// merge sightings of the same species on the same day
// into the first sighting of the  species on that day
// The number of whales seen in subsequent sightings is added
// to the number of whales seen in the first sighting.
// The subsequent sightings are deleted from the list
// and all associated memory freed.

// compare equality of string 'species'
int same_species(struct pod *pod, struct pod *comp) {
   return (strcmp(pod->species, comp->species) == 0);
}

// check dates are the same
int same_day(struct date *day, struct date *comp) {
   return (day->year  == comp->year &&
           day->month == comp->month &&
           day->day   == comp->day);
}

// check same day and same date
int same_day_species(struct pod *pod, struct pod *comp) {
   return (same_species(pod, comp) && same_day(pod->when, comp->when));
}

// delete a pod strct
void delete_pod(struct pod *curr) {
   // don't delete if null
   if (!curr) return;
   free(curr->species);
   free(curr->when);
   free(curr);
}

// helper for main
struct pod *merge_helper(struct pod *comp, struct pod *curr) {
   if (curr) {
      struct pod *next = curr->next;
      if (same_day_species(comp, curr)) {
         comp->how_many += curr->how_many;
         delete_pod(curr);
         curr = merge_helper(comp, next);
      } else {
         curr->next = merge_helper(comp, next);
      }
   }
   return curr;
}

// main
void merge_day_whales(struct pod *first_pod) {
    struct pod *curr = first_pod;
    while (curr->next) {
      curr->next = merge_helper(curr, curr->next);
      curr = curr->next;
    }
}