Double Linked List Code Explain

Hello to everyone.
Im trying to learn some more things about the linked lists.
I have here two structures Users and Ratings the ratings is double linked list and link to users.

Can someone explain me exactly what they do every line...i have many things in my mind about, but i want to clear everything from an expert

Code:

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USER *user = usertable[slot];
(user->num_ratings)++;          // here is a counter
RATING_NODE *prev = NULL;
RATING_NODE *curr = user->ratings;
while (curr != NULL)
{
if (rnode->item_id < curr->item_id)
  break;                        // ok here checking the input item value with existing in the nodes
prev = curr;                    // search serialise every next
curr = curr->next;
}

if (prev == NULL) {
  rnode->next = user->ratings;
  user->ratings->prev = rnode;
  user->ratings = rnode;
} else if (curr == NULL) {
  prev->next = rnode;
  rnode->prev = prev;
} else {
  prev->next = rnode;
  rnode->prev = prev;
  rnode->next = curr;
  curr->prev = rnode;
}

}

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Thanks everyone for your time!.

Code:

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USER *user = usertable[slot];            // Find our user based on slot #
(user->num_ratings)++;                  // Increase the number of ratings
RATING_NODE *prev = NULL;                // We are going to keep track of the previous item, so we can do the insert
RATING_NODE *curr = user->ratings;      // Start at the head of the user's 'ratings' list


// We are going to walk the list until either
// * The current nodes's item_id is greater than the rating we are adding
// * We reach the end of the list
while (curr != NULL)                    // While we have not reached the end of the list
{
    if (rnode->item_id < curr->item_id)  // If we have reahed the point where we want to insert
      break;                            // then drop out of the loop


    prev = curr;                        // Move onto the next item in the list
    curr = curr->next;
}


// We now have three different places to insert
// - At the start of the list
// - In the middle of the list
// - At the end of the list
// It seems somebody has forgotten the 4th case - if the list is completely empty...
if (prev == NULL) {                      // Was prev never assigned anything but the initial NULL?                                       
  rnode->next = user->ratings;          // We are inserting at the head of the list
  user->ratings->prev = rnode;          // THIS LINE SEEM WRONG.. a NULL pointer access if user->ratings is NULL???
  user->ratings = rnode;
} else if (curr == NULL) {              // Did we walk off the end of the list?
  prev->next = rnode;                  // Insert at the end of the list - need to update 2 pointers
  rnode->prev = prev;
} else {                                // Both prev and curr are not NULL, so we are in the middle of the list
  prev->next = rnode;                  // Insert into the middle of the list - need to update 4 pointers
  rnode->prev = prev;
  rnode->next = curr;
  curr->prev = rnode;
}

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Thank you Hamster.
You clear my thoughts, fortunately they follow your instructions.