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function determine_left_index(len){
    /*
    If the number of locations doesn't divide equally
    into three perform a function on that number that
    will determine the index we should point to.
    Example:
        len = 20        // twenty photo objects
        len % 3 == 0    // returns false, 20 does not divide equally into 3
        len/3           // returns 6.666...
        (len/3)|0       // returns 6 which says we have six screens of three photos
        (((len/3)|0)*3) // returns 18 which says we should point our left index
                        // at 18 and the visited locations will display the photos
                        // at index 18 and 19.
    */
    if (len % 3 === 0){ 
        return photo_objects.length - 3;
    } else {
        return (((len/3)|0)*3);
    }   
}