trigger auto_Account_assignment on Contact(before insert) {
    Set<String> emailDomains = new Set<String>();

    for (Contact l : Trigger.New) {
        if (l.Email == null) {
            continue;
        }   

        emailDomains.add(l.Email.split('@').get(1));
    }

    emailDomains.remove(null);
           
    if (!emailDomains.isEmpty()) {
        Map<String, Account> emailDomainToAccountMap = new Map<String, Account>();

        for (Account a : [ 
            select Name,
                Id
            from Account
            where Name in :emailDomains
        ]) {
            emailDomainToAccountMap.put(a.Name, a);
        }   

        for (Contact l : Trigger.New) {
            if (l.Email == null) {
                continue;
            }

            String domain = l.Email.split('@').get(1);

            if (!emailDomainToAccountMap.containsKey(domain)) {
                continue;
            }

            l.AccountId= emailDomainToAccountMap.get(domain).Id;
        }
    }
}

@isTest
private class AccountTestClass {
 static testMethod void validateAId() {
Account a= new Account();
a.name='test';
//I am adding only the name but please add all the required fields so that above account record is properly inserted
insert a;
Contact tempc = new contact();
tempc.LastName='TestContact';
tempc.email='primary@test.com';
insert tempc;
Id AI= a.id;
Contact check=[select id,accountid from contact where id in :AI];
system.assert(check.AccountId,AI);
 }
}