What does this code do?

Question

function mystery($y, $m, $d) {

  $a = 0;
  $b = 0;
  $c = 0;

  if($m < 3) {
    $a = $m + 10;
    $b = ($y-1) % 100;
    $c = ($y-1) / 100;
  }
  else {
    $a = $m - 2;
    $b = $y % 100;
    $c = $y / 100;
  }

  $w = (700 + (((26*$a)-2)/10)+$d+$b+$b/4+$c/4-(2*$c))%7;
  echo $w;

}

One of my tutorial questions asks what the function calculates. I can go through and explain every calculation, but I'm sure that's not what we're expected to do. Is there any obvious use I'm not seeing?

It looks to me like it could be a checksum algorithm, because it always seems to generate a digit between 0 and 6.

ps, it was originally written in Java but I ported it to PHP for simplicity when I typed it into my computer to test. I can re-type the Java version in if anyone would prefer it.

Answer 1

What does %7 suggest?

Answer 2

Think about dates and what Marcelo posted. Here is compilable java. Try running the program with various inputs and see what you come up with.

class mys {
    public static void main(String[] args) {
          int y= Integer.parseInt(args[0]);
          int m= Integer.parseInt(args[1]);
          int d = Integer.parseInt(args[2]);
          int a = 0;
          int b = 0;
          int c = 0;

          if(m < 3) {
            a = m + 10;
            b = (y-1) % 100;
            c = (y-1) / 100;
          }
          else {
            a = m - 2;
            b = y % 100;
            c = y / 100;
          }

          int w = (700 + (((26*a)-2)/10)+d+b+b/4+c/4-(2*c))%7;
          System.out.println(w);
    }
}