Salesforce trigger to generate invoice number, reset each year

gistfile1.java

trigger InvoiceTrigger on Invoice__c (before insert) {
  if (trigger.isBefore && trigger.isInsert)
  {
    // ---- GENERATE INVOICE NUMBER, AUTO RUNNING NUMBER, RESET EVERY YEAR BASED ON CREATED DATE --- //
    // ---- INVOICE NUMBER FORMAT : {0000} --//
    Integer iCharLen = 4; // character length for number
    Integer iLastNo = 0; // information last running number this year
    Integer iThisYear = Date.Today().year();
    
    // search latest invoice number
    String strTemp = '0';
    String strZero = '0';
    List<Invoice__c> lsInvoice = new List<Invoice__c>([SELECT Id, Name FROM Invoice__c WHERE CALENDAR_YEAR(CreatedDate) = :iThisYear LIMIT 1 ]);
    if (lsInvoice.size() > 0) strTemp = lsInvoice[0].Name; // in this case, invoice number only contains numeric value : 001, 003, etc
    iLastNo = Integer.valueOf(strTemp);
    
    // start generate number
    for(Invoice__c Inv: trigger.new)
    {
      iLastNo++;
      strTemp = String.valueOf(iLastNo);
      if (strTemp.length() < iCharLen) strTemp = strZero.repeat(iCharLen - strTemp.length()) + strTemp; // add 0 prefix
      Inv.Name = strTemp;
      
    } 
  }
}