trigger UpdateActivityDetailsOnAccount on Event (after insert, after update, after delete) {

    Set <ID> acctIDs = new Set <ID> ();
    Set <ID> contactIDs = new Set <ID> ();
    Map <ID, DateTime> lastVisitDateMap = NEW Map <ID, DateTime> ();
    Map <ID, DateTime> scheduledMeetingMap = NEW Map <ID, DateTime> ();
    Map <ID, String> lastActivityTypeMap = NEW Map <ID, String> ();
    Map <ID, DateTime> lastActivityDateMap = NEW Map <ID, DateTime> ();
    
    if (!Trigger.isDelete) {
        for (Event e: Trigger.NEW) {
            if (e.WhatId != NULL) {
                if (String.valueOf(e.WhatId).startsWith('001')) {
                    acctIDs.add(e.WhatId);
                } 
            }
            if (e.WhoId != NULL) {
                if (String.valueOf(e.WhoId).startsWith('003')) {
                    contactIDs.add(e.WhoId);
                } 
            }
        }
    }
    if (Trigger.isDelete) {
        for (Event e: Trigger.OLD) {
            if (e.WhatId != NULL) {
                if (String.valueOf(e.WhatId).startsWith('001')) {
                    acctIDs.add(e.WhatId);
                } 
                if (e.WhoId != NULL) {
                    if (String.valueOf(e.WhoId).startsWith('003')) {
                        contactIDs.add(e.WhoId);
                    } 
                }
            }
        }
    }
    
    if (!contactIDs.isEmpty()) {
        List <Contact> allRelatedContacts = [ 
                                                SELECT AccountID FROM Contact 
                                                    WHERE ID IN: contactIDs
                                            ];
        for (Contact con: allRelatedContacts) {
            if (con.AccountID != NULL)
                acctIDs.add(con.AccountID);
            
        }
    }
    
    /**** To Capture Last Meeting Date ********/
    for(Account accHistory: [ SELECT 
                                (SELECT 
                                    o.What.Name , o.WhatId, o.IsTask, 
                                    o.StartDateTime, o.Subject, o.Account.Name 
                                FROM ActivityHistories o 
                                WHERE IsTask = FALSE
                                    AND Subject = 'Meeting'
                                Order By StartDateTime Desc 
                                LIMIT 1) 
                            FROM Account WHERE ID IN: acctIDs]) {
        
        if (!accHistory.ActivityHistories.isEmpty()) {
            for (ActivityHistory lastMeeting: accHistory.ActivityHistories) {
                lastVisitDateMap.put(accHistory.ID, lastMeeting.StartDateTime);
            }
        }                   
    }
    
    /**** To Capture Next Meeting Date ********/
    for(Account accHistory: [ SELECT 
                                (SELECT 
                                    o.What.Name , o.WhatId, o.IsTask, 
                                    o.StartDateTime, o.Subject, o.Account.Name 
                                FROM OpenActivities o 
                                WHERE IsTask = FALSE
                                    AND Subject = 'Meeting'
                                Order By StartDateTime ASC 
                                LIMIT 2) 
                            FROM Account WHERE ID IN: acctIDs]) {
        
        if (!accHistory.OpenActivities.isEmpty()) {
            for (OpenActivity nextMeeting: accHistory.OpenActivities) {
                scheduledMeetingMap.put(accHistory.ID, nextMeeting.StartDateTime);
            }
        }                   
    }
    
    /**** To Capture Last Activity Type & Date********/
    for(Account accHistory: [ SELECT 
                                (SELECT 
                                    o.What.Name , o.WhatId, o.IsTask, 
                                    o.StartDateTime, o.Subject, o.Account.Name 
                                FROM ActivityHistories o 
                                WHERE IsTask = FALSE
                                Order By StartDateTime Desc 
                                LIMIT 1) 
                            FROM Account WHERE ID IN: acctIDs]) {
        
        if (!accHistory.ActivityHistories.isEmpty()) {
            for (ActivityHistory lastTypeActivity: accHistory.ActivityHistories) {
                lastActivityTypeMap.put(accHistory.ID, lastTypeActivity.Subject);
                lastActivityDateMap.put(accHistory.ID, lastTypeActivity.StartDateTime);
            }
        }                   
    }
    
    List<Account> accList = NEW List<Account> ();
    for (ID accID: lastVisitDateMap.keySet()) {
        
        Account acc = NEW Account(ID = accID);
            acc.Last_Visit_Date__c = lastVisitDateMap.get(accID);
            acc.Scheduled_Visit_Date__c = scheduledMeetingMap.get(accID);
            acc.Last_Activity_Type__c = lastActivityTypeMap.get(accID);
            acc.Last_Activity_Date__c = lastActivityDateMap.get(accID);
        accList.add(acc);
        
    }
    Update accList;
    
}