Determining if a Field Has Changed in an Update Trigger

/ ==================================================================================
//   Object: biuContact
//   Author: John Westenhaver
// Comments: Automatically assign ownership of a Contact to the owner of the parent
//           Account.
// ==================================================================================
//  Changes: 2010-05-05 Initial version.
// ==================================================================================

trigger biuContact on Contact (before insert, before update)
{
   set accountIds = new set();

   // Get a list of Account IDs where the Owner ID has changed.
   for (Contact c : system.trigger.new)
   {
      if (system.trigger.isInsert)
      {
         // By definition, on insert, the Owner ID has changed.
         if (c.AccountId != null) accountIds.add(c.AccountId);
      }
      else if (system.trigger.isUpdate)
      {
         // Only check if the Owner ID is changed.
         if (c.OwnerId != system.trigger.oldMap.get(c.Id).OwnerId)
         {
            if (c.AccountId != null) accountIds.add(c.AccountId);
         }
      }
   }

   // Get a map of Accounts.
   map accountMap = new map(
      [SELECT Id, Name, OwnerId FROM Account WHERE Id IN :accountIds]);
 
   // Loop over all Contacts and update the Owner if
   // the Contact owner differs from the Account owner.
   Account a;
   for (Contact c : system.trigger.new)
   {
      a = accountMap.get(c.AccountId);
      if (a != null)
      {
         if (a.OwnerId != c.OwnerId) c.OwnerId = a.OwnerId;
      }
   }
}