#include <stdio.h>
#include <math.h>
#include <stdlib.h>
#define SEC_PARAM 10

/* candidate pseudorandom generator */
unsigned char * G(unsigned char input[SEC_PARAM]) {
  int i;
  unsigned char last;
  unsigned char *output = malloc((SEC_PARAM+1)*(sizeof(unsigned char)));

  last = 0;

  for (i=0; i<SEC_PARAM; i++) {
    output[i]=input[i];
    if (i%2) last ^= input[i];
  }

  output[SEC_PARAM]=last;

  return output;
}

/* Code for a distinguisher */
unsigned char D(unsigned char input[SEC_PARAM+1]){

  /* Your code goes here.
     Note: buffer overflows are out of scope... */

  return 0; // you can remove this line
}

main(){
  int i, j, total;
  FILE *randfile;
  float pr_a, pr_b;
  unsigned char guess;
  unsigned char input[SEC_PARAM];
  unsigned char *string = malloc((SEC_PARAM+1)*(sizeof(unsigned char)));

  randfile = fopen("/dev/urandom", "r");
  // urandom gives us a source of random numbers.
  // Note that urandom is NOT safe for real-world cryptographic applications

  total=0;

  for (i=0; i<10000; i++){
    // string chosen at random
      for (j=0; j<SEC_PARAM+1; j++)
	fscanf(randfile, "%c", &string[j]);
      guess = D(string);
      if (guess==1) total++;
  }
  pr_a = (float) total/10000; // Pr[D(string)=1]
  printf("pr_a = %f, ", pr_a);

  total=0;

  for (i=0; i<10000; i++){
    // string generated pseudorandomly
    for (j=0; j<SEC_PARAM; j++)
      fscanf(randfile, "%c", &input[j]);
      string = G(input);
      guess = D(string);
      if (guess==1) total++;
  }
  pr_b = (float) total/10000; // Pr[D(G(input))=1]
  printf("pr_b = %f\n", pr_b);

  printf("Your distinguisher's advantage was %f\n", fabs(pr_b-pr_a));

  fclose(randfile);

}