Hi would anyone happen to know how to write a testmethod for my trigger below:

trigger rollupTrigger on Commitment_Draw__c (after insert, after update, after delete)
{
    double totalSum = 0;
    Commitment_Draw__c[] c = trigger.new;
    if(Trigger.isUpdate || Trigger.isInsert)
    {
       for(Commitment_Draw__c cd : c)
       {
        for(Opportunity parent : [select Id,Name from Opportunity where Id = :cd.Opportunity__c])
        {
         for(Commitment_Draw__c childObject : [select Id,Opportunity__c,Amount__c from Commitment_Draw__c where Opportunity__c = :parent.Id])
            {
                  totalSum += childObject.Amount__c;
            }
            parent.Rollup__c = totalSum;  
            update parent;                    
        }
      }
    }
    else if(Trigger.isDelete)
    {
        Commitment_Draw__c [] cold = Trigger.old;
      for(Commitment_Draw__c oldobj :cold) 
      {
        for (Opportunity parentObject : [select Id,Name from Opportunity where Id= :oldobj.Opportunity__c])
        {
          for (Commitment_Draw__c childObject : [select Id,Opportunity__c,Amount__c from Commitment_Draw__c where Opportunity__c = :parentObject.Id])  
          {
             totalSum += childObject.Amount__c; 
          }
            parentObject.Rollup__c = totalSum;  
            update parentObject;                     
         }  
      }
    }
}