knapsack bktr + plot

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <time.h>
#define MAX(a, b) a > b ? a : b
#define NMAX 15
#define NMIN 5

typedef struct {
  int weight, gain;
} Item;

int ksBktr(int n, int k, const Item items[]) {
  uint16_t state, lastState = (1 << n) - 1;
  int ans = 0;
  for (state = 0; state <= lastState; state++) {
    int weight = 0, gain = 0;
    for (int bit = 0; bit < NMAX; bit++) {
      Item item = items[bit];
      if ((state & (1 << bit)) && weight + item.weight <= k) {
        weight += item.weight;
        gain += item.gain;
      }
    }
    ans = MAX(ans, gain);
  }
  return ans;
}

double getSecs(clock_t start, clock_t end) {
  return ((double) (end - start)) / CLOCKS_PER_SEC;
}

int main() {
  //modify the input from here if you want
  const int k = 10;
  const Item items[NMAX + 1] = {{3,7},{3,4},{1,2},{1,9},{2,4},{1,5}};

  FILE *f = popen("gnuplot --slow", "w");
  if (f == NULL) {
    perror("popen error: ");
    exit(EXIT_FAILURE);
  }//*/

  fprintf(f, "plot '-'\n");
  clock_t start, end;
  for (int i = NMIN; i <= NMAX; i++) {//plot 5-15
    start = clock();
    int ans = ksBktr(i, k, items);
    end = clock();
    printf("n=%d: %d\n", i, ans);
    double secs = getSecs(start, end);
    fprintf(f, "%d %lf\n", i, secs);
  }

  fprintf(f, "e\n");
  fprintf(f, "set title \"time(n)\"\n");
  fprintf(f, "set xlabel \"n\"\n");
  fprintf(f, "set ylabel \"time\"\n");
  if (fflush(f) == EOF) {
    perror("");
    fclose(f);
    exit(-1);
  }

  puts("PRESS ENTER TO EXIT...");
  getchar();
  if (fclose(f) == EOF) {
    perror("");
  }//*/
  return 0;
}