global class SendEmailToDueDateTask implements Database.Batchable<sObject>  {
    map<string,list<task>> userEmailTasklistmap = new map<string,list<task>>();
    
    global Database.QueryLocator start(Database.BatchableContext BC){
        return Database.getQueryLocator([SELECT ID,createddate,what.id,Owner.Email,OwnerId,owner.name,Status,ActivityDate,Subject from Task WHERE Status != 'Completed' and owner.isactive = true and owner.profile.id = '00eD0000001Qs3R']);
    }
    
    global void execute(Database.BatchableContext BC, List<Task> scope){
        for(Task Tsk : scope){
            if(!userEmailTasklistmap.Containskey(tsk.owner.email)){
                userEmailTasklistmap.put(tsk.owner.email, new list<task>());
            }
            userEmailTasklistmap.get(tsk.owner.email).add(tsk);
            
          }  
        List<Messaging.SingleEmailMessage> mails = new List<Messaging.SingleEmailMessage>();
    
            for(string email : userEmailTasklistmap.keyset()){
                
                Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
                list<string> toAddresses = new list<string>();
                toAddresses.add(email);
                mail.setToAddresses(toAddresses);
                mail.setSubject('Details of tasks due for today');                
                String username = userEmailTasklistmap.get(email)[0].owner.name;
                String htmlBody = '';
                
                htmlBody = '<table width="100%" border="0" cellspacing="0" cellpadding="8" align="center" bgcolor="#F7F7F7">'+
                            +'<tr>'+
                              +'<td style="font-size: 14px; font-weight: normal; font-family:Calibri;line-height: 18px; color: #333;"><br />'+
                                   +'<br />'+
                                    +'Dear '+username+',</td>'+
                            +'</tr>'+
                            +'<tr>'+
                                +'<td style="font-size: 14px; font-weight: normal; font-family:Calibri; line-height: 18px; color: #333;">Following tasks are pending. Please update the status.</td>'+
                            +'</tr>'+
                        +'</table>';
 
                htmlBody +=  '<table border="1" style="border-collapse: collapse"><tr><th>Related To</th><th>Subject</th><th>Created Date</th><th> Due Date</th></tr>';
                for(task tsk : userEmailTasklistmap.get(email)){
                    
                    String duedate = '';
                    if (tsk.ActivityDate != null)
                        duedate = tsk.ActivityDate.format();                    
                    else
                        duedate = '';
                    String Subject = tsk.subject;
                    datetime dt = tsk.createddate;
                    string createddate = dt.format('M/d/yyyy');
                    //string what = https://ap2.salesforce.com/tsk.what.id;
                    string View = URL.getSalesforceBaseUrl().toExternalForm()+'/'+ tsk.id;
                    string Clickhere = View;  
                    htmlBody += '<tr><td>' + Clickhere + '</td><td>' + Subject + '</td><td>' + createddate + '</td><td>' + duedate + '</td></tr>';                    
                }
                 htmlBody += '</table><br>';
                 mail.sethtmlBody(htmlBody);
                 mails.add(mail);                    
            }
             if(mails.size()>0)
             Messaging.sendEmail(mails);
    }
    global void finish(Database.BatchableContext BC){        
    }
}

@isTest 

public class SendEmailToDueDateTaskTest { 

static testMethod void testMethod1(){ 

        Profile pro = [SELECT Id FROM Profile WHERE Name='Standard User'];  
        User usr = new User(Alias = 'standt', Email='standarduser@tt.com',  
        EmailEncodingKey='UTF-8', LastName='Testing', LanguageLocaleKey='en_US',  
        LocaleSidKey='en_US', ProfileId = pro.Id,  
        TimeZoneSidKey='America/Los_Angeles', UserName='standarduser@tt.com'); 

        System.runAs(usr) { 
            Account acc = new Account(); 
                acc.Name = 'Test Account'; 
                insert acc; 
                acc=[SELECT id,Name FROM account WHERE id=:acc.Id]; 
                System.assertEquals(acc.Name,'Test Account'); 
            Task tsk = new Task(); 
                tsk.whatId = acc.Id;  
                tsk.Subject = 'Testing'; 
                tsk.Status = 'In Progress'; 
                tsk.Priority = 'Normal'; 
                tsk.ActivityDate = System.today(); 
                insert tsk ; 
                tsk=[SELECT id,Status FROM Task WHERE id=:tsk.Id]; 
                System.assertEquals(tsk.Status ,'In Progress'); 
            } 
            Test.StartTest(); 
                Database.executeBatch (new SendEmailToDueDateTask (),200); 
            Test.StopTest(); 
    } 
 }