given 3 ints, return the median

main.c

int median3(int a, int b, int c) {
  int index = 
    ((a < b) << 0) |
    ((b < c) << 1) |
    ((c < a) << 2);
  switch (index) {
    case 0: unreachable();
    case 1: return c;
    case 2: return a;
    case 3: return b;
    case 4: return b;
    case 5: return a;
    case 6: return c;
    case 7: unreachable();
  }
  unreachable();
}