The birthday clues

Problem

For Emiliano's eighth birthday, his aunt filled the backyard with shoe boxes numbered from 0, and inside each one she left a note with the number of the box to open next. Emiliano opens box 0 first, reads the note, goes to the box it names, and keeps going. The game ends when a note sends him to a box he already opened: that one does not count again.

You get notes, a list with at least one whole number: notes[i] is the number on the note in box i, always between 0 and the length minus 1, and it may be the same box. Several notes may send him to the same box, and some boxes may never be opened.

Return, as a whole number, how many different boxes Emiliano opened.

With [2, 0, 3, 1] he opens box 0, which sends him to 2; box 2 sends him to 3, box 3 to 1, and box 1 back to 0, which he already opened: you return 4.

With [1, 1, 0] he opens boxes 0 and 1, and box 1 sends him to itself: you return 2.

Examples

• The example

  [2, 0, 3, 1] → 4

• A note that points to its own box

  [1, 1, 0] → 2

• A single box

  [0] → 1

• Two boxes stay closed

  [1, 0, 3, 2] → 2

• Back to a box in the middle

  [1, 2, 3, 1, 0, 4] → 4

Besides these, the challenge has hidden tests that are revealed when you submit your solution.

You start with this

def opened_boxes(notes):
    pass

function openedBoxes(notes) {
}