here is a class - can anyone help me to explain the functionality of this apex class so that i can write test class for it..


public with sharing class CuAp_ContractDebtTransferCtrlr {
    private static final String CLASS_NAME = 'CuAp_ContractDebtTransferCtrlr';
    private static final String DEBUG = '[' + CLASS_NAME + '] ';
    private static final List<String> transferReasonField = new List<string>{ GlUt_APIConstantsBillTransfer.TRANSFERREASONAPINAME};
        
    private static final List<String> contractFields = new List<string>{ 
       GlUt_APIConstantsContracts.CONTRACTNAMEAPINAME,
       GlUt_APIConstantsContracts.BILLINGADDRESSAPINAME,
       GlUt_APIConstantsContracts.CONTRACTBALANCEAPINAME
    };
        
    private static final List<String> billCalculationFields = new List<string>{ 
       GlUt_APIConstantsBillCalculation.STATUSAPINAME
    };
                    
        
    @AuraEnabled
    public static List<List<GlAp_FieldProps>> retrieveTransferReasonDetails() {
        List<List<GlAp_FieldProps>> twoColFormattedList = new List<List<GlAp_FieldProps>>();
        try {
            List<GLAp_FieldProps> transferReasonFieldProps = new List<GLAp_FieldProps>();
            transferReasonFieldProps = GlAp_LightningDataAccess.getObjectsFieldProperties(GlUt_APIConstantsBillTransfer.BILLTRANSFERAPINAME, transferReasonField);
            for(integer i=0; i < transferReasonFieldProps.size(); i = i+2){
                List<GlAp_FieldProps> lstEntry = new List<GlAp_FieldProps>();
                integer secondEntryIndex = i+1;
                lstEntry.add(transferReasonFieldProps[i]);
                if(secondEntryIndex < transferReasonFieldProps.size()){
                    lstEntry.add(transferReasonFieldProps[secondEntryIndex]);
                }
                twoColFormattedList.add(lstEntry);
            }
        } catch (exception e) {
            GlUt_Logger.logException(e);
        }
        finally {
            GlUt_Logger.writeLogsToDatabase();
        }
        return twoColFormattedList;
    }
    
    @AuraEnabled
    public static List<List<GlAp_FieldProps>> retrieveContractFieldDetails() {
        List<List<GlAp_FieldProps>> twoColFormattedList = new List<List<GlAp_FieldProps>>();
        // Get the field properties for the Location object
        List<GLAp_FieldProps> contractProperties = new List<GLAp_FieldProps>();
        try {
            contractProperties = GlAp_LightningDataAccess.getObjectsFieldProperties(GlUt_APIConstantsContracts.CONTRACTAPINAME, contractFields);
     
            GlAp_FieldProps objFieldProp1 = new GlAp_FieldProps(true, false, true, false, CuAp_Constants.AMOUNTTOTRANSFER,CuAp_Constants.AMOUNTTOTRANSFERLABEL,
                                                               'number', false, false, GlUt_APIConstantsContracts.CONTRACTAPINAME);

            contractProperties.add(objFieldProp1);
            
            for(GlAp_FieldProps fieldProp : contractProperties ){
                if (fieldProp.fieldName == GlUt_APIConstantsContracts.CONTRACTBALANCEAPINAME )
                    fieldProp.fieldLabel = LoAp_Constants.DEBTAMOUNTLABEL;
                
                fieldProp.showLabel = true;
            }
            
            for(integer i=0; i < contractProperties.size(); i = i+2){
                List<GlAp_FieldProps> lstEntry = new List<GlAp_FieldProps>();
                integer secondEntryIndex = i+1;
                lstEntry.add(contractProperties[i]);
                if(secondEntryIndex < contractProperties.size()){
                    lstEntry.add(contractProperties[secondEntryIndex]);
                }
                twoColFormattedList.add(lstEntry);
            }
        } catch (exception e) {
            GlUt_Logger.logException(e);
        }
        finally {
            GlUt_Logger.writeLogsToDatabase();
        }
        return twoColFormattedList;
    }
    
    @AuraEnabled
    public static BillingContract__c retrieveContractDetails(String ContractId) {
        BillingContract__c objBillingContract ;
        if(string.isNotBlank(ContractId)){
            objBillingContract = (BillingContract__c)GlAp_LightningDataAccess.getRecord(GlUt_APIConstantsContracts.CONTRACTAPINAME,
                                                                                        ContractId,contractFields);
        }
        return objBillingContract;
    }
    
    @AuraEnabled
    public static string transferDebts(String contractFromTransferID, String contractToTransferID, String transferReason, String amountToTransfer, string billCalculationId) {
        String responseCode;
        CuAp_BillTransferObject objBTData = new CuAp_BillTransferObject();
        objBTData.BillTransfers = new List<CuAp_BillTransferObject.BillTransfers>();
        
        CuAp_BillTransferObject.BillTransfers objBillTransfer = new CuAp_BillTransferObject.BillTransfers(contractToTransferID,contractFromTransferID,
                                                                                                          transferReason,amountToTransfer,billCalculationId);
        objBTData.BillTransfers.add(objBillTransfer);
        String JSONPayload = JSON.serialize(objBTData);
        try{
            HttpResponse response = BlUt_HerokuWebservices.makeCallout(BlUt_HerokuWebservices.DEBT_TRANSFER, 2000, 'application/json', JSONPayload, 'PUT');
            // handle response
            if (response.getStatusCode() == 200) {
                if(billCalculationId != null ){
                    BillCalculation__c objBillCalculation = (BillCalculation__c)GlAp_LightningDataAccess.getRecord(GlUt_APIConstantsBillCalculation.BILLCALCULATIONAPINAME,
                                                                                        billCalculationId,billCalculationFields);
                    
                    objBillCalculation.Status__c = Label.BlIn_BillCalculationStatusPending;
                    objBillCalculation  = (BillCalculation__c) GlUt_FieldSecurity.cleanseFields(objBillCalculation, false);
                    update objBillCalculation;
                }
                responseCode = Label.GlUt_OK;
            }
            else {
                throw new AuraHandledException(Label.BlAp_BillCalcRequestError + response.getStatus());
            }
        }
        catch (exception e) {
            GlUt_Logger.logException(e);
            throw new AuraHandledException(e.getMessage());
        }
        finally {
            GlUt_Logger.writeLogsToDatabase();
        }
        return responseCode;
    }
    
    @AuraEnabled
    public static Id createBillCalculation(string contractId){
        BillCalculation__c billCalculationObj = GlUt_BillCalculationUtilities.createBillCalculation(contractId,Label.GlUt_Transfer,Label.BlIn_BillCalculationStatusNew);
        return billCalculationObj.Id;
    }
    
    
    public class DebtTransferContainerWrapper{
        @AuraEnabled
        public List<List<GlAp_FieldProps>> transferReasonList;
        @AuraEnabled
        public List<List<GlAp_FieldProps>> billingContractFieldList;
        @AuraEnabled
        public BillingContract__c billingContractObj;
        @AuraEnabled
        public String  billingAddressFormatted;
        
        public DebtTransferContainerWrapper(){
              this.transferReasonList = new List<List<GlAp_FieldProps>>();
              this.billingContractFieldList = new List<List<GlAp_FieldProps>>();
              this.billingContractObj = new BillingContract__c();
          }
    }
   
    @AuraEnabled
    public static DebtTransferContainerWrapper getInfoToDisplay(string billingContractRecordId) {
        DebtTransferContainerWrapper objWrapper = new DebtTransferContainerWrapper();
        if(billingContractRecordId != null ){
            objWrapper.transferReasonList = retrieveTransferReasonDetails();
            objWrapper.billingContractFieldList = retrieveContractFieldDetails();
            objWrapper.billingContractObj = retrieveContractDetails(billingContractRecordId);
            objWrapper.billingAddressFormatted =   retrieveContractDetails(billingContractRecordId).BillingAddress__c.replace('<br>', '\r\n');
        }
        return objWrapper;
    }
}