global class BooksToAuthorsBatch implements Database.Batchable<SObject> {

    global BooksToAuthorsBatch() {

    }

    /**
     * @description gets invoked when the batch job starts
     * @param context contains the job ID
     * @returns the record set as a QueryLocator object that will be batched for execution
     */
    global Database.QueryLocator start(Database.BatchableContext context) {
        //return all books that don't have junction with writer and have author field filled
        return Database.getQueryLocator('SELECT Id, Name,  Author__c FROM Book__c WHERE Id NOT IN (SELECT Book__c FROM BooksToWriters__c) AND Author__c != '' ');
    }

    /**
     * @description gets invoked when the batch job executes and operates on one batch of records. Contains or calls the main execution logic for the batch job.
     * @param context contains the job ID
     * @param scope contains the batch of records to process.
     */
    global void execute(Database.BatchableContext context, List<Book__c> scope) {
        Map<Id, List<Writer__c>> booksAndWriters = new Map<Id, List<Writer__c>>();
        Set<Writer__c> writers = new Set<Writer__c>();

        Map<Id, Set<String>> booksAndWritersNames = new Map<Id, Set<String>>();

        //get all writers names for each book
        Set<String> writersNames = new Set<String>();
        for(Book__c book : scope){
            List<String> splitedNames = book.Author__c.split(', ');
            writersNames.addAll(splitedNames);

            if(!booksAndWritersNames.contains(book){
                booksAndWritersNames.put(book, splitedNames);
            }
            else {
                booksAndWritersNames.get(book).put(splitedNames);
            }
        }

        //get all writers records that already in org and their names
        Set<String> writersExistNames = new Set<String>();
        for(Writer__c  writer : [SELECT Id, Name FROM Writer__c WHERE Name IN :writersNames]){
            writers.add(writer);
            writersExistNames.add(writer.Name);
        }

        //create writers that doesn't exists
        List<Writer__c> newWritersToInsert = new List<Writer__c>();
        for(String writer : writersNames.removeAll(writersExistNames)){
            Writer__c newWriter = new Writer__c(Name = writer);
            newWritersToInsert.add(newWriter);
            writers.add(newWriter);
        }
        insert newWritersToInsert;

        List<BooksToWriters__c> booksToWritersToInsert = List<BooksToWriters__c>();
        for(Book__c book :booksAndWritersNames.keySet()){
            for(Writer__c writer : writers){
                if(booksAndWritersNames.get(book).contains(writer.Name)){
                    booksToWritersToInsert.add(new BooksToWriters__c(Book__c = book, Writer__c = writer));
                }
            }
        }
    }

    /**
     * @description gets invoked when the batch job finishes. Place any clean up code in this method.
     * @param context contains the job ID
     */
    global void finish(Database.BatchableContext context) {
        Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
        mail.setToAddresses(new String[]{'sidoretsyura@gmail.com'});
        mail.setReplyTo('batch@acme.com');
        mail.setSenderDisplayName('Batch Processing');
         mail.setSubject('Batch Process Completed');
         mail.setPlainTextBody('Batch Process has completed');
         Messaging.sendEmail(new Messaging.SingleEmailMessage[] { mail });

    }
}