trigger RecordSteps on Record__c (before insert,after delete) {
    
    /* This 'IF' handles serial number assignments for all new inserted records */
    if(Trigger.isBefore && Trigger.isInsert){
		/* This set holds parent objects Ids */
        Set<Id> setParentIds=new Set<Id>();
        for(Record__c record:(List<Record__c>)Trigger.new)
            setParentIds.add(record.Object__c );
			
        /* Map to hold current count for each parent */
        Map<Id,Decimal> mapParentToCurrentCount=new Map<Id,Decimal>();
        
        /* Get the current count for each parent */
        for(AggregateResult result: [SELECT MAX(Seq__c),Object__c FROM Record__c WHERE Object__c IN :setParentIds GROUP BY Object__c])
            mapParentToCurrentCount.put((Id)result.get('Object__c'),(Decimal)result.get('expr0'));
        
        /* Assign record number for each new record according to current count of its parent */
        for(Record__c record:Trigger.new){
            Decimal currentCount=mapParentToCurrentCount.get(record.Object__c);
            currentCount=currentCount==null?1:(currentCount+1);
            record.Seq__c=currentCount;
            mapParentToCurrentCount.put(record.Object__c,currentCount);
        }
    }
    /* This 'ELSE If' handles serial number updation as some records get deleted */
    else if(Trigger.isAfter && Trigger.isDelete){
	
        Set<Id> setParentIds=new Set<Id>();
        Decimal lowestDeletedSerialNumber;
        Map<Id,List<Decimal>> mapParentToLstDeletedRecords=new Map<Id,List<Decimal>>();
        
        for(Record__c record:(List<Record__c>)Trigger.old){
			/* Get the parents' Ids */
            setParentIds.add(record.Object__c );
            
            if(mapParentToLstDeletedRecords.get(record.Object__c)==null)
                mapParentToLstDeletedRecords.put(record.Object__c,new List<Decimal>());
            
			/* Store each deleted serial number parent wise */
            mapParentToLstDeletedRecords.get(record.Object__c).add(record.Seq__c);
            
			/* Sort the deleted numbers so that we would not miss anything */
            mapParentToLstDeletedRecords.get(record.Object__c).sort();
            
			/* Infer lowest serial number, we could use it in next query */
            if(lowestDeletedSerialNumber==null||lowestDeletedSerialNumber>record.Seq__c)
                lowestDeletedSerialNumber=record.Seq__c;
                           
        }
        List<Record__c> lstRecordsRetrived = [SELECT Seq__c,Object__c FROM Record__c WHERE Object__c IN :setParentIds AND Seq__c > :lowestDeletedSerialNumber ORDER BY Seq__c DESC];
        
		/* Release unused memory */
        setParentIds=null;
        
        Map<Id,Record__c> mapIdToRecordsToBeUpdated=new Map<Id,Record__c>();
        
		/* Update serial numbers for records whose serial number was greater than the deleted serial number for given parent */
        for(Id parentId:mapParentToLstDeletedRecords.keyset()){
            for(Integer i=mapParentToLstDeletedRecords.get(parentId).size()-1;i>=0;i--){
                for(Record__c record:lstRecordsRetrived){
                    if(record.Object__c==parentId&&record.Seq__c>mapParentToLstDeletedRecords.get(parentId).get(i)){
                        record.Seq__c--;
						/* Update only records which get changed */
                        if(!mapIdToRecordsToBeUpdated.keyset().contains(record.Id))
                            mapIdToRecordsToBeUpdated.put(record.Id,record);
                    }
                }
            }
        }
        
		/* Release unused memory */
        mapParentToLstDeletedRecords=null;
        
        if(!mapIdToRecordsToBeUpdated.values().isEmpty())
            update mapIdToRecordsToBeUpdated.values();
       
    }
}