Salesforce trigger to generate invoice number, each account has its own running number, format {AccountNumber}-{YYYY}-{MM}-{000}

trigger_autonumber3.java

trigger InvoiceTrigger3 on Invoice__c (before insert) {
  if (trigger.isBefore && trigger.isInsert)
  {
    // ---- GENERATE INVOICE NUMBER, AUTO RUNNING NUMBER, EACH ACCOUNT HAS ITS OWN RUNNING NUMBER --- //
    // ---- INVOICE NUMBER FORMAT : {ACCOUNTNUMBER}-{YYYY}-{MM}-{000} --//
    // ---- ASSUME THAT ACCOUNT NUMBER IS MANDATORY -- //
    // ---- BECAUSE WE HAVE YEAR AND MONTH INFORMATION IN THE NUMBER, EASIER IF WE CREATE A FIELD FOR THE NUMBER [ACC RUNNING NO] --//
    Integer iCharLen = 3; // character length for number
    Integer iLastNo = 0; // information last running number this year
    String strZero = '0';
    
    // search list of account
    String strTemp;
    Map<Id, Integer> mapAccLast = new Map<Id, Integer>(); // map of last number for an account
    for (Invoice__c Inv: trigger.new)
    {
      mapAccLast.put(Inv.Account__c, 0); // add default last number
    }
    
    // search latest invoice number
    List<String> arrStr = new List<String>();
    Set<Id> setAccId = mapAccLast.keyset();
    Map<Id, String> mapAccNumber = new Map<Id, String>();// map account number for each account id
    for (AggregateResult ar : [Select Account__c, MAX(Acc_Running_No__c) last_no
                               From Invoice__c
                               WHERE IsDeleted = false AND Account__c IN : setAccId
                               GROUP BY Account__c
                              ])
    {
        strTemp = String.valueOf(ar.get('last_no'));
        mapAccLast.put(String.valueOf(ar.get('Account__c')), Integer.valueOf(strTemp));
        
    }
    // get information of account number
    for (Account A: [SELECT Id, AccountNumber FROM Account WHERE Id IN : setAccId])
    {
        mapAccNumber.put(A.Id, A.AccountNumber);
    }
    
    // start generate number
    String strNo = '';
    for(Invoice__c Inv: trigger.new)
    {
      iLastNo = 0; // init
      
      if (mapAccLast.containsKey(Inv.Account__c)) iLastNo = mapAccLast.get(Inv.Account__c);
      iLastNo++; // update to next number
      strTemp = String.valueOf(iLastNo);
      if (strTemp.length() < iCharLen) strTemp = strZero.repeat(iCharLen - strTemp.length()) + strTemp; // add 0 prefix
      
      strNo  = '';
      //if (mapAccNumber.containsKey(Inv.Account__c)) 
      strNo += mapAccNumber.get(Inv.Account__c) + '-';
      strNo += String.ValueOf(Inv.Invoice_Date__c.year()) + '-';
      strNo += (Inv.Invoice_Date__c.month() < 10) ? '0' + String.ValueOf(Inv.Invoice_Date__c.month()) : String.ValueOf(Inv.Invoice_Date__c.month());
      strNo += '-' + strTemp;
      Inv.Name = strNo; // update field
      Inv.Acc_Running_No__c = iLastNo;
      
      mapAccLast.put(Inv.Account__c, iLastNo); // update map info      
    } 
  }
}