
            e_sub += signal[i] * signal[i+k];
        //printf("[k:%i i:%i] %i\n", k, i, signal[i] * signal[i+k]);
        }
        // Power of two
        e_total += pow(e_sub,2);
    }
    f_y = pow((float)SIZE,2)/(2 * e_total);

    // Save best value
    if (f_y >= best_value) {
        print_array(n,f_y);
        best_value = f_y;
        best_number = n;
    }

}

// Clean up mess
free(signal);
return EX_OK;
}