Question

I am working on a PHP application that finds all possible ways to swap cards. each user has at least one card. When a user request a card, the application shows him all possible ways to swap his card in excange for the card that he requested.

Lets say that:

user 1 has card type A
user 2 has card type B 
user 3 has card type C 

And lets assume that:

user 1 wants card type C
user 2 wants card type A
user 3 wants card type B

If user 1 requests card C, the only solution is that:

user 1 gives user 2 his card
user 2 gives user 3 his card
user 3 gives user 1 his card

But what if there were two other users:

user 4 has card type B & wants card type A
user 5 has card type C & wants card type B

Now, If user 1 requests card C, there is one other solution besides the one above which is that :

user 1 gives user 4 his card
user 4 gives user 5 his card
user 5 gives user 1 his card

There will be NO limit in terms of how many cards a user may have or request. So far, I created two SQL tables. The table “cards” keeps track of each userID and cardID. The Table “requests” keeps track of each userID and the desired cardID.

Answer 1

I wouldn’t use pure PHP for this. Rather I would create a MySQL table which stores a record for each user, recording which card that user has as well as which card he wants. Then, I would run something like this:

$sql = "SELECT * FROM users";
$result = $this->db->query();
$users = $result->getAll(); //A list of all the users.

foreach ($users as $user)
{
   $sql = "SELECT * FROM users WHERE cardId = '$user->wantedCardId'";
   $result = $this->db->query($sql);

   if (! $result->has_rows())
   {
      echo "No other users with $user->wantedCardId were found.";
      continue;
   }

   $cardHolder = $result->row();
   echo: "User $cardHolder->id gives $user->id his card";
}

If I were to do this using plain PHP, I would do something like this:

//Populate an array containing a list of all the users and their cards.

$users = array();

$user[1] = new StdClass();;
$user[1]->cardId = 2;
$user[1]->wantedId = 3;

$user[2] = new StdClass();
$user[2]->cardId = 3;
$user[2]->wantedId = 2;
// .....

foreach ($users as $userId=>$user)
{
   //Run a secondary loop through the users to find those that have the card that
   //this user wants.
   foreach ($users as $holderId=>$cardHolder)
   {
      if ($cardHolder->cardId != $user->wantedId)
         continue;
      echo "User $holderId gives $userId his card";
   }
}